Vascular flows and transpiration affect peach (Prunus persica Batsch.) fruit daily growth

The relative contributions of xylem, phloem, and transpiration to fruit growth and the daily patterns of their flows have been determined in peach, during the two stages of rapid diameter increase, by precise and continuous monitoring of fruit diameter variations. Xylem, phloem, and transpiration contributions to growth were quantified by comparing the diurnal patterns of diameter change of fruits, which were then girdled and subsequently detached. Xylem supports peach growth by 70%, and phloem 30%, while transpiration accounts for approximately 60% of daily total inflows. These figures and their diurnal patterns were comparable among years, stages, and cultivars. Xylem was functional at both stage I and III, while fruit transpiration was high and strictly dependent on environmental conditions, causing periods of fruit shrinkage. Phloem imports were correlated to fruit shrinkage and appear to facilitate subsequent fruit enlargement. Peach displays a growth mechanism which can be explained on the basis of passive unloading of photoassimilates from the phloem. A pivotal role is played by the large amount of water flowing from the tree to the fruit and from the fruit to the atmosphere.     

Vascular functioning and the water balance of ripening kiwifruit (Actinidia chinensis) berries

Indirect evidence suggests that water supply to fleshy fruits during the final stages of development occurs through the phloem, with the xylem providing little water, or acting as a pathway for water loss back to the plant. This inference was tested by examining the water balance and vascular functioning of ripening kiwifruit berries (Actinidia chinensis var. chinensis 'Hort16A') exhibiting a pre-harvest 'shrivel' disorder in California, and normal development in New Zealand. Dye labelling and mass balance experiments indicated that the xylem and phloem were both functional and contributed approximately equally to the fruit water supply during this stage of development. The modelled fruit water balance was dominated by transpiration, with net water loss under high vapour pressure deficit (D(a)) conditions in California, but a net gain under cooler New Zealand conditions. Direct measurement of pedicel sap flow under controlled conditions confirmed inward flows in both the phloem and xylem under conditions of both low and high D(a). Phloem flows were required for growth, with gradual recovery after a step increase in D(a). Xylem flows alone were unable to support growth, but did supply transpiration and were responsive to D(a)-induced pressure fluctuations. The results suggest that the shrivel disorder was a consequence of a high fruit transpiration rate, and that the perception of complete loss or reversal of inward xylem flows in ripening fruits should be re-examined.     

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.     

Vascular Development and Sap Flow in Apple Pedicels

Xylem and phloem tissues of the pedicel of apple fruit increasein cross-sectional area throughout development. The increasein phloem is similar in the two cultivars examined (Cox's OrangePippin and Royal Gala) and reflects a steadily increasing phloemsap flow to the fruit. The increase in xylem tissue is due toa proliferation of non-conducting, structural, components sinceclose examination reveals no increase in the number of vesselelements from just after flowering onwards. The greater number,and the larger diameter, of the vessels in Cox's explains theinitially higher xylem conductance found in this cultivar. In vitro measurements of xylem exudation reveal a decline duringthe growing season in the xylem conductance of both cultivarsand an increasing proportion of fruit (particularly in Cox's)in which the xylem comes to be totally non-conducting. Thisobservation is in line with previously reported measurementsof xylem sap flow in vivo. The straightforward techniques used in this study offer a feasiblealternative to more arduous methods of assessing xylem and phloemsap flows and their balance during growth.Copyright 1994, 1999Academic Press Apple, xylem, phloem, vascular development, sap flow, Malus domestica Borkh     

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.     

Xylem, Phloem and Transpiration Flows in a Grape: Application of a Technique for Measuring the Volume of Attached Fruits to High Resolution Using Archimedes' Principle

The minute changes in volume of a grape berry which occur fromhour to hour were measured non-destructively in the field usingreadily available and cheap laboratory equipment and a modernelectronic balance. The method, applicable even to small (approximately10 g) fruits, is based on Archimedes' principle and gave a resolutionof about 1 part in 1 000 by measuring the buoyant upthrust experiencedby a berry when immersed in water. Volume data from control,pedicel-steamed, and detached berries were used to calculatethe magnitudes and directions of the fluid flows which tookplace through the stalk of the phloem and xylem streams andthrough the skin in the transpiration stream. In the latter stages of fruit development, after the onset ofripening, net volume growth more or less ceases in grapes althoughtheir rate of sugar import is at its strongest. Cessation ofvolume growth comes about because the strong inflow of sugarywater in the phloem is closely balanced in part by transpirationalwater loss through the skin and in part by the backflow of xylemwater to the parent vine. This xylem backflow appears to persistthroughout the diurnal cycle. The net backflow direction of the xylem stream, together withthe inability of the phloem stream to carry certain ions (notablycalcium), may explain how some mineral imbalance disorders arisein the later stages of fruit development. The intense manner in which fruiting sinks compete with vegetativesinks in Vitis finds its explanation in the breakdown of apoplast:symplast compartmentation in the berry which occurs around thetime of onset of ripening. The breakdown exposes the terminalsieve tubes of the berry to a highly negative water potentialenvironment, serving to increase both the speed and the concentrationof the translocation stream. Key words: Archimedes' principle, volume measurement, mineral nutrition, xylem, phloem, assimilate partitioning, fruit splitting     

