Analysis of Growth and Water Relations of Tomato Fruits in Relation to Air Vapor Pressure Deficit and Plant Fruit Load

The influence of air vapor pressure deficit (VPD) and plant fruit load on the expansion and water relations of young tomato fruits grown in a glasshouse were evaluated under summer Mediterranean conditions. The contributions of phloem, xylem and transpiration fluxes to the fruit volume increase were estimated at an hourly scale from the growth curves of intact, heat-girdled and detached fruits, measured using displacement transducers. High VPD conditions reduced the xylem influx and increased the fruit transpiration, but hardly affected the phloem influx. Net water accumulation and growth rate were reduced, and a xylem efflux even occurred during the warmest and driest hours of the day. Changes in xylem flux could be explained by variations in the gradient of water potential between stem and fruit, due to changes in stem water potential. Misting reduced air VPD and alleviated the reduction in fruit volume increase through an increase in xylem influx and a decrease in fruit transpiration. Under low fruit load, the competition for assimilates being likely reduced, the phloem flux to fruits increased, similarly to the xylem and transpiration fluxes, without any changes in the fruit water potential. However, different diurnal dynamics among treatments assume variable contributions of turgor and osmotic pressure in F3 and F6 fruits, and hypothetical short-term variations in the water potential gradient between stem and fruit, preventing xylem efflux in F3 fruits.     

High light decreases xylem contribution to fruit growth in tomato

Recently, contradicting evidence has been reported on the contribution of xylem and phloem influx into tomato fruits, urging the need for a better understanding of the mechanisms involved in fruit growth. So far, little research has been performed on quantifying the effect of light intensity on the different contributors to the fruit water balance. However, as light intensity affects both transpiration and photosynthesis, it might be expected to induce important changes in the fruit water balance. In this study, tomato plants (Solanum lycopersicum L.) were grown in light and shade conditions and the fruit water balance was studied by measuring fruit growth of girdled and intact fruits with linear variable displacement transducers combined with a model‐based approach. Results indicated that the relative xylem contribution significantly increased when shading lowered light intensity. This resulted from both a higher xylem influx and a lower phloem influx during the daytime. Plants from the shade treatment were able to maintain a stronger gradient in total water potential between stem and fruits during daytime, thereby promoting xylem influx. It appeared that the xylem pathway was still functional at 35 days after anthesis and that relative xylem contribution was strongly affected by environmental conditions.     

Expansion rate of young tomato fruit growing on plants at positive water potential

Changes in tomato fruit expansion rate and carbohydrate content have been assessed during treatments designed to alter the carbon import rate. Because fruit expansion is sensitive to plant water status, the relationship with carbon import is difficult to assess, and thus, the diameter growth rate of young fruit was measured on plants maintained at positive water potentials. The detached top metre of a tomato plant was supplied with water, through the cut stem base, at a pressure of 0.08 MPa. Developing fruit on the stem continued to grow at high rates for up to 2 d. Fruit diameter growth rate after plant detachment was directly proportional to temperature. Plants acclimated to different continuous irradiances for 5 d before detachment gave fruit growth rates after plant detachment which were directly proportional to the irradiance up to 7 MJ m⁻²d⁻¹ photosynthetically active radiation (PAR). In continuous darkness, fruit growth rate remained unchanged for 20 h and then declined to less than 40% of the original rate over the following 30 h. On re-exposure to light, about 5 h elapsed before fruit growth rate increased but the growth rate stabilized at approximately 50% of the rate in continuously illuminated plants. During darkness, both fruit starch and hexose content decreased in comparison to illuminated controls, but on re-illumination, carbohydrate content increased before carbon was allocated to structural growth. Heat-killing the phloem of the fruit pedicel caused an immediate, but temporary, cessation of growth. After a partial recovery, expansion growth continued, but more slowly than in untreated fruit and at steadily declining rates. Starch and hexose sugars were not used to provide substrates for growth and starch synthesis was maintained. Continuing cell expansion was assumed to have been supported by water import via the xylem. Thus, fruit expansion may be related to carbon accumulation in most circumstances, but the changing allocation of imported carbon to storage and cell expansion may modify this relationship.     

