Grain filling in wheat crops grown in semiarid regions may depend more on stored water-soluble carbohydrates (WSC) than on current photosynthesis. We evaluated the hypothesis that internode WSC content, specific content (WSC content/internode length), and concentration (WSC content/internode weight) of genotypes affect accumulation and mobilization of stem WSC. Genotypic variation for internode WSC-related traits was measured on the main stem at 10-d intervals in 11 diverse wheats grown under well-watered and droughted field conditions across 2 yr. Relationships among internode WSC-related traits were determined. Date of harvest was the most important factor affecting internode WSC-related traits followed by genotype, irrigation, and year. Genotype 3 date of harvest was the most important interaction. Drought reduced WSC content, specific content, and concentration in different internodes, except WSC concentration in peduncles. Mobilized WSC from peduncle, penultimate, and lower internodes ranged from 70 to 244 mg, from 95 to 227 mg, and from 175 to 450 mg, respectively. The lower internodes provided 51% of the stem mobilized WSC. Mobilized WSC was higher in well-watered than in droughted conditions for penultimate (164 vs. 135 mg) and lower internodes (274 vs. 244 mg). Drought improved mobilization efficiency in the peduncle, penultimate, and lower internodes by 33, 17, and 11%, respectively. Postanthesis maximum WSC content was highly correlated (r 5 0.89 to 0.99) with the amount of WSC mobilized in different internodes, and could be used as a selection criterion to stabilize grain yield under stressful environments.

Wheat crops grown in dryland areas may depend more on stem reserves for grain filling than on current photosynthesis. We evaluated the hypothesis that internode length, weight, and specific weight of genotypes affect accumulation and mobilization of stem reserves. This knowledge might complement selection in stressful environments. Genotypic variation for internode characteristics and their effects on dry matter accumulation and mobilization were measured at 10‐d intervals in 11 diverse wheat cultivars grown under well‐watered and droughted field conditions across 2 yr. Relationships among internode characteristics and accumulation and mobilization of stem reserves were determined. The main effect of year, irrigation, genotype, and harvest date and genotype × harvest date interaction were significant. Internode length, weight, and specific weight were reduced under drought. Mobilized dry matter from peduncle, penultimate, and the lower internodes ranged from 43 to 171, 81 to 272, and from 198 to 474 mg, respectively. Mobilized dry matter was less in well‐watered than in droughted conditions for peduncle (93 vs. 110 mg) but not for penultimate (173 vs. 143 mg) and the lower internodes (331 vs. 304 mg). Drought increased mobilization efficiency, expressed as percentage of maximum dry mater mobilized, in the peduncle, penultimate, and the lower internodes by 65, 11, and 5%, respectively. Stem maximum specific weight was correlated (r = 0.64) with stem mobilized dry matter. Balanced partitioning of stem length into upper and lower internodes and internode maximum specific weight are important in genotypic accumulation and mobilization of stem reserves in wheat.

Avocado (Persea americana Mill.) tissues contain high levels of the seven-carbon (C7) ketosugar mannoheptulose and its polyol form, perseitol. Radiolabeling of intact leaves of `Hass' avocado on `Duke 7' rootstock indicated that both perseitol and mannoheptulose are not only primary products of photosynthetic CO2 fixation but are also exported in the phloem. In cell-free extracts from mature source leaves, formation of the C7 backbone occurred by condensation of a three-carbon metabolite (dihydroxyacetone-P) with a four-carbon metabolite (erythrose-4-P) to form sedoheptulose-1,7-bis-P, followed by isomerization to a phosphorylated d-mannoheptulose derivative. A transketolase reaction was also observed which converted five-carbon metabolites (ribose-5-P and xylulose-5-P) to form the C7 metabolite, sedoheptulose-7-P, but this compound was not metabolized further to mannoheptulose. This suggests that C7 sugars are formed from the Calvin Cycle, not oxidative pentose phosphate pathway, reactions in avocado leaves. In avocado fruit, C7 sugars were present in substantial quantities and the normal ripening processes (fruit softening, ethylene production, and climacteric respiration rise), which occurs several days after the fruit is picked, did not occur until levels of C7 sugars dropped below an apparent threshold concentration of ≈20 mg·g-1 fresh weight. The effect of picking could be mimicked by girdling the fruit stalks, which resulted in ripening on the tree. Again, ripening followed a decline in C7 sugars to below an apparent threshold level. Taken together, these data indicate that the C7 sugars play important roles in carbon allocation processes in the avocado tree, including a possible novel role as phloem-mobile ripening inhibitors.