Theoretical analysis of systematic errors introduced by a pedicel-girdling technique used to estimate separately the xylem and phloem flows.

The water budget of fruits was analysed by means of a biophysical model of fruit growth, built and calibrated recently for peaches [Prunus persica (L.) Batsch]. This analysis was applied to the evaluation of systematic errors introduced by a pedicel-girdling method (with the observations being treated by means of a subtractive technique) used to separate the contributions of xylem and phloem flow to the total water inflow to the fruit. The flows were considered as solution transport through composite membranes and were calculated by means of equations drawn from non-equilibrium thermodynamics. The total inflow of water was simulated as dependent on the water status in the tree. The hourly time step was used for the simulation. The flows obtained by simulation of the pedicel-girdled fruit were compared with those found by simulation of the intact-pedicel fruit. The error introduced by the pedicel-girdling technique was evaluated theoretically and was shown to vary during the day, ranging from very small (relative error of 3-7%) at the period when the rate of fruit growth is maximal to 100% when the fruit volume does not change. The vascular flows obtained from the "girdling experiments" are discussed in relation to the possible theoretically estimated errors.     

Time lags for xylem and stem diameter variations in a Scots pine tree

Diameter variations in the xylem and whole stem (i.e. over bark) stem of a Scots pine (Pinus sylvestris L.) tree were measured at four heights over a 23 d period at 5 min intervals. Cross‐correlation analysis was used to calculate time lags between the measurements. Xylem diameter measurements at the different heights had time lags varying from 10 to 50 min, measurements at the lower heights lagging behind the most. This result was in good agreement with the cohesion theory of transpiration. For the whole stem diameter measurements, the treetop lagged behind all other heights and the shortest lags were midway along the stem. Changes in whole stem diameter always lagged behind those of xylem stem diameter (30–110 min), and at all heights. The considerable differences in the behaviour of xylem and whole stem diameter support the Münch hypothesis of phloem flow. Time lags calculated separately for the shrinkage (morning) and swelling (afternoon) periods indicated shorter time lags during the swelling periods. The non‐destructive methods used show promise in the simultaneous study of flow dynamics of xylem and phloem in trees.     

Apple tree water relations studied by means of the relative rate of water flow in the trunk xylem

The results of long-term investigations of variations of rates of water transport through the trunk xylem, its diameter, the leaf water potential and the transpiration rate of the apple tree showed that the daily rhythm of the relative rate of moisture flow in the trunk xylem is an obvious index of the state of the apple tree water exchange. This enables us to determine the period of its unbalance at intensive transpiration as well as the level of the forming water deficit with high accuracy and operativeness. Moreover, by the daily curve of the relative rate of xylem flow one can judge the role of contribution of the trunk reservoir to transpiration.     

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.     

Developmental changes in the diurnal water budget of the grape berry exposed to water deficits

The diurnal water budget of developing grape (Vitis vinifera L.) berries was evaluated before and after the onset of fruit ripening (veraison). The diameter of individual berries of potted ‘Zinfandel’ and ‘Cabernet Sauvignon’ grapevines was measured continuously with electronic displacement transducers over 24 h periods under controlled environmental conditions, and leaf water status was determined by the pressure chamber technique. For well-watered vines, daytime contraction was much less during ripening (after veraison) than before ripening. Daytime contraction was reduced by restricting berry or shoot transpiration, with the larger effect being shoot transpiration pre-veraison and berry transpiration post-veraison. The contributions of the pedicel xylem and phloem as well as berry transpiration to the net diurnal water budget of the fruit were estimated by eliminating phloem or phloem and xylem pathways. Berry transpiration was significant and comprised the bulk of water outflow for the berry both before and after veraison. A nearly exclusive role for the xylem in water transport into the berry was evident during pre-veraison development, but the phloem was clearly dominant in the post-veraison water budget. Daytime contraction was very sensitive to plant water status before veraison but was remarkably insensitive to changes in plant water status after veraison. This transition is attributed to an increased phloem inflow and a partial discontinuity in berry xylem during ripening.     

