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Abstract

The CO2 assimilation characteristics of walnut leaves (Juglans regia L.) were measured on excised branches using controlled, open-system, infrared gas analysis techniques in the laboratory and on large bearing trees with a CO2 depletion method in the field. The mean maximum rate of net CO2 assimilation measured by both techniques was 1.3 nM CO2 cm-2s-l on a leaf area basis of 6.0 nm CO2 mg N-1s-1 on a leaf nitrogen basis. Leaves approached light saturation at 600-800 µEs-1m-2, and exhibited an otpimum range for CO2 assimilation at 18 to 26°C. CO2 assimilation increased linearly with increases in intercellular CO2 concentrations between 60-250 µl liter-1. The daily pattern of field CO2 assimilation was highly correlated with leaf conductance to H2O but exhibited a midday depression that was independent of the daily pattern of incident photosynthetic photon flux density at the surface of the leaves.

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1 Associate Professor. 2 Graduate Research Associate. 3 Graduate Research Assistant. Texas Agricultural Experiment Station Journal Series TA 23792. We are grateful to DeRuiter Seeds, Columbus, Ohio, for a gift of `Fidelio' cucumber seed. The cost of

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Supported by the College of Agricultural and Life Sciences, Univ. of Wisconsin, Madison, and National Aeronautics and Space Administration grant NCC-2-301. The cost of publishing this paper was defrayed in part by the payment of page charges. Under

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Abbreviations: A, net CO 2 assimilation; chl, chlorophyll; Ci, intercellular CO 2 concentration; E, transpiration; GI, growth index, GLM, general linear model; gs, stomatal conductance; LT, leaf temperature; RLWC, relative leaf water content; SLW

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Authors: and

1 Graduate student. 2 Professor. Appreciation is expressed to James Stuckey and the Southeastern Fruit and Tree Nut Laboratory, Byron, Ga. for use of trees. The cost of publishing this paper was defrayed in part by the payment of page

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Abstract

Application of a complete nutrient solution (CNS) on apple seedling leaves reduced stomatal conductance (gs). Tween 20 and CaCl2 were components of the CNS which induced gs reduction. Tween 20 alone, however, did not cause stomatal closure, but CaCl2 (24.8 mm) had a consistent, negative effect on gs when applied alone. Application of CaCl2 in combination with one of the other macrocomponents of the CNS (MgSO4, urea, or K2SO4 + KH2PO4) produced less consistent gs reductions indicating that the CaCl2 effect on gs can be modified by the presence of these compounds. Urea, MgSO4, or K2SO4 + KH2PO4 had little effect on gs when applied separately. Application of MgCl2 or KCl, which were not the CNS components, decreased and had no effect on gs, respectively. In addition to gs reduction, CaCl2 sprays reduced net photosynthesis (Pn). The equivalence of intercellular CO2 concentration in sprayed and unsprayed seedlings implied that the Pn drop following CaCl2 sprays resulted from decreased capacity of mesophyll for CO2 fixation and not from reduction in the stomatal aperture. Two possible explanations for stomata closure are discussed: a direct effect of CaCl2 on stomata and an indirect effect of CaCl2 spray through changes in mesophyll CO2 fixation capacity. Reductions in gs and Pn following treatments with different salts were not associated with visible leaf injury.

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Fertigation of young Fuji/M26 apple trees (Malus domestica Borkh.) with different nitrogen concentrations by using a modified Hoagland solution for 6 weeks resulted in a wide range of leaf nitrogen content in recently expanded leaves (from 0.9 to 4.4 g·m–2). Net photosynthesis at ambient CO2, carboxylation efficiency, and CO2-saturated photosynthesis of recently expanded leaves were closely related to leaf N content expressed on both leaf area and dry weight basis. They all increased almost linearly with increase in leaf N content when leaf N < 2.4 g·m–2, leveled off when leaf N increased further. The relationship between stomatal conductance and leaf N content was similar to that of net photosynthesis with leaf N content, but leaf intercellular CO2 concentration tended to decrease with increase in leaf N content, indicating non-stomatal limitation in leaves with low N content. Photosynthetic nitrogen use efficiency was high when leaf N < 2.4 g·m–2, but decreased with further increase in leaf N content. Due to the correlation between leaf nitrogen and phosphorus content, photosynthesis was also associated with leaf P content, but to a lesser extent.

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Leaf gas exchange of avocado (Persea americana Mill.) and mango (Mangifera indica L.) trees in containers and in an orchard (field-grown trees) was measured over a range of photosynthetic photon fluxes (PPF) and ambient CO2 concentrations (Ca ). Net CO2 assimilation (A) and intercellular partial pressure of CO2 (Ci) were determined for all trees in early autumn (noncold-stressed leaves) when minimum daily temperatures were ≥14 °C, and for field-grown trees in winter (cold-stressed leaves) when minimum daily temperatures were ≤10 °C. Cold-stressed trees of both species had lower maximum CO2 assimilation rates (Amax ), light saturation points (QA ), CO2 saturation points (CaSAT ) and quantum yields than leaves of noncold-stressed, field-grown trees. The ratio of variable to maximum fluorescence (Fv/Fm ) was ≈50% lower for leaves of cold-stressed, field-grown trees than for leaves of nonstressed, field-grown trees, indicating chill-induced photoinhibition of leaves had occurred in winter. The data indicate that chill-induced photoinhibition of A and/or sink limitations caused by root restriction in container-grown trees can limit carbon assimilation in avocado and mango trees.

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Own-rooted one-year-old `Concord' grapevines were fertigated twice weekly for 11 weeks with 1, 10, 20, 50, OR 100 μmol iron (Fe) from ferric ethylenediamine di (o-hydroxyphenylacetic) acid in a complete nutrient solution. As Fe supply increased, leaf total Fe content did not change, whereas active Fe (extracted by 2, 2'-dipyridyl) and total chlorophyll content increased curvilinearly. CO2 assimilation and stomatal conductance increased curvilinearly with increasing active Fe, whereas intercellular CO2 concentrations decreased linearly. Activities of key Calvin cycle enzymes, Rubisco, NADP-glyceraldehyde-3-phosphate dehydrogenase, phosphoribulokinase, stromal fructose-1,6-bisphosphatase (FBPase), and a key enzyme in sucrose synthesis, cytosolic FBPase, all increased linearly with increasing active Fe. No difference was found in the activities of ADP-glucose pyrophosphorylase and sucrose phosphate synthase of leaves between the lowest and the highest treatments, whereas slightly lower activities were observed in the middle Fe treatments. Content of 3-phosphoglycerate increased curvilinearly with increased active Fe, whereas glucose-6-phosphate and fructose-6-phosphate did not change. Glucose, fructose, sucrose, starch, and total non-structural carbohydrates at both dusk and pre-dawn increased with increasing active Fe. Carbon export from starch breakdown during the night, calculated as the difference between dusk and predawn levels, increased as active Fe increased. 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 non-structural carbohydrates in source leaves, and therefore, are source limited.

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