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  • Author or Editor: Lailiang Cheng x
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Sorbitol is the primary photosynthetic end product in the leaves of many tree fruit species in the Rosaceae family, but its physiological role remains unclear. In this study, we determined the effect of decreased sorbitol synthesis on the antioxidant system that scavenges reactive oxygen species (ROS) in apple leaves. Sorbitol synthesis was decreased in apple leaves by antisense inhibition of aldose-6-phosphate reductase activity. Dehydroascorbate reductase (DHAR), glutathione reductase, and catalase (CAT) activities increased in the leaves of the transgenic plants with decreased sorbitol synthesis, whereas superoxide dismutase, ascorbate peroxidase, NADH dependent and NADPH dependent monodehydroascorbate reductase activity did not show significant changes. Ascorbate and glutathione concentrations were higher in leaves of the transgenic plants compared with the control. The effect of decreased sorbitol synthesis on the antioxidant enzyme activity was dependent on leaf developmental stages. Larger changes in the enzyme activities of CAT, DHAR, and GR were observed in the old leaves than in the young leaves. These results suggest that sorbitol may play a role in ROS scavenging in apple leaves.

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Reserve N and carbohydrate levels of bench-grafted Fuji/M26 plants were altered by fertigation with seven N concentrations from 30 June to 1 Sept. in combination with or without 3% foliar urea application in mid-October. The plants were harvested after natural leaf fall and stored at 2 °C. One set of plants were destructively sampled in January for reserve N and carbohydrates analysis, and the remaining plants were transplanted into a N-free medium in the spring and supplied with or without 5 mM 15N-ammonium nitrate in a Hoagland solution for 60 days after budbreak. Plants fertigated with higher N concentrations had higher reserve N content and lower carbohydrate concentrations. Foliar urea application increased whole plant N content and decreased reserve carbohydrate concentration at each given N concentration used in fertigation. Regardless of N supply in the spring, total new shoot and leaf growth of plants fertigated with N was closely related to the amount of reserve N but not reserve carbohydrates. Plants treated with foliar urea had more new shoot and leaf growth than the fertigated controls. By pooling all the data concerning reserve N used for growth regardless of the spring N supply, a linear relationship was found between the amount of reserve N used for new shoot and leaf growth and the total amount of N. We conclude that the growth of apple nursery plants in the spring is mainly determined by reserve N, not reserve carbohydrates. The amount of reserve N used for new shoot and leaf growth in the spring is dependent on the total amount of reserve and is not affected by the current N supply.

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`Concord' grapevines (Vitis labruscana Bailey) can readily develop iron deficiency-induced leaf chlorosis when grown on calcareous or high pH soils. Iron (Fe) chelates are often applied to the soil to remedy chlorosis but can vary in their stability and effectiveness at high pH. We transplanted own-rooted 1-year-old `Concord' grapevines into a peat-based medium adjusted to pH 7.5 and fertigated them with 0, 0.5, 1.0, 2.0, or 4mg·L–1 Fe from Fe-EDDHA [ferric ethylenediamine di (o-hydroxyphenylacetic) acid] to determine the effectiveness of this Fe chelate for alleviating Fe deficiency-induced chlorosis at high pH. Vines were sampled midseason for iron, chlorophyll, CO2 assimilation, and photosystem II quantum efficiency (PSII) and at the end of the season for leaf area, dry weight, and cane length. We found that leaf total Fe concentration was similar across all treatments, but active Fe (extracted with 0.1 n HCl) concentration increased as the rate of Fe-EDDHA increased. Chlorophyll concentration increased curvilinearly as applied Fe increased and was highly correlated with active Fe concentration. CO2 assimilation, stomatal conductance, and PSII were very low without any supplemental Fe and increased rapidly in response to Fe application. Total leaf area, foliar dry weight, and cane length all increased as Fe application increased to 1 mg·L–1 Fe, but above this rate, a further increase in Fe did not significantly increase growth. Our results demonstrate that Fe-EDDHA is very effective in alleviating Fe deficiency-induced leaf chlorosis in `Concord' grapevines grown at high pH, which provides a foundation for continuing research related to the optimum rate and timing of application of Fe-EDDHA in `Concord' vineyards on calcareous soils. Compared with total Fe, leaf “active Fe” better indicates the actual Fe status of `Concord' vines.