Field evaluation of water transport in grape berries during water deficits

The net flow in vascular and transpirational components of the grape berry water budget was evaluated during water deficits imposed at different stages of fruit development. Diurnal fluctuations in berry diameter were measured on field-grown grapevines ( Vitis vinifera L. cv. Cabernet Sauvignon) by using electronic displacement transducers. Water deficits were imposed by withholding irrigation, and water potentials of mid-shoot leaves, basal stem xylem and clusters were determined with a pressure chamber. The relative net flows through pedicel xylem and phloem and through berry transpiration were estimated pre-veraison and post-veraison. The xylem functioned nearly exclusively in providing net inflow pre-veraison, while the phloem was clearly dominant post-veraison. Accordingly, the amplitude of diurnal contraction was markedly smaller post-veraison than pre-veraison. The amplitude of diurnal contraction increased dramatically with decreasing plant water status pre-veraison, yet exhibited little sensitivity to low vine water status post-veraison. Measurements of the difference in water potential between clusters and source stems did not provide evidence of a gradient that would elicit significant water movement from the cluster to the stem at any time of the day. This was true for both irrigated and non-irrigated vines, although the non-irrigated vines exhibited a smaller gradient favoring inflow throughout much of the day. The gradient for xylem water transport to the cluster was considerably smaller post-veraison than pre-veraison. The results showed that berry transpiration functioned as the primary pathway for water loss both pre- and post-veraison.     

An analysis of the accumulation of water and dry matter in tomato fruit

Abstract Previously published data from tomato plants grown in nutrient solutions having one of three electrical conductivities (2, 12 and 17 mS cm^?1) were analysed. The rate of water import into the fruit, and the proportion of this conducted by the xylem stream were calculated from the daily rates of transpiration and the net accumulation of water and calcium. The rate of water import decreased as the conductivity of the nutrient solution rose, the maximum daily import rates in the third week after pollination being 3.2, 3.0 and 1.8 g fruit^?1 d^?1 for fruit grown at 2, 12 and 17 mS cm^?1, respectively. During fruit development, the proportion of water imported via the xylem fell from 8–15% to 1–2% at maturity. The principal source of water for tomato fruit growth was phloem sap. Based on the daily rates of net dry matter accumulation, respiration and phloem water import, the calculated dry matter concentration of the phloem sap declined from 7 to 3%, or from 12.5 to 7.8% during fruit development in low or high salinity, respectively. The similar dry matter accumulation of fruit grown at different salinities was due to changes in both volume and concentration of phloem sap. Potassium salts in tomato fruit were calculated lo have contributed –0.29, –0.48 and –0.58 MPa to total fruit osmotic potential in the 2, 12 and 17 mS cm^?1 treatments, respectively, which accounted for 38% or 49% of the measured total osmotic potential of the 2 mS cm^?1 or 17 mS cm^?1 treatments. The contribution of hexoses to total fruit osmotic potential in the young fruit was from about –0.1 to –0.2 MPa at all salinities. The osmotic potential of tomato fruit is regulated more by potassium salts than by hexoses.     

Modeling xylem and phloem water flows in trees according to cohesion theory and Münch hypothesis

Water and solute flows in the coupled system of xylem and phloem were modeled together with predictions for xylem and whole stem diameter changes. With the model we could produce water circulation between xylem and phloem as presented by the Münch hypothesis. Viscosity was modeled as an explicit function of solute concentration and this was found to vary the resistance of the phloem sap flow by many orders of magnitude in the possible physiological range of sap concentrations. Also, the sensitivity of the predicted phloem translocation to changes in the boundary conditions and parameters such as sugar loading, transpiration, and hydraulic conductivity were studied. The system was found to be quite sensitive to the sugar-loading rate, as too high sugar concentration, (approximately 7 MPa) would cause phloem translocation to be irreversibly hindered and soon totally blocked due to accumulation of sugar at the top of the phloem and the consequent rise in the viscosity of the phloem sap. Too low sugar loading rate, on the other hand, would not induce a sufficient axial water pressure gradient. The model also revealed the existence of Münch “counter flow”, i.e., xylem water flow in the absence of transpiration resulting from water circulation between the xylem and phloem. Modeled diameter changes of the stem were found to be compatible with actual stem diameter measurements from earlier studies. The diurnal diameter variation of the whole stem was approximately 0.1 mm of which the xylem constituted approximately one-third.     

Gradients and dynamics of inner bark and needle osmotic potentials in Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L. Karst)

Preconditions of phloem transport in conifers are relatively unknown. We studied the variation of needle and inner bark axial osmotic gradients and xylem water potential in Scots pine and Norway spruce by measuring needle and inner bark osmolality in saplings and mature trees over several periods within a growing season. The needle and inner bark osmolality was strongly related to xylem water potential in all studied trees. Sugar concentrations were measured in Scots pine, and they had similar dynamics to inner bark osmolality. The sucrose quantity remained fairly constant over time and position, whereas the other sugars exhibited a larger change with time and position. A small osmotic gradient existed from branch to stem base under pre‐dawn conditions, and the osmotic gradient between upper stem and stem base was close to zero. The turgor in branches was significantly driven by xylem water potential, and the turgor loss point in branches was relatively close to daily minimum needle water potentials typically reported for Scots pine. Our results imply that xylem water potential considerably impacts the turgor pressure gradient driving phloem transport and that gravitation has a relatively large role in phloem transport in the stems of mature Scots pine trees.     

