Contrasted responses to carbohydrate limitation in tomato fruit at two stages of development

The metabolic consequences of long‐term carbohydrate depletion have been well documented in many sink organs but not extensively in fruit. Therefore, in the present study the response to sugar limitation in tomato fruit (Lycopersicon esculentum Mill.) was investigated at two developmental stages; during the cell division and cell expansion phases. First, the response in excised fruit cultured in vitro was characterized. Sugar depletion caused an arrest of growth and an exhaustion of carbon reserves. The proteins that were degraded and the nitrogen released was transiently stored as asparagine and glutamine in both developmental stages and also as γ ‐aminobutyric acid (GABA) in expanding fruit. Fruit at the cell division stage appeared to be more sensitive to sugar limitation. The response to sugar depletion was then characterized in fruit from plants submitted to extended darkness. In planta, the effects of sugar‐limitation were similar to those described in vitro but much more attenuated, especially in expanding fruit, which still accumulated dry matter. The expression of cell cycle genes, sugar‐ and nitrogen‐related genes was reduced by darkness. Only asparagine synthetase gene expression was induced in both dark‐treated fruit. Together the present data revealed that the effects of the carbon limitation are more pronounced in the youngest fruits as it is probably controlled by the relative sink strength of the fruit.     

Water relations and growth of tomato fruit pericarp tissue

The water relations of young tomato fruit pericarp tissue were examined and related to tissue expansion. The relationship between bulk turgor pressure and tissue expansion (as change in fresh mass or length of tissue) was determined in slices of pericarp cut from young, growing fruit by incubation in different osmotic concentrations of polyethylene glycol 6000 or mannitol. The bulk turgor of this tissue was low (about 0.2 MPa), even in fruit from plants that were otherwise fully turgid, whether measured psychrometrically or by length change in osmotic solutions. The rate of tissue growth at maximum turgor was less than that at moderate turgor unless calcium was added to the incubation medium. However, added calcium also decreased the rate of growth at lower turgor pressures. Yield turgor was < 0.1 MPa, but it was increased by the addition of calcium ions. Electrolyte leakage from tissue was greatest at maximum turgor pressure but was decreased by the addition of calcium ions or osmoticum. Tissue growth was unaffected by a range of plant growth regulators (IAA, abscisic acid, benzyladenine and GA₃) but was inhibited, particularly at high turgor, by low concentrations of malic or citric acid. The low turgor pressure of pericarp tissue could be due to the presence of apoplastic solutes within the pericarp, and evidence for this is discussed. Thus, fruit tissue may be able to maintain optimal expansion rates only at moderate turgor and low calcium concentration.     

Differential expression of expansin gene family members during growth and ripening of tomato fruit

cDNA clones encoding homologues of expansins, a class of cell wall proteins involved in cell wall modification, were isolated from various stages of growing and ripening fruit of tomato (Lycopersicon esculentum). cDNAs derived from five unique expansin genes were obtained, termed tomato Exp3 to Exp7, in addition to the previously described ripening-specific tomato Exp1 (Rose et al. (1997) Proc Natl Acad Sci USA 94: 5955–5960). Deduced amino acid sequences of tomato Exp1, Exp4 and Exp6 were highly related, whereas Exp3, Exp5 and Exp7 were more divergent. Each of the five expansin genes showed a different and characteristic pattern of mRNA expression. mRNA of Exp3 was present throughout fruit growth and ripening, with highest accumulation in green expanding and maturing fruit, and lower, declining levels during ripening. Exp4 mRNA was present only in green expanding fruit, whereas Exp5 mRNA was present in expanding fruit but had highest levels in full-size maturing green fruit and declined during the early stages of ripening. mRNAs from each of these genes were also detected in leaves, stems and flowers but not in roots. Exp6 and Exp7 mRNAs were present at much lower levels than mRNAs of the other expansin genes, and were detected only in expanding or mature green fruit. The results indicate the presence of a large and complex expansin gene family in tomato, and suggest that while the expression of several expansin genes may contribute to green fruit development, only Exp1 mRNA is present at high levels during fruit ripening.     