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The reaction catalyzed by ADP-glucose pyrophosphorylase (AGPase) to form ADP-glucose is a regulatory and rate-limiting step in starch synthesis in plants. In response to decreased sorbitol synthesis, starch synthesis was up-regulated in the transgenic apple plants. In this study, we examined both redox and metabolite regulation of AGPase to understand the mechanism responsible for the up-regulation of starch synthesis. No difference in the monomerization/dimerization of apple leaf AGPase small subunits was observed between the transgenic plants and the untransformed control. NADP-dependent malate dehydrogenase, indicative of chloroplastic redox status, did not show significant change in the transgenic plants either. Determination of key metabolites with nonaqueous fractionation indicated that concentrations of hexose phosphates (mainly glucose-6-phosphate and fructose-6-phosphate) were higher in both the cytosol and chloroplasts of the transgenic plants than in the control, whereas 3-phosphoglycerate (PGA) concentration in the chloroplast was not higher in the transgenic plants. We conclude that accumulation of hexose-phosphates results in a decrease in inorganic phosphate (Pi) concentration and an increase in PGA/Pi ratio in the chloroplast, leading to up-regulation of starch synthesis via activating AGPase.

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Considering starch synthesis was enhanced in leaves of transgenic apple trees with decreased sorbitol synthesis, we hypothesized that starch degradation must be up-regulated correspondingly to maintain carbon supply to sink tissues. Compared with the untransformed control, mature leaves of the transgenic plants had a larger drop in starch concentration between dusk and pre-dawn, higher maltose concentration, and higher activities of two key enzymes in starch degradation: -amylase and cytosolic glucosyltransferase during the day and night. 14C-maltose and 14C-glucose were fed to the apple leaves to study the fate of starch breakdown products in the synthesis of sorbitol and sucrose. Under light, a larger proportion of both 14C-maltose and 14C-glucose were converted to sorbitol than to sucrose in the untransformed control, whereas conversion of 14C-maltose and 14C-glucose to sucrose predominated over that to sorbitol in the transgenic apple leaves. The leaf samples fed with 14C-maltose and 14C-glucose in the dark are still being analyzed, but it appears that sucrose is the main product in both the untransformed control and the transgenic plants. These results support the hypothesis that starch degradation is up-regulated in the transgenic plants.

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Plants grown on calcareous soils often exhibit symptoms of Fe-deficiency induced chlorosis despite a high content of total Fe in the leaf tissue. Iron is transported in the xylem primarily as the ferric citrate (Fe-Citr) chelate, and changes in pH, HCO - 3, and Citr can lead to the formation of different Fe-Citr species. Understanding how Fe dissociates from these chelates may help explain why Fe is immobilized in the leaves. The goal was to quantify Fe mobilization (Fe-Mob) from Fe-Citr in an assay system buffered at pH 5, 6, or 7 when: 1) the molar ratio of HCO - 3 to Fe increased in a 1 Fe: 1 Citr system; 2) the molar ratio of Citr increased in a 1 Fe: 3 HCO - 3 system; and 3) solutions were photoreduced (PR) or left in the dark. For non-PR solutions, Fe-Mob from Fe-Citr using 500 μmol NADH was the greatest at the 1 Fe: 0 HCO - 3-level, and decreased as HCO - 3 increased. Fe-Mob also decreased as buffer pH increased from 5 to 7. Increasing the Citr ratio was effective in increasing Fe-Mob, but the effect decreased as buffer pH increased from 5 to 7. PR solutions behaved quite differently. In the 1 Fe: 1 Citr system, little to no Fe-Mob was detected at any buffer pH. However, there were already large pools of Fe2+ in solution, which decreased as HCO - 3 increased, irrespective of buffer pH. Increasing the Citr ratio greatly increased Fe-Mob in the 1 Fe: 3 HCO - 3 system, and mobilization decreased as buffer pH increased. Increasing Citr did not increase the amount of Fe2+ in solution. This work illustrates that increasing the HCO - 3: Fe ratio can lead to an immobilization of Fe, and that increasing the Citr ratio can aid in Fe-Mob from Fe-Citr when the HCO - 3: Fe ratio is high. Increasing the Citr ratio, however, does not increase the amount of PR Fe2+.