Changes in vascular and transpiration flows affect the seasonal and daily growth of kiwifruit (Actinidia deliciosa) berry

Background and Aims

The kiwifruit berry is characterized by an early stage of rapid growth, followed by a relatively long stage of slow increase in size. Vascular and transpiration flows are the main processes through which water and carbon enter/exit the fruit, determining the daily and seasonal changes in fruit size. This work investigates the biophysical mechanisms underpinning the change in fruit growth rate during the season.

Methods

The daily patterns of phloem, xylem and transpiration in/outflows have been determined at several stages of kiwifruit development, during two seasons. The different flows were quantified by comparing the diurnal patterns of diameter change of fruit, which were then girdled and subsequently detached while measurements continued. The diurnal courses of leaf and stem water potential and of fruit pressure potential were also monitored at different times during the season.

Key Results

Xylem and transpiration flows were high during the first period of rapid volume growth and sharply decreased with fruit development. Specific phloem import was lower and gradually decreased during the season, whereas it remained constant at whole-fruit level, in accordance with fruit dry matter gain. On a daily basis, transpiration always responded to vapour pressure deficit and contributed to the daily reduction of fruit hydrostatic pressure. Xylem flow was positively related to stem-to-fruit pressure potential gradient during the first but not the last part of the season, when xylem conductivity appeared to be reduced.

Conclusions

The fruit growth model adopted by this species changes during the season due to anatomical modifications in the fruit features.     

MRI of long-distance water transport: a comparison of the phloem and xylem flow characteristics and dynamics in poplar, castor bean, tomato and tobacco

We used dedicated magnetic resonance imaging (MRI) equipment and methods to study phloem and xylem transport in large potted plants. Quantitative flow profiles were obtained on a per-pixel basis, giving parameter maps of velocity, flow-conducting area and volume flow (flux). The diurnal xylem and phloem flow dynamics in poplar, castor bean, tomato and tobacco were compared. In poplar, clear diurnal differences in phloem flow profile were found, but phloem flux remained constant. In tomato, only small diurnal differences in flow profile were observed. In castor bean and tobacco, phloem flow remained unchanged. In all plants, xylem flow profiles showed large diurnal variation. Decreases in xylem flux were accompanied by a decrease in velocity and flow-conducting area. The diurnal changes in flow-conducting area of phloem and xylem could not be explained by pressure-dependent elastic changes in conduit diameter. The phloem to xylem flux ratio reflects what fraction of xylem water is used for phloem transport (Münch's counterflow). This ratio was large at night for poplar (0.19), castor bean (0.37) and tobacco (0.55), but low in tomato (0.04). The differences in phloem flow velocity between the four species, as well as within a diurnal cycle, were remarkably small (0.25-0.40 mm s(-1)). We hypothesize that upper and lower bounds for phloem flow velocity may exist: when phloem flow velocity is too high, parietal organelles may be stripped away from sieve tube walls; when sap flow is too slow or is highly variable, phloem-borne signalling could become unpredictable.     

How do salinity and water stress affect transport of water,assimilates and ions to tomato fruits?

The study was conducted in order to determine whether water stress affects the accumulation of dry matter in tomato fruits similarly to salinity, and whether the increase in fruit dry matter content is solely a result of the decrease in water content. Although the rate of water transport to tomato fruits decreased throughout the entire season in saline water irrigated plants, accumulation rates of dry matter increased significantly. Phloem water transport contributed 80–85% of the total water transport in the control and water-stressed plants, and over 90% under salinity. The concentration of organic compounds in the phloem sap was increased by 40% by salinity. The rate of ions transported via the xylem was also significantly increased by salinity, but their contribution to fruit osmotic adjustment was less. The rate of fruit transpiration was also markedly reduced by salinity. Water stress also decreased the rate of water transport to the tomato fruit and increased the rate of dry matter accumulation, but much less than salinity. The similar changes, 10–15%, indicate that the rise in dry matter accumulation was a result of the decrease in water transport. Other parameters such as fruit transpiration rates, phloem and xylem sap concentration, relative transport via phloem and xylem, solutes contributing to osmotic adjustment of fruits and leaves, were only slightly affected by water stress. The smaller response of these parameters to water stress as compared to salinity could not be attributed to milder stress intensity, as leaf water potential was found to be more negative. Measuring fruit growth of girdled trusses, in which phloem flow was inactive, and comparing it with ungirdled trusses validated the mechanistic model. The relative transport of girdled as compared to ungirdled fruits resembled the calculated values of xylem transport.     

