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Margarita R. Villagarcia, Wanda W. Collins, and C. David Raper Jr.

Soil N availability is an important component in storage root production of sweetpotato [Ipomoea batata (L.) Lam.]. A controlled-environment experiment was conducted to characterize effects of N availability on patterns of dry matter, nonstructural carbohydrates, and N accumulation, and to determine possible components of N use efficiency that vary between two genotypes of sweetpotato. Rooted cuttings of `Jewel' and MD810 were transplanted into pots filled with sand and kept in a growth chamber for 72 days. Plants were watered during the first 30 days with a complete nutrient solution that contained 14 mm NO3 - and then for the next 42 days with one of three complete nutrient solution that contained either 2, 8, or 14 mm NO3 -. At 30, 44, 58, and 72 days after transplanting, three plants from each cultivar and treatment combination were sampled and separated into leaves, stems plus petioles, fibrous roots, and storage roots. Each plant fraction was freeze-dried, weighed, ground, and analyzed for total N, soluble sugars, and starch. Availability of N in the substrate, which limited dry matter accumulation at 2 mm NO3 -, was nonlimiting at 8 and 14 mm NO3 -. In both genotypes, net assimilation rate, efficiency of N use (i.e., increments of dry matter accumulated per increment of N taken up), and proportion of dry matter allocated to storage roots were greater for N-stressed (2 mm NO3 -) than N-replete (8 and 14 mm NO3 -) plants. For the N-stressed plants, however, efficiency of N use was greater in MD810 than in `Jewel'. Although rate of NO3 - uptake per unit fibrous root mass was similar in the two genotypes under the N stress treatment, MD810 had greater uptake rate than `Jewel' under nonlimiting availability of NO3- in the substrate. The increased rate of uptake under nonlimiting NO3 - supplies apparently was related to enhanced rates of carbohydrate transport from shoots to roots. As tissue concentration of N declined in response to the lowest application of NO3 -, shoot growth was limited prior to, and to a greater extent than, the photosynthetic rate. The resulting relative decline in sink activity of shoots thus presumably increased the availability of carbohydrates for transport to roots.

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Ruth Lavon, Eliezer E. Goldschmidt, Rafael Salomon, and Andre Frank

Carbohydrate content and related enzyme activities were determined in leaves of rough-lemon (Citrus volkameriana Ten. & Pasq) plants grown under K, Mg, and Ca deficiencies. Starch content was lower and soluble sugar significantly higher in K-deficient than in control leaves. Magnesium- and Ca-deficient leaves, on the other hand, accumulated large amounts of starch. Electron micrographs also showed a reduction in the number and size of starch grains in chloroplasts of K-deficient leaves, while those of Mg- and Ca-deficient leaves were filled with large starch grains. Total amylase activity increased 3- to 7-fold in K-deficient leaves, with maltose as the major product and small amounts of maltotriose. Electrophoretic separation of amylase isozymes on native gels containing starch or amylopectin showed higher band intensities in K-deficient leaves. Examination of the capacity of leaf extracts to use amylopectin vs. β-limit dextrin indicated that mainly 8-amylase was involved. Acid invertase activity increased 7-fold in K-deficient leaves, while alkaline invertase showed little change.

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Mubarak S. Khalafalla and David A. Palzkill

Total nonstructural carbohydrates (TNC), starch, total soluble sugars, sucrose, and proline concentrations were monitored for 18 months in leaf tissue of two jojoba [Simmondsia chinensis (Link) Schneider] clones that differ in frost susceptibility. Seasonal changes in TNC and starch concentrations, with maxima in the winter and minima in summer, were significant. Sugar levels decreased from fall to spring and increased during early summer. The more frost-resistant clone (C-1) had significantly higher sugar concentrations during most of the study than the less frost-resistant clone (C-2). Proline concentrations largely followed the trends found for TNC. The C-1 clone had the higher levels of proline, except when C-2 was frost-injured. Growth trends were similiar between C-1 and C-2, with a major growth flush from March to May. Relatively high levels of starch preceded growth flushes.