Diurnal Patterns of Transport and Accumulation of Minerals in Fruiting Plants of Lupinus angustifolius L.

Fluctuations in mineral elements id xylem (tracheal) sap, fruitphloem sap, leaflets and dmloping fruits were studied in a fieldpopulation of Lupinus angustifolius L. by three-hourly samplingover a 39 h period. Elements usually reached maximum contentsor concentrations at or near noon, minimum levels during thenight. Amplitudes of diurnal fluctuations in minerals lay withinthe range ±4–33 per cent of the mean content ofleaflets, and ±17–157 per cent of the mean concentrationsin xylem and phloem sap. Most minerals elements fluctuatcd inphase with daily changes in sugar level of phloem sap and drymatter and carbohydrate fluctuations of leaflets, suggestinga coupling of translocation of photosynthate and minerals fromthe leaflets. Rates of import of minerals by shoots wereestimatedfrom shoot transpiration and mineral concentrations in trachealsap. Average day time rates of import of most elements were12–25 times those at night. Translocation of minerals,nitrogen and carbon to fruits also exhibited diurnal periodicity,average rates of import king three to seven times higher inthe day than at night. A model of transport based on the carbonand water economy of the fruit suggested that P, K, Fe, Zn,Mn and Cu were imported predominantly by phloem. Estimates ofvascular import accounted for 87–104 per cent of the fruit'sactual increment of these elements. Na and Ca were gauged tobe imported mainly by xylem, Mg almost equally by xylem andphloem. However, large discrepancies existed for these threeelements between estimated vascular import and actual intakeby the fruit. Lupinus angustifolius L., mineral transport, accumulation, fruits, xylem sap, phloem sap, transpiration     

Phloem and Xylem Transport in the Supply of Minerals to a Developing Legume (Lupinus albus L.) Fruit

The economy of carbon, nitrogen, water and mineral elementsin fruits of Lupinus albus L. was studied by measuring accumulationof these quantities in the developing fruit and estimating itstranspirational losses and CO₂ exchanges. Combining this informationwith data on levels of mineral elements in the xylem sap andphloem sap supplying the fruit, it was possible to test whethertransport based on mass inflow through xylem and phloem wouldaccount for the observed intake of elements. A model of transportbased on water and carbon intake suggested that vascular intakeduring the fruit's life matched the recorded increment for mineralsto within ± 15 per cent for N, Na, Zn, Fe and Cu, andto within ± 23 per cent for P, K and S. However, estimatedvascular intake of Ca, Mg and Mn accounted for less than one–thirdof the recorded intake by the fruit, inadequacy of vascularintake being especially great early in growth. Transport inphloem accounted for more than 80 per cent of the fruit's vascularintake of C, N and S, and 70–80 per cent of its P, K,Mg and Zn. Xylem contributed 68 per cent of the vascular inputof Ca, 59 per cent of the Na, and 34–38 per cent of theFe, Mn and Cu. Enclosure and darkening of fruits reduced levelsof Ca and Fe but increased levels of N, P, K and Zn in fruitdry matter relative to unenclosed, illuminated fruits. Resultswere related to previous observations on fruit functioning. Lupinus albus, legume fruit, mineral supply, phloem, xylem     

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.     

Sugar demand of ripening grape berries leads to recycling of surplus phloem water via the xylem

We tested the common assumption that fleshy fruits become dependent on phloem water supply because xylem inflow declines at the onset of ripening. Using two distinct grape genotypes exposed to drought stress, we found that a sink‐driven rise in phloem inflow at the beginning of ripening was sufficient to reverse drought‐induced berry shrinkage. Rewatering accelerated berry growth and sugar accumulation concurrently with leaf photosynthetic recovery. Interrupting phloem flow through the peduncle prevented the increase in berry growth after rewatering, but interrupting xylem flow did not. Nevertheless, xylem flow in ripening berries, but not berry size, remained responsive to root or shoot pressurization. A mass balance analysis on ripening berries sampled in the field suggested that phloem water inflow may exceed growth and transpiration water demands. Collecting apoplastic sap from ripening berries showed that osmotic pressure increased at distinct rates in berry vacuoles and apoplast. Our results indicate that the decrease in xylem inflow at the onset of ripening may be a consequence of the sink‐driven increase in phloem inflow. We propose a conceptual model in which surplus phloem water bypasses the fruit cells and partly evaporates from the berry surface and partly moves apoplastically to the xylem for outflow.     