Xylem, Phloem and Transpiration Flows in Developing Apple Fruits

Xylem, phloem and transpiration flows were measured in developingfruits of the apples Royal Gala and Cox's Orange Pippin at early,mid and late stages during their development. Fruit volume growthwas first computed from measurements of diameter made usinga system of sensitive displacement transducers. Xylem, phloemand transpiration flow components (of which fruit volume growthis the integral) were then separated using a scheme of treatmentswhich disabled one or other flow at a time. Changes observed during development in the patterns of the threeflows are in general agreement with expectations based on lessdirect observations (loc. cit. Ferguson and Watkins, 1989).Recognizing the distinctive mineral compositions of xylem andphloem streams, the changes also accommodate, and seem to explain,published observations of mineral accumulation in apple. Significant differences in the pattern of xylem and phloem flowwere observed between the varieties examined. These may explainvarietal differences in susceptibility to the mineral imbalancedisorder bitter-pit. Xylem flows were shown to reverse at times; that is they flowedfrom fruit to tree. This occurred particularly during periodsof high evaporative demand. Xylem reversal is of importanceto the overall water economy of a tree but may also have specialimportance to the mineral composition of the fruit. Key words: Mineral nutrition, bitter-pit, apple, xylem, phloem, transpiration, water balance     

梨枣在果实生长期对土壤水势的响应

以4年生梨枣为试验材料,在果实生长期设置了4个土壤水势水平,研究不同处理梨枣茎秆直径生长、光合速率、蒸腾速率、叶片相对含水量以及果实数量对土壤水势的响应,探讨了梨枣果实生长期适宜的土壤水势范围。结果表明:1)在果实缓慢生长期,茎秆直径生长缓慢;土壤水势高于-84 kPa时能显著地降低落果率。2)果实快速生长期,茎秆直径日最大值和叶片相对含水量能反映梨枣的水分状况;适当的控制土壤水势能显著的提高叶片的水分利用效率;土壤水势高于-84 kPa时果实快速生长期出现坐果现象。3)果实生长期前期的土壤水势低至-461 kPa会影响果实生长期叶片的功能和后期的坐果。因此,梨枣果实生长期的适宜的土壤水势范围为-41—-84 kPa,提高了叶片水分利用效率,提高了单果重,不影响产量。     

Starch synthesis in tomato remains constant throughout fruit development and is dependent on sucrose supply and sucrose synthase activity

Studies designed to investigate the cellular pathway of phloem unloading were conducted on two tomato lines with either high or low fruit invertase activities. Experiments were based on determination of the degree to which ³H label from [³H]-(fructosyl)-sucrose was randomized between fructose and glucose following exposure of excised fruit to a pulse of labelled sucrose delivered through pedicels. Fruit from the low invertase line harvested 10, 20 and 40 d after anthesis had similar sucrose uptake kinetics to the high invertase line. A positive correlation was found between sucrose synthase activity and sucrose uptake in both low and high invertase lines. In contrast, no correlation was observed between acid or neutral invertase activities and sucrose uptake. Within the putative apoplasmic sap collected from fruit, label in [³H]-(fructosyl)-sucrose was randomized between the free hexoses and sucrose hexose moieties. Label asymmetry was retained in sucrose on arrival within the tissues. Randomization patterns were similar in both the low and high acid invertase lines. These data support the view that sucrose imported into the fruit was not exposed to extracellular hydrolysis. This suggests that movement from the phloem is likely to occur predominantly through a symplastic pathway. About 25% of the sucrose taken up by the fruit was converted into starch regardless of fruit age, suggesting that starch turnover remains constant throughout fruit development and that starch synthesis was dependent on sucrose supply.     