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Cindy L. Flinn and Edward N. Ashworth

The accumulation of total soluble sugars (TSS) and starch and their relationship to flower bud hardiness were studied in three Forsythia taxa: Forsythia ×intermedia `Spectabilis', Forsythia ×intermedia `Lynwood', and F. suspensa. Taxon hardiness was based on the mean temperature at which low temperature exotherms (LTEs) occurred during thermal analysis. Ethanol-extracted soluble sugars were quantified with anthrone, and starch was enzymatically digested and quantified with Trinder reagent. Qualitative changes in sugar content were determined with high-performance liquid chromatography and co-chromatography of authentic standards. Quantitative and qualitative changes in sugar content, similar for the three taxa, were observed in conjunction with fluctuations in flower bud hardiness, although neither TSS nor starch were correlated with mean LTE temperature. TSS was higher in acclimated than nonacclimated buds. However, after deacclimation began, sugars continued to increase with mean LTE temperature. Buds lacked starch except for a brief period during deacclimation. Galactose, stachyose, raffinose, and an unidentified carbohydrate were positively correlated with hardiness (P = 0.005, 0.001, 0.005, and 0.001, respectively).

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Gary W. Stutte, Tara A. Baugher, Sandra P. Walter, David W. Leach, D. Michael Glenn, and Thomas J. Tworkoski

A study was conducted to quantify the effects of rootstock and training system on C allocation in apple. Dry-matter distribution was determined at harvest in 5-year-old `Golden Delicious' apple (Malus domestica Borkh.) trees on four rootstocks (MM.111 EMLA, M.7a, M.26 EMLA, and M.9 EMLA) and in three training systems (three-wire palmette, free-standing central leader, and nonpruned). Mobilizable carbohydrate content was determined at harvest and leaf fall in trees from the same planting on MM.111 EMLA and M.9 EMLA in all three training systems. Training system effects interacted with rootstock effects in dry weights of branches and of fruit. Nonpruned system shoot and fruit dry weights reflected known rootstock vigor; whereas, pruned system (three-wire and central leader) shoot dry weights were greatest and fruit dry weights were lowest in trees on M.7a. Rootstock affected the partitioning of dry matter between above- and below-ground tree components, with MM.111 EMLA accumulating significantly more dry matter in the root system than trees on the other rootstocks. Trees in the central leader and the three-wire palmette systems partitioned more dry weight into nonbearing 1-year shoots than trees in the nonpruned system. Root starch content at harvest was greater in trees on MM.111 EMLA than on M.9 EMLA, and root sucrose and sorbitol were less in trees on MM.111 EMLA compared to M.9 EMLA. At leaf fall, starch in young roots was equal in trees on both rootstocks, and sorbitol again was lower in trees on MM.111 EMLA. Harvest starch content of roots, shoots, and branches was lower in nonpruned than in pruned trees. At leaf fall, root, shoot, and branch starch content increased in nonpruned and central leader-trained trees but did not increase in three-wire palmette-trained trees.

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Paul E. Cappiello and Gary J. Kling

Cornus sericea L. rooted cuttings were held in cold storage for 60 days and then transferred to a growth chamber in hydroponic culture. Roots and shoot tips were sampled during storage and through resumption of vegetative growth. Samples were analyzed for abscisic acid (ABA), indole-3-acetic acid (IAA), zeatin, zeatin riboside, glucose, fructose, sucrose, and starch. Budbreak was associated with increasing levels of the cytokinins and IAA, and decreasing levels of sucrose and starch in the shoot tips. Regeneration of new roots was preceded by an increase in the cytokinins and IAA, and a decrease in ABA in roots. Root sucrose increased nearly two times 1 week after budbreak and starch content generally decreased throughout the experiment. The results agree, in general, with previous reports indicating decreasing levels of ABA and increasing levels of cytokinins to be associated with root regeneration and budbreak. They also indicate that, of the four carbohydrates studied, sucrose levels changed most dramatically during the root regeneration and budbreak processes.