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Based on the curvilinear relationship between carboxylation efficiency and leaf N in apple leaves, we hypothesized that deactivation of Rubisco accounts for the lack of response of photosynthesis to increasing leaf N under high N supply. A wide range of leaf N content (from 1.0 to 5.0 g·m–2) was achieved by fertigating bench-grafted Fuji/M26 apple trees for 6 weeks with different N concentrations using a modified Hoagland solution. Analysis of photosynthesis in response to intercellular CO2 under both 21% and 2% O2 indicated that photosynthesis at ambient CO2 was mainly determined by the activity of Rubisco. Measurements of Rubisco activity revealed that initial Rubisco activity increased with leaf N up to 3.0 g·m–2, then leveled off with further rise in leaf N, whereas total Rubisco activity increased linearly with increasing leaf N throughout the leaf N range. As a result, Rubisco activation state decreased with increasing leaf N. Photosynthesis at ambient CO2 and carboxylation efficiency were both linearly correlated with initial Rubisco activity, but showed curvilinear relationships with total Rubisco activity and leaf N. As leaf N increased, photosynthetic nitrogen use efficiency declined with decreasing Rubisco activation state.

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Ribulose bisphosphate carboxylase/oxygenase (Rubisco) initiates the photosynthetic carbon metabolism;therefore, its activity has been measured in many physiological studies. However, information on in vitro Rubisco activity from leaves of deciduous fruit crops is very limited and the reported activities are suspiciously low. We measured Rubisco activity in crude extracts of leaves of apple, pear, peach, cherry, and grape by using a photometric method in which RuBP carboxylation was enzymically coupled to NADH oxidation. Replacing polyvinylpyrrolidone with polyvinylpolypyrrolidone in the extraction solution significantly increased extractable Rubisco activity. Depending on species, freezing leaf discs in liquid nitrogen followed by storage at –80°C for only 24 hr reduced both initial and total Rubisco activity to 5% to 50% of that obtained from fresh leaves. Initial Rubisco activity from fresh leaf tissues of all species was well correlated with maximum Rubisco activity (Vcmax) estimated from gas exchange; an exception was pear, where initial Rubisco activity was higher than Vcmax. In most cases, initial Rubisco activity was approximately two to three times higher than net photosynthesis.

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Bench-grafted Fuji/M26 trees were fertigated with seven nitrogen concentrations (0, 2.5, 5.0, 7.5, 10, 15, and 20 mm) by using a modified Hoagland solution from 30 June to 1 Sept. In Mid-October, plants in each N treatment were divided into three groups. One group was destructively sampled to determine background tree N status before foliar urea application. The second group was painted with 3% 15N-urea solution twice at weekly interval on both sides of all leaves while the third group was left as controls. All the fallen leaves from both the 15N-treated and control trees were collected during the leaf senescence process and the trees were harvested after natural leaf fall. Nitrogen fertigation resulted in a wide range of tree N status in the fall. The percentage of whole tree N partitioned into the foliage in the fall increased linearly with increasing leaf N content up to 2.2 g·m–2, reaching a plateau of 50% to 55% with further rise in leaf N. 15N uptake and mobilization per unit leaf area and the percentage of 15N mobilized from leaves decreased with increasing leaf N content. Of the 15N mobilized back to the tree, the percentage of 15N partitioned into the root system decreased with increasing tree N status. Foliar 15N-urea application reduced the mobilization of existing N in the leaves regardless of leaf N status. More 15N was mobilized on a leaf area basis than that from existing N in the leaves with the low N trees showing the largest difference. On a whole-tree basis, the increase in the amount of reserve N caused by foliar urea treatment was similar. We conclude that low N trees are more effective in utilizing N from foliar urea than high N trees in the fall.

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New roots of Malus domestica Borkh MM106 apple rootstock were divided into two categories, 1) feeder roots and 2) extension roots based on morphology and their ability to take up NH4 +, were studied. The roots were harvested in August from 1-year-old potted plants growing under natural conditions in Corvallis, Ore. Extension roots were thicker and longer than feeder roots. Average diameter and length were 0.89 and 45.29 mm for extension roots and 0.27 and 5.36 mm for feeder roots. Root special length (cm/g FW) and surface area (cm2/g FW) were 11.94 and 33.17 for extension roots and 108.97 and 93.38 for feeder roots. Maximum uptake rate, Imax, Km, and root absorption power, α (α = Imax•1/Km), for NH4 + absorption were 6.875, 0.721, and 9.48 for extension roots and 4.32, 0.276, and 15.63 for feeder roots. Feeder roots had stronger affinity to NH4 + (low Km) and higher NH4 + absorption power (high α value) than extension roots. The feeder roots were better able to uptake NH4 + at lower external solution concentrations than extension roots according to the nutrient depletion curve, which indicates feeder roots being more efficient than extension roots in nutrient absorption when NH4 + availability was low.

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