A Space-time Model of Carbon Translocation along a Shoot Bearing Fruits

A carbon-based model is described of the source-sink relationshipsof a stem bearing fruits in space and time and focusing on growthvariability along the branch. The novelty of the model comesfrom the aggregation of physiological processes taking intoaccount spatial aspects. The stem is represented as a set ofcompartments (metamers) connected to source (leafy shoots) andsink (fruits) compartments. Each leafy shoot forms one compartment.The fruit consists of three compartments involved in translocation(cytoplasm), structure (cell wall) and storage (vacuole). Physiologicalprocesses considered are photosynthesis, respiration of fruitsand leaves, translocation of assimilates and fruit growth. Assimilateproduction is regulated by sink strength. Carbon translocationbetween two compartments depends on the gradient of assimilateconcentration. The gradient induces carbon translocation fromthe most to the least concentrated compartment, except for thevacuole compartment into which translocation is possible whateverthe concentration gradient. Fruit growth, in terms of freshweight, results from the phloem water supplied to the fruitaccording to the concentration gradient between the fruit andthe stem. The model is calibrated for peach trees by comparingobserved and simulated fruit dry and fresh weights for a shootwith normal fruit load. The model simulates variability betweenpeach fruits and the effect of contrasting fruit loads. Accordingto this model, photosynthesis increases and assimilate concentrationsin leaves and phloem decrease with decreasing leaf:fruit ratioas reported in the literature. Simulated concentrations of assimilatesin the phloem range from 2 to 14%. Simulated concentration gradientsand specific mass transfer for peach trees range from 0.05 to0.17 g cm^-3m^-1and from 0 to 3 g cm^-2h^-1, respectively, and areof the same order of magnitude as those reported for variousother tree species. The model is used to analyse the effectof fruit position relative to the leaves. Copyright 1999 Annalsof Botany Company Peach tree, Prunus persica (L.) Batsch, model, carbohydrates, translocation, source-sink, fruit.     

Model-assisted evaluation of crop load effects on stem diameter variations and fruit growth in peach

Key message

The paper identifies and quantifies how crop load influences plant physiological variables that determine stem diameter variations to better understand the effect of crop load on drought stress indicators.

Abstract

Stem diameter (D ^stem) variations have extensively been applied in optimisation strategies for plant-based irrigation scheduling in fruit trees. Two D ^stem derived water status indicators, maximum daily shrinkage (MDS) and daily growth rate (DGR), are however influenced by other factors such as crop load, making it difficult to unambiguously use these indicators in practical irrigation applications. Furthermore, crop load influences the growth of individual fruits, because of competition for assimilates. This paper aims to explain the effect of crop load on DGR, MDS and individual fruit growth in peach using a water and carbon transport model that includes simulation of stem diameter variations. This modelling approach enabled to relate differences in crop load to differences in xylem and phloem water potential components. As such, crop load effects on DGR were attributed to effects on the stem phloem turgor pressure. The effect of crop load on MDS could be explained by the plant water status, the phloem carbon concentration and the elasticity of the tissue. The influence on fruit growth could predominantly be explained by the effect on the early fruit growth stages.     

Carbon and water balances for young fruits of platyopuntias

Questions relating to transpired versus retained water for fruits, the xylem versus the phloem as water supplier to the fruits, and the importance of fruit photosynthesis for fruit dry mass gain were examined in the field for 6 species of platyopuntias ( Nopalea cochenillifera , Opuntia ficus-indica , O. megacantha , O. robusta , O. streptacantha and O. undulata ), cacti with flattened stem segments (cladodes). For plants with fruits midway between floral bud appearance and fruit maturation, transpiration was greater at night for the cladodes, as expected for Crassulacean acid metabolism (CAM) plants, but greater during the daytime for the fruits of all 6 species. Nevertheless, net CO₂ uptake by fruits of these platyopuntias occurred predominantly at night, as expected for CAM plants. The water potential of the young fruits (average of −0.41 MPa) was higher than that of the cladodes (average of −0.60 MPa), indicating that water entered the fruits via the phloem rather than via the xylem. Solution entry into the fruits via the phloem supplied the water lost by transpiration and allowed for increases in fruit fresh mass (daily transpiration averaged 3.2-fold higher than daily water content increases), while the accumulating solutes were apparently polymerized to account for the higher water potentials of the fruits compared with the cladodes. The phloem thus acts as the sole supplier of water and the main supplier of dry mass (90%) to such young fruits of platyopuntias.     