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Li-Song Chen, Brandon R. Smith, and Lailiang Cheng

Own-rooted 1-year-old `Concord' grapevines (Vitis labruscana Bailey) were fertigated twice weekly for 11 weeks with 1, 10, 20, 50, or 100 μm iron (Fe) from ferric ethylenediamine di (o-hydroxyphenylacetic) acid (Fe-EDDHA) in a complete nutrient solution. As Fe supply increased, leaf total Fe content did not show a significant change, whereas active Fe (extracted by 2,2′-dipyridyl) content increased curvilinearly. Chlorophyll (Chl) content increased as Fe supply increased, with a greater response at the lower Fe rates. Chl a: b ratio remained relatively constant over the range of Fe supply, except for a slight increase at the lowest Fe treatment. Both CO2 assimilation and stomatal conductance increased curvilinearly with increasing leaf active Fe, whereas intercellular CO2 concentrations decreased linearly. Activities of key enzymes in the Calvin cycle, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), NADP-glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphoribulokinase (PRK), stromal fructose-1,6-bisphosphatase (FBPase), and a key enzyme in sucrose synthesis, cytosolic FBPase, all increased linearly with increasing leaf active Fe. No significant difference was found in the activities of ADP-glucose pyrophosphorylase (AGPase) and sucrose phosphate synthase (SPS) of leaves between the lowest and the highest Fe treatments, whereas slightly lower activities of AGPase and SPS were observed in the other three Fe treatments. Content of 3-phosphoglycerate (PGA) increased curvilinearly with increasing leaf active Fe, whereas glucose-6-phosphate (G6P), fructose-6-phosphate (F6P), and the ratio of G6P: F6P remained unchanged over the range of Fe supply. Concentrations of glucose, fructose, sucrose, starch, and total nonstructural carbohydrates (TNC) at both dusk and predawn increased with increasing leaf active Fe. Concentrations of starch and TNC at any given leaf active Fe content were higher at dusk than at predawn, but both glucose and fructose showed the opposite trend. No difference in sucrose concentration was found at dusk or predawn. The export of carbon from starch breakdown during the night, calculated as the difference between dusk and predawn measurements, increased as leaf active Fe content increased. The ratio of starch to sucrose at both dusk and predawn also increased with increasing leaf active Fe. In conclusion, Fe limitation reduces the activities of Rubisco and other photosynthetic enzymes, and hence CO2 assimilation capacity. Fe-deficient grapevines have lower concentrations of nonstructural carbohydrates in source leaves and, therefore, are source limited.

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Li-Song Chen and Lailiang Cheng

To determine the cause of a characteristic zonal chlorosis of `Honeycrisp' apple (Malus ×domestica Borkh.) leaves, we compared CO2 assimilation, carbohydrate metabolism, the xanthophyll cycle and the antioxidant system between chlorotic leaves and normal leaves. Chlorotic leaves accumulated higher levels of nonstructural carbohydrates, particularly starch, sorbitol, sucrose, and fructose at both dusk and predawn, and no difference was found in total nonstructural carbohydrates between predawn and dusk. This indicates that carbon export was inhibited in chlorotic leaves. CO2 assimilation and the key enzymes in the Calvin cycle, ribulose 1,5-bisphosphate carboxylase/oxygenase, NADP-glyceraldehyde-3-phosphate dehydrogenase, phosphoribulokinase, stromal fructose-1,6-bisphosphatase, and the key enzymes in starch and sorbitol synthesis, ADP-glucose pyrophosphorylase, cytosolic fructose-1,6-bisphosphatase, and aldose 6-phosphate reductase were significantly lower in chlorotic leaves than in normal leaves. However, sucrose phosphate synthase activity was higher in chlorotic leaves. In response to a reduced demand for photosynthetic electron transport, thermal dissipation of excitation energy (measured as nonphotochemical quenching of chlorophyll fluorescence) was enhanced in chlorotic leaves under full sun, lowering the efficiency of excitation energy transfer to PSII reaction centers. This was accompanied by a corresponding increase in both xanthophyll cycle pool size (on a chlorophyll basis) and conversion of violaxanthin to antheraxanthin and zeaxanthin. The antioxidant system, including superoxide dismutase and ascorbate peroxidase and the ascorbate pool and glutathione pool, was up-regulated in chlorotic leaves in response to the increased generation of reactive oxygen species via photoreduction of oxygen. These findings support the hypothesis that phloem loading and/or transport is partially or completely blocked in chlorotic leaves, and that excessive accumulation of nonstructural carbohydrates may cause feedback suppression of CO2 assimilation via direct interference with chloroplast function and/or indirect repression of photosynthetic enzymes.