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.     

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.     

Developmental changes in tomato fruit composition in response to water deficit and salinity

Processing tomato ( Lycopersicon esculentum Mill. cv. UC82B) plants were subjected to moderate levels of water deficit and salinity (Na₂SO₄/CaCl₂) in sand culture. Fruit water content and the relative contributions of organic and inorganic constituents to fruit solute potential (_Ψ) and soluble solids content were determined throughout development. Fruit _Ψ averaged –0.63, –0.86 and –0.77 MPa in the control, salinity and water deficit plants, respectively. Reduced net water import and maintenance of solute accumulation, irrespective of water import, accounted for the reductions in _Ψ of stressed fruits. Mineral ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻ and SO^2-₄) contributed –0.31 MPa to _Ψ in salinized fruit, compared with –0.19 MPa in control and water deficit treatments. Changes in net carbon accumulation were not observed among treatments, despite considerable differences in fruit K⁺ status. Starch accumulation in immature fruit was increased and hexose accumulation was decreased by both salinity and water deficit. Maximum starch levels were negatively correlated with total fruit _Ψ, but were independent of fruit K⁺. Organic acid levels were generally higher throughout development in salinized plants, relative to control plants, and correlated with increased inorganic cation rather than anion accumulation in these fruits.     

Analysis of the tomato fruit growth response to temperature and plant fruit load in relation to cell division, cell expansion and DNA endoreduplication

BACKGROUND AND AIMS: To better understand the regulation of fruit growth in response to environmental factors, the effects of temperature and plant fruit load on cell number, cell size and DNA endoreduplication were analysed. METHODS: Plants were grown at 20/20 degrees C, 25/25 degrees C and 25/20 degrees C day/night temperatures, and inflorescences were pruned to two ('2F') or five ('5F') flowers. KEY RESULTS AND CONCLUSIONS: Despite a lower fruit growth rate at 20/20 degrees C, temperature did not affect final fruit size because of the compensation between cell number and size. The higher cell number at 20/20 degrees C (9.0 x 10(6) against 7.9 x 10(6) at 25/25 degrees C and 7.7 x 10(6) at 25/20 degrees C) resulted from an extended period of cell division, and the smaller cell size was due to a shorter period of expansion rather than a lower expansion rate. By contrast, the lower fruit growth rate and size of 5F fruits compared with 2F fruits resulted from the slow down of cell expansion, whereas the number of cells was hardly affected in the proximal fruit. However, within the inflorescence the decreasing gradient of fruit size from proximal to distal fruits was due to a decrease in cell number with similar cell size. Fruit size variations within each treatment were always positively correlated to variations in cell number, but not in cell size. Negative correlations between cell size and cell number suggested that cells of tomato pericarp can be seen as a population of competing sinks. Mean ploidy was slightly delayed and reduced in 5F fruits compared with 2F fruits. It was highest at 25/25 degrees C and lowest at 25/20 degrees C. Treatments did not affect ploidy and cell size in similar ways, but within each treatment, positive correlations existed between mean ploidy and cell size, though significant only in the 2F-25/20 treatment.     

Control of kernel weight and kernel water relations by post-flowering source-sink ratio in maize