Cell expansion and endoreduplication show a large genetic variability in pericarp and contribute strongly to tomato fruit growth

Postanthesis growth of tomato (Solanum lycopersicon) as of many types of fruit relies on cell division and cell expansion, so that some of the largest cells to be found in plants occur in fleshy fruit. Endoreduplication is known to occur in such materials, which suggests its involvement in cell expansion, although no data have demonstrated this hypothesis as yet. We have analyzed pattern formation, cell size, and ploidy in tomato fruit pericarp. A first set of data was collected in one cherry tomato line throughout fruit development. A second set of data was obtained from 20 tomato lines displaying a large weight range in fruit, which were compared as ovaries at anthesis and as fully grown fruit at breaker stage. A remarkable conservation of pericarp pattern, including cell layer number and cell size, is observed in all of the 20 tomato lines at anthesis, whereas large variations of growth occur afterward. A strong, positive correlation, combining development and genetic diversity, is demonstrated between mean cell size and ploidy, which holds for mean cell diameters from 10 to 350 microm (i.e. a 32,000-times volume variation) and for mean ploidy levels from 3 to 80 C. Fruit weight appears also significantly correlated with cell size and ploidy. These data provide a framework of pericarp patterning and growth. They strongly suggest the quantitative importance of polyploidy-associated cell expansion as a determinant of fruit weight in tomato.     

Water relations of tropical dry forest flowers: pathways for water entry and the role of extracellular polysaccharides

Many trees in tropical dry forests flower during the dry season when evaporative demand is high and soil water levels are low. In this study the factors influencing the water balance of flowers from three species of dry forest trees were examined. Flowers had greater mucilage contents than leaves, high intrinsic and absolute capacitances, long time constants for water exchange and high transfer resistances. Flower water potentials were higher than in leaves and did not fluctuate over the lifespan of the flower. Flower water content also remained constant even though evaporation rates were high, suggesting that water was being supplied from the stem. In two of the species, the water potential gradient between flowers and leaves was opposite to that necessary for water transport from stem to flowers through the xylem, and it was therefore hypothesized that water may enter the flower through the phloem. Calculations showed that nectar production in these flowers could drive a sink of sufficient magnitude to allow water input via the phloem equal to water lost from the flower to the atmosphere.     

Most Water in the Tomato Truss Is Imported through the Xylem,Not the Phloem: A Nuclear Magnetic Resonance Flow Imaging Study

In this study, we demonstrate nuclear magnetic resonance flow imaging of xylem and phloem transport toward a developing tomato (Solanum lycopersicum) truss. During an 8-week period of growth, we measured phloem and xylem fluxes in the truss stalk, aiming to distinguish the contributions of the two transport tissues and draw up a balance between influx and efflux. It is commonly estimated that about 90% of the water reaches the fruit by the phloem and the remaining 10% by the xylem. The xylem is thought to become dysfunctional at an early stage of fruit development. However, our results do not corroborate these findings. On the contrary, we found that xylem transport into the truss remained functional throughout the 8 weeks of growth. During that time, at least 75% of the net influx into the fruit occurred through the external xylem and about 25% via the perimedullary region, which contains both phloem and xylem. About one-half of the net influx was lost due to evaporation. Halfway through truss development, a xylem backflow appeared. As the truss matured, the percentage of xylem water that circulated into the truss and out again increased in comparison with the net uptake, but no net loss of water from the truss was observed. The circulation of xylem water continued even after the fruits and pedicels were removed. This indicates that neither of them was involved in generating or conducting the circulation of sap. Only when the main axis of the peduncle was cut back did the circulation stop.Fruits are terminal organs that depend completely on long-distance transport to supply them with sugars and water for growth. Water is imported by means of both the xylem and the phloem, whereas sugars are only imported by means of the phloem. Fruits have to compete for water with the rest of the plant, and for that reason, xylem influx is expected to be sensitive to changes in plant water potential. Xylem influx into fruits may thus be lower during the day and higher during the night. When in the apoplast the water potential is especially low, for instance, when the plant is transpiring a lot of water during a hot day, fruits may even experience a xylem efflux and lose water to the vegetative parts of the plant (Johnson et al., 1992; Guichard et al., 2005). It has been suggested that in several species, in order to reduce the sensitivity of fruits to changes in plant water status, during fruit development the xylem connection between fruit and plant is reduced or even severed (Findlay et al., 1987; Lang, 1990; Creasy et al., 1993; Lang and Ryan, 1994; van Ieperen et al., 2003; Drazeta et al., 2004). In contrast to the xylem, the phloem is expected to be relatively insensitive to diurnal changes in water potential (Ehret and Ho, 1986; Ho et al., 1987). For instance, in the main stem of a number of plants, the phloem was found not to respond to diurnal differences in plant water status, whereas the xylem did (Peuke et al., 2001; Windt et al., 2006).The tomato (Solanum lycopersicum) plant has been the subject of many studies dealing with long-distance transport to fruits and has been chosen as a model system in this study as well. It has been estimated that in tomato fruits, about 80% to 90% of the influx of sap takes place by means of the phloem (Ho et al., 1987; Plaut et al., 2004; Guichard et al., 2005). It has been proposed that the low xylem contribution is due to the presence of some form of restriction in the xylem connection between plant and fruit, possibly in the knuckle (Lee, 1989; van Ieperen et al., 2003). Despite the expected low xylem contribution and limited conductivity of the xylem connection between plant and fruit, fruits have been shown to exhibit a diurnal pattern of growth. In most cases, fruits have been observed to grow fastest at night (Lee, 1989; Grange, 1995; van de Sanden and Uittien, 1995; Guichard et al., 2005). The opposite has been found to occur as well (Ehret and Ho, 1986; Pearce et al., 1993), but in these cases, the faster daytime growth was probably caused by a low diurnal stress environment. In a number of studies, even an efflux of xylem sap and fruit shrinkage during the day was reported (Johnson et al., 1992; Leonardi et al., 1999, 2000). It has been proposed that, if the phloem and xylem operate under different diurnal cycles or if their relative contributions can be modified in any way by adjusting the environmental conditions in a greenhouse, it might become possible to control and regulate fruit yield as well as fruit quality and taste.Considering the importance of fruit for the world''s food production, surprisingly little is known about the dynamics of sap flow to fruits. Since the conception of the cohesion tension theory (Dixon and Joly, 1894) and the Munch pressure flow hypothesis (Münch, 1930), there has been a decent theoretical understanding of the basic forces that govern phloem and xylem flow. It has already been attempted to apply this understanding to model fruit growth for a variety of fruits and applications (e.g. Daudet et al., 2002). However, many of the parameters that are needed to model long-distance transport to fruits are currently outside of experimental reach. First, little is known about the pressure and water potential gradients that drive flow to fruits. The xylem and the phloem are extremely sensitive to invasive experimentation and are easily disturbed, and the water potentials in the fruits’ symplast and apoplast are difficult to assess. Second, it is not clear whether xylem and phloem sap only enters the fruit (unidirectional flow), or if return flow is possible as well, and if it is, under which conditions it may occur. As the results of this study show, NMR flow imaging can provide answers to these important questions.