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Morris Ingle, Mervyn C. D'Souza, and E.C. Townsend

Firmness, soluble solids concentration (SSC), starch index (SI), internal ethylene concentration (IE), and titratable acid concentration (TA) of `York Imperial' apple (Malus ×domestica Borkh.) fruit changed linearly with harvest date between 152 and 173 days after full bloom (DAFB). Firmness was positively correlated with TA, SSC was correlated with SI, and SI was negatively correlated with TA. After 150 days of refrigerated-air (RA) storage, there was no relationship between DAFB at harvest and firmness or superficial scald, but the malic acid concentration declined linearly and storage decay increased linearly with DAFB. Firmness had declined to a plateau and was not correlated with any variable at harvest. Malic acid concentration after CA storage was correlated with DAFB, firmness, SSC, and SI; scald was correlated with firmness and SI; and decay was correlated with DAFB, firmness, SSC, and SI. During 150 days of controlled-atmosphere (CA) storage (2.5% O2, 1.0% CO2), firmness and TA decreased as a linear function of DAFB. Percentage of fruit with scald and scald rating changed quadratically with DAFB, and decay increased linearly with DAFB. After 150 days of CA, firmness was correlated with DAFB, SI, and IE at harvest; TA was correlated with DAFB, firmness, SSC, TA, and SI; scald was correlated with firmness and SI; and decay was correlated with DAFB, SSC, and scald index at harvest. During 250 days of CA storage, firmness, TA, scald, and decay changed linearly with DAFB in only 1 or 2 years out of 3. Formulas were created to predict firmness after CA within 10 to 12 N (2.0–2.5 lb-f) and TA to within 25%.

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O. Ayari, M. Dorais, and A. Gosselin

Daily and seasonal variations of photosynthetic activity, chlorophyll a (Chl-a) fluorescence and foliar carbohydrate content were studied in situ on greenhouse tomato (Lycopersicon esculentum Mill. `Trust') plants grown under CO2 enrichment and supplemental lighting. The objective of this study was to assess the effect of seasonal variation of the photosynthetic photon flux (PPF) on photosynthetic efficiency of tomato plants and to determine the presence or absence of photosynthetic down-regulation under greenhouse growing conditions prevailing in northern latitudes. During winter, the fifth and the tenth leaves of tomato plants showed low, constant daily photosynthetic activity suggesting a source limitation under low PPF. In winter, the ratio of variable to maximum Chl-a fluorescence in dark adapted state (Fv/Fm) remained constant during the day indicating no photoinhibition occurred. In February, an increase in photosynthetic activity was followed by a decline during March, April, and May accompanied by an increase in sucrose and daily starch concentrations and constant but high hexose level. This accumulation was a long-term response to high PPF and CO2 enrichment which would be caused by a sink limitation. Thus, in spring we observed an in situ downregulation of photosynthesis. The ratio Fv/Fm decreased in spring compared to winter in response to increasing PPF. The daily decline of Fv/Fm was observed particularly as a midday depression followed by a recovery towards the end of the day. This indicated that tomato leaves were subject to a reversible inhibition in spring. Fv/Fm was lower in March than in April and May even though PPF was higher in April and May than in March. These results suggest that tomato plants develop an adaptive and protective strategy as PPF increases in spring.