Continuous monitoring of steady-state carbon dioxide exchange rates in mature muskmelon (Cucumis melo L.) leaves showed diurnal patterns of photosynthesis and respiration that were translated into distinct patterns of accumulation and phloem export of soluble sugars and amino acids. Leaf soluble sugar patterns in general followed the pattern of photosynthetic activity observed in the leaf, whereas starch accumulated steadily throughout the light period. Sugar and starch levels declined through the dark phase. Phloem exudate analysis revealed that diurnal levels of the major transport sugars (stachyose and sucrose) in the phloem did not appear to correlate directly with the photosynthetic activity of the leaf but instead were inversely correlated with leaf starch accumulation and degradation. The amino acid pool in leaf tissues remained constant throughout the diurnal period; however, the relative contribution of individual amino acids to the total pool varied with the diurnal photosynthetic and respiratory activity of the leaf. In contrast, the phloem sap amino acid pool size was substantially larger in the light than in the dark, a result primarily due to enhanced export of glutamine, glutamate, and citrulline during the light period. The results indicate that the sugar and amino acid composition of cucurbit phloem sap is not constant but varies throughout the diurnal cycle in response to the metabolic activities of the source leaf.Rates of assimilate production in source leaves are determined both by biochemical regulation of key biosynthetic and degradative enzymes and by regulation of compartmentalization events that distribute assimilates between storage (chloroplast, vacuole) and transport (phloem) compartments (3,22). Export of photosynthetically fixed carbon from a source leaf is, therefore, dependent on the functioning of many complex metabolic events that control the production of phloem-mobile assimilates such as sucrose and delivery of these solutes to the phloem (3,6,18). In some plants, a direct correlation can be found between sucrose levels in the leaf and the rate of carbon export from the leaf, suggesting that the rate of phloem transport is controlled directly by the rate of sucrose synthesis (3,6,18 levels have no correlation with rates of export, suggesting possibly that it may not be the synthesis of sucrose but rather its delivery to the phloem system that is important in determining export rates (3,22). This may indicate a role for vacuolar transport processes in the control of export (3).At present, virtually all of our current information concerning biochemical regulation of carbon partitioning comes from plants that translocate sucrose exclusively (18). Recently, a renewed interest has been shown in carbon partitioning in those species that, in addition to sucrose, also synthesize and export the raffinose oligosaccharide, stachyose. The biochemistry of carbon partitioning between phloem-mobile and storage metabolites is far more complicated in these plants because of the additi...

Changes in soluble sugar and starch reserves in avocado (Persea americana Mill. on `Duke 7' rootstock) fruit were followed during growth and development and during low temperature storage and ripening. During the period of rapid fruit size expansion, soluble sugars accounted for most of the increase in fruit tissue biomass (peel: 17% to 22%, flesh: 40% to 44%, seed: 32% to 41% of the dry weight). More than half of the fruit total soluble sugars (TSS) was comprised of the seven carbon (C7) heptose sugar, D-mannoheptulose, and its polyol form, perseitol, with the balance being accounted for by the more common hexose sugars, glucose and fructose. Sugar content in the flesh tissues declined sharply as oil accumulation commenced. TSS declines in the seed were accompanied by a large accumulation of starch (≈30% of the dry weight). During postharvest storage at 1 or 5 °C, TSS in peel and flesh tissues declined slowly over the storage period. Substantial decreases in TSS, and especially in the C7 sugars, was observed in peel and flesh tissues during fruit ripening. These results suggest that the C7 sugars play an important role, not only in metabolic processes associated with fruit development, but also in respiratory processes associated with postharvest physiology and fruit ripening.