Estimating Long-Distance Transport to Fruits

So far, the most important methods to estimate xylem and phloem influx in fruits have been the subtractive method (Lang and Thorpe, 1989) and the mineral accumulation method (Ho et al., 1987). In the subtractive method, the contribution of xylem and phloem are estimated by heat girdling the pedicel (fruit stalk) of a fruit. Heat girdling destroys the sieve tubes, stopping phloem influx, while the xylem is assumed to remain intact and functional. By comparing the growth of nongirdled fruits to that of girdled fruits, the phloem contribution can be estimated. The most critical assumption in this method is that the xylem sap flow is not affected by heat girdling. However, the validity of this assumption is not evident. First, because xylem and phloem flow to fruits are coupled. Xylem influx is driven by a water potential difference between the xylem and the fruit symplast, which is maintained by osmotically active compounds (sugars), which in turn are imported by means of the phloem. Fishman et al. (2001) showed that the coupling between phloem and xylem influx could give rise to significant errors when using the pedicel girdling technique. A second reason is that heat girdling may profoundly affect xylem function. The xylem tissue may apparently escape heat girdling unscathed, as demonstrated by Guichard et al. (2005), but if the surrounding cells are damaged, it is not unlikely that functional damage will occur. For instance, it has been proposed that the cells that surround the xylem protect it against embolisms by preventing the entry of air (Hacke and Sperry, 2001). Van Ieperen et al. (2003) found that in the tomato pedicel, the abscission zone is the site of highest xylem resistance and that only a few xylem conduits traverse it. If an obstruction would occur in these conduits, either by embolisms or by particles of debris, it could significantly affect xylem resistance and have large implications for xylem transport to the fruit.In the second method, the mineral composition of the fruit is used to estimate the relative xylem and phloem contribution. Ho et al. (1987) measured calcium accumulation, net water import, and fruit respiration in tomato fruits. The xylem contribution to fruit growth was then estimated based on a number of assumptions: (1) the calcium content of phloem sap can be neglected compared to that of xylem sap; (2) the calcium content of xylem sap is similar to that measured in root stump exudate; and (3) xylem backflow from fruits does not occur. However, in view of current knowledge, the first and third assumptions are questionable. Calcium is used in signal transduction and as such is known to be present in the phloem. The question is, in what concentration. In phloem sap exudate of castor bean (Ricinus communis) and eucalyptus (Eucalyptus globulus), calcium concentrations have been found that were about 66% and 25% of the concentration in the root stump exudate, respectively (Pate et al., 1998; Peuke et al., 2006). In Banksia prionotes, the calcium concentration in phloem exudate was even found to be 10 times higher than that of the xylem sap (Pate and Jeschke, 1995). It should be noted that in these studies phloem sap was harvested by cutting. This may have elicited a wounding response, causing elevated calcium levels in the phloem (Knoblauch et al., 2001). Still, we argue that these findings illustrate that the calcium concentration in the phloem cannot be assumed to be negligible, especially when the majority of influx of sap is thought to take place via the phloem. The assumption that backflow does not take place also may not hold. In a number of studies, backflow from tomato fruits has already been observed, especially under summer conditions or high vapor deficit (Johnson et al., 1992; Leonardi et al., 1999, 2000; Guichard et al., 2005). The subtractive and the mineral accumulation method thus are likely to be subject to large systematic errors. Better methods to estimate or measure long-distance transport to fruits are needed.