Generalized Münch coupling between sugar and water fluxes for modelling carbon allocation as affected by water status

A model of within-plant carbon allocation is proposed which makes a generalized use of the Münch mechanism to integrate carbon and water functions and their involvement in growth limitations. The plant is envisioned as a branched network of resistive pathways (phloem and xylem) with nodal organs acting as sources and sinks for sucrose. Four elementary organs (leaf, stem, fruit, root) are described with their particular sink functions and hydraulic attributes. Given the rates of photosynthesis and transpiration and the hydraulic properties of the network as inputs, the model calculates the internal fluxes of water and sucrose. Xylem water potential (Psi), phloem sucrose concentration (C) and turgor pressure (P) are calculated everywhere in the network accounting for osmotic equilibrium between apoplasm and symplasm and coupled functioning of xylem and phloem. The fluxes of phloem and xylem saps are driven by the gradients of P and Psi, respectively. The fruit growth rate is assumed as turgor pressure dependent. To demonstrate its ability to address within-plant competition, the model is run with a simple-branched structure gathering three leaves, eight stem segments, three competing growing fruits and one root. The model was programmed with P-Spice, a software specifically designed for simulating electrical circuits but easily adaptable to physiology. Simulations of internal water fluxes, sucrose concentrations and fruit growth rates are given for different conditions of soil water availability and hydraulic resistances (sensitivity analysis). The discussion focuses on the potential interest of this approach in functional--structural plant models to address water stress-induced effects.     

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.     

盐胁迫对苹果器官中钙镁铁锌含量的影响

以盆栽2年生富士苹果树(砧木为平邑甜茶M.hupehensisReld)为试材,研究了盐胁迫对苹果矿质营养平衡的影响.结果表明,在盐胁迫下,苹果各器官不同时期的单位干样中Ca、Mg、Fe和Zn含量的平均值没有明显变化,但各元素与Na的比值明显下降,特别是在高盐(3‰NaCl)胁迫下下降更为明显,从而破坏了树体内元素平衡.在无盐和盐胁迫下,苹果各器官中Ca含量的顺序为主干韧皮部＞叶片、新梢＞根＞主干木质部;Mg含量为新梢、根＞主干木质部、主干韧皮部、叶片;Fe含量为根＞叶片＞主干韧皮部、新梢＞主干木质部;Zn含量为新梢＞叶片＞根、主干韧皮部＞主干木质部.与对照相比,器官中各元素含量在胁迫期间表现出不同程度的波动性.     

Giant Flowers of Southern Magnolia Are Hydrated by the Xylem

Flowering depends upon long-distance transport to supply water for reproductive mechanisms to function. Previous physiological studies suggested that flowers operated uncoupled from stem xylem transport and received water primarily from the phloem. We demonstrate that the water balance of Southern magnolia (Magnolia grandiflora) flowers is regulated in a manner opposite from that of previously examined flowers. We show that flowers of Southern magnolia rely upon relatively efficient xylem hydraulic transport to support high water demand during anthesis. We measured rapid rates of perianth transpiration ranging from twice to 100 times greater than previous studies. We found that relatively efficient xylem pathways existed between the xylem and flower. Perianth hydraulic conductance and the amount of xylem to transpirational surface area ratios of flowers were both approximately one-third those measured for leafy shoots. Furthermore, we observed that perianth tissues underwent significant diurnal depressions in water status during transpiring conditions. Decreases in water potential observed between flowers and vegetative tissues were consistent with water moving from the stem xylem into the flower during anthesis. Xylem hydraulic coupling of flowers to the stem was supported by experiments showing that transpiring flowers were unaffected by bark girdling. With Southern magnolia being a member of a nearly basal evolutionary lineage, our results suggest that flower water balance represents an important functional dimension that influenced early flower evolution.     

Water translocation in Kalanchoë daigremontiana during periods of drought

Abstract. Kalanchoë daigremontiana strongly reduced daily water loss within 6 d of drought using CAM to restrict transpiration and net CO₂ uptake to the dark period.
Water translocation from old to young leaves of the plant was an additional mechanism which reduced the negative effects of drought on the water relations of young leaves. Excision of old leaves after 7–9 d of drought resulted in a decrease in the water content of young leaves. This was observed despite a decrease in transpirational water loss from young leaves. Water content in young leaves increased slightly in plants with all their leaves in place.
The dry weight of young leaves clearly increased during the experimental period when old leaves were present, but it remained relatively constant in plants without old leaves. Obviously, in addition to water, solutes were transported from old to young leaves of the plant via the phloem. Xylem tension was higher in young compared to old leaves; thus, water translocation could have occurred via xylem elements.
Since transport of organic matter in the phloem is also linked to water flow, phloem transport additionally may contribute effectively to the balance of the water budget in young leaves.