The maize (Zea mays L.) kernel undergoes large changes in water content during its development. Whether such changes regulate the pattern of kernel development or are simply a consequence of it has not yet been established because other factors, such as assimilate supply, can also affect the rate and duration of kernel growth. This study was conducted to determine whether variation in kernel weight (KW) in response to source-sink treatments is mediated by a change in kernel water relations. Two hybrids were sown at three stand densities (one, eight and 18 plants m-2), and kernel numbers were restricted to control the post-flowering source-sink ratio within each stand density. Kernel development and water relations [water content, water potential (psiw), osmotic potential (psis) and turgor] were monitored throughout grain filling. Final KW varied from 253 to 372 mg per kernel in response to source-sink treatments. For both genotypes, variation in KW was a result of a change in kernel growth rate (r2 = 0.91; P < 0.001) and not in the duration of kernel filling. Final KW was closely correlated with maximum kernel water content (r2 = 0.94; P < 0.001) achieved during rapid dry matter accumulation. However, variation in KW was not reflected in kernel water status parameters (psiw, psis or turgor), which remained fairly stable across treatments. These results indicate that maximum water content provides an easily quantifiable measure of kernel sink capacity in maize. Kernel water status parameters may affect the duration of grain filling, but have no discernible impact on kernel growth rate.     

Night temperature has a minimal effect on respiration and growth in rapidly growing plants

BACKGROUND AND AIMS: Carbon gain depends on efficient photosynthesis and adequate respiration. The effect of temperature on photosynthetic efficiency is well understood. In contrast, the temperature response of respiration is based almost entirely on short-term (hours) measurements in mature organisms to develop Q(10) values for maintenance and whole-plant respiration. These Q(10) values are then used to extrapolate across whole life cycles to predict the influence of temperature on plant growth. METHODS: In this study, night temperature in young, rapidly growing plant communities was altered from 17 to 34 degrees C for up to 20 d. Day temperature was maintained at 25 degrees C. CO(2) gas-exchange was continuously monitored in ten separate chambers to quantify the effect of night-temperature on respiration, photosynthesis and the efficiency of carbon gain (carbon use efficiency). KEY RESULTS: Respiration increased only 20-46 % for each 10 degrees C rise in temperature (total respiratory Q(10) of between 1.2 to about 1.5). This change resulted in only a 2-12 % change in carbon use efficiency, and there was no effect on cumulative carbon gain or dry mass. No acclimation of respiration was observed after 20 d of treatment. CONCLUSIONS: These findings indicate that whole-plant respiration of rapidly growing plants has a small sensitivity to temperature, and that the sensitivity does not change among the species tested, even after 20 d of treatment. Finally, the results support respiration models that separate respiration into growth and maintenance components.     

Acclimation of respiration to temperature and CO2 in seedlings of boreal tree species in relation to plant size and relative growth rate

The role of acclimation of dark respiration to temperature and CO₂ concentration and its relationship to growth are critical in determining plant response to predicted global change. We explored temperature acclimation of respiration in seedlings of tree species of the North American boreal forest. Populus tremuloides, Betula papyrifera, Larix laricina, Pinus banksiana, and Picea mariana plants were grown from seed in controlled-environments at current and elevated concentrations of CO₂ (370 and 580 μmol mol^–1) in combination with three temperature treatments of 18/12, 24/18, and 30/24 °C (light/dark period). Specific respiration rates of roots and shoots acclimated to temperature, damping increases in rates across growth-temperature environments compared to short-term temperature responses. Compared at a standard temperature, root and shoot respiration rates were, on average, 40% lower in plants grown at the highest compared to lowest growth temperature. Broad-leaved species had a lower degree of temperature acclimation of respiration than did the conifers. Among species and treatment combinations, rates of respiration were linearly related to size and relative growth rate, and relationships were comparable among growth environments. Specific respiration rates and whole-plant respiratory CO₂ efflux as a proportion of daily net CO₂ uptake increased at higher growth temperatures, but were minimally affected by CO₂ concentration. Whole-plant specific respiration rates were two to three times higher in broad-leaved than coniferous species. However, compared to faster-growing broad-leaved species, slower-growing conifers lost a larger proportion of net daily CO₂ uptake as respiratory CO₂ efflux, especially in roots. Interspecific variation in acclimation responses of dark respiration to temperature is more important than acclimation of respiration to CO₂ enrichment in modifying tree seedling growth responses to projected increases in CO₂ concentration and temperature.