NMR Flow Imaging

Over the last 10 years, it has been demonstrated that NMR flow imaging can provide an excellent tool to measure xylem and phloem transport (Van As, 2007). NMR flow imaging does not only give information about the average flow velocity, such as heat pulse based methods do, but gives access to all properties of the flowing water, such as the flow conducting area, the distribution of flow velocities, and the volume flow, all on a per pixel basis (Scheenen et al., 2000b). So far, studies have been conducted measuring flow in the stem of a variety of plants, ranging from castor bean seedlings (Köckenberger et al., 1997) to fully developed tomato, castor bean, and tobacco (Nicotiana tabacum) plants, and a small poplar tree (Populus spp.; Windt et al., 2006). The technique has been used to study the diurnal variation in long-distance transport (Peuke et al., 2001; Windt et al., 2006), the effects of cold girdling (Peuke et al., 2006), and xylem embolism repair (Scheenen et al., 2007) and has been used as a reference technique to provide detailed velocity maps for comparison with different heat pulse methods (e.g. Helfter et al., 2007; D. Chavarro, C.W. Windt, M.W. Lubczynski, J. Roy, and H. Van As, unpublished data). These studies have in common that flow was only measured in the main stem of the plant. This is a convenient place to do flow imaging for a variety of reasons. In comparison with other flow-conducting structures in the plant, the stem is large, sturdy, and stable. It conducts the largest fluxes, and the xylem and phloem can be easily distinguished on the basis of their direction of flow. These properties make imaging xylem and phloem transport relatively easy.

Aims and Research Questions

In this study, we used NMR flow imaging to measure long-distance transport to fruits. As a model plant, tomato was chosen. The anatomy of the tomato truss, as well as the dimensions of the magnetic resonance imaging (MRI) device and its components, made it impossible to image the pedicel of a single fruit. The pedicels were too short and too close together to fit them with the radio frequency (RF) coil that is needed for MRI. For this reason, we chose to perform flow imaging on the peduncle of tomato, measuring the transport toward the entire developing truss. After fitting the plant in the imager, it was impossible to remove the plant without damaging it. The plant was therefore left in the imager and allowed to grow there for 8 weeks. In this period, we continuously monitored long-distance transport into the truss, aiming to answer the following questions: (1) can xylem and phloem flow into the truss be visualized and distinguished; (2) what transport tissues conduct sap into the truss during truss development; (3) is phloem and xylem transport into the truss unidirectional, or does backflow occur; and (4) can NMR flow imaging be used to draw up a quantitative balance of xylem and phloem influx into the truss?     

Tomato sap flow, stem and fruit growth in relation to water availability in rockwool growing medium

Background and aims

Irrigation strategies for glasshouse tomato are often based on solar radiation sums. However, due to new energy-saving climate control, current strategies might result in inappropriate irrigation. Because of the limited water buffering capacity of soilless growing media like rockwool, this could have adverse effects on fruit production and quality. We present an overview of tomato plant ecophysiological responses to substrate water availability to allow the evaluation of mechanistic hypotheses about internal plant water storage and depletion and reversible stem-fruit water transport.

Methods

The hydraulic properties of the growing medium were determined and plant water uptake, stem and fruit diameter variations were studied.

Results

A low substrate matric suction (?2 to ?3?kPa) had a significant effect on stem and fruit growth dynamics. The substrate water retention curve indicated a sharp decrease in hydraulic conductivity, limiting the water availability for plant roots significantly.

Conclusions

The hydraulic properties of the growing medium are of utmost importance for plant water uptake, and should therefore be incorporated in plant models describing water flow. Internally stored water responds instantaneously to varying water availability and rates of water backflow from tomato fruits can be quite substantial.