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Lailiang Cheng, Sunghee Guak, and Leslie H. Fuchigami

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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Anthony W. Whiley, Christopher Searle, Bruce Schaffer, and B. Nigel Wolstenholme

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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P.C. Andersen, J.G. Norcini, and G.W. Knox

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

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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Amauri Alves Nery and Adonai Gimenez Calbo

1 Research assistant. 2 Researcher. This investigation was supported by research grants from the Brazilian Scientific and Technological Council (CNPq). We are indebted to Linda Styer Caldas and Washington L.C. Silva for their critical reading of the

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Li-Song Chen and Lailiang Cheng

One-year-old grapevines (Vitis labrusca L. `Concord') were supplied twice weekly for 5 weeks with 0, 5, 10, 15, or 20 mm nitrogen (N) in a modified Hoagland's solution to generate a wide range of leaf N status. Both light-saturated CO2 assimilation at ambient CO2 and at saturating CO2 increased curvilinearly as leaf N increased. Although stomatal conductance showed a similar response to leaf N as CO2 assimilation, calculated intercellular CO2 concentrations decreased. On a leaf area basis, activities of key enzymes in the Calvin cycle, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), NADP-glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphoribulokinase (PRK), and key enzymes in sucrose and starch synthesis, fructose-1,6-bisphosphatase (FBPase), sucrose phosphate synthase (SPS), and ADP-glucose pyrophosphorylase (AGPase), increased linearly with increasing leaf N content. When expressed on a leaf N basis, activities of the Calvin cycle enzymes increased with increasing leaf N, whereas activities of FBPase, SPS, and AGPase did not show significant change. As leaf N increased, concentrations of glucose-6-phosphate (G6P), fructose-6-phosphate (F6P), and 3-phosphoglycerate (PGA) increased curvilinearly. The ratio of G6P/F6P remained unchanged over the leaf N range except for a significant drop at the lowest leaf N. Concentrations of glucose, fructose, and sucrose at dusk increased linearly with increasing leaf N, and there was no difference between predawn and dusk measurements. As leaf N increased, starch concentration increased linearly at dusk, but decreased linearly at predawn. The calculated carbon export from starch degradation during the night increased with increasing leaf N. These results showed that 1) grapes leaves accumulated less soluble carbohydrates under N-limitation; 2) the elevated starch level in low N leaves at predawn was the result of the reduced carbon export from starch degradation during the night; and 3) the reduced capacity of CO2 assimilation in low N leaves was caused by the coordinated decreases in the activities of key enzymes involved in CO2 assimilation as a result of direct N limitation, not by the indirect feedback repression of CO2 assimilation via sugar accumulation.

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Lailiang Cheng and Leslie H. Fuchigami

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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Abdul K. Janoudi, Irvin E. Widders, and James A. Flore

Laboratory, Michigan State Univ., East Lansing, MI 48824. 2 Associate Professor. To whom reprint requests should be addressed. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper

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Milton E. McGiffen Jr., John B. Masiunas, and Morris G. Huck

Eastern black nightshade (Solanum ptycanthum) and black (Solanum nigrum) nightshade are difficult to control in tomato, interfering with harvest and decreasing fruit quality and yield. In irrigated tomatoes, soil water depletion was greater as nightshade density increased. However, tomato yield loss due to black nightshade was greatest at the lower weed densities. As density increases, photosynthetic activity (photosynthetic rates, stomatal conductance, intercellular CO2 concentration, and stomatal resistance) of black nightshade is more affected than eastern black nightshade. Photosynthetic activity of tomato is the least affected. In greenhouse experiments where water was denied for approximately a week prior to measurement, tomatoes were more sensitive to water stress than were nightshades. Nightshades were more adapted to drought stress than were tomatoes.

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Qibing Wang and Jianjun Chen*

Ficus benjamina is considered to have a high degree of morphological and physiological plasticity in response to light levels. In this study, leaf area and thickness, specific leaf area (SLA), chlorophyll content, and photosynthetic characteristics of Ficus benjamina `Common'; grown in a shaded greenhouse under four maximum photosynthetic photon flux densities (PPFDs) of 150, 250, 450, or 650 μmol·m-2·s-1 were investigated. Results showed that plants grown under 450 and 650 PPFDs had higher SLA and leaf thickness but smaller leaf areas than those grown under 150 and 250 PPFDs. Total chlorophyll content per unit leaf area decreased as PPFDs increased. Net photosynthetic rates (Pn) increased from 2.7 μmol·m-2·s-1 under 150 PPFD to 5.7 μmol·m-2·s-1 under 450 PPFD, then slightly decreased to 5.5 μmol·m-2·s-1 under 650 PPFD. The highest net photosynthetic rate was not associated with higher intercellular CO2 concentrations (Ci) and stomatal conductance (gs) as plants grown under 250 PPFD had the highest (Ci) (259 ppm) and gs (0.1 mol·m-2·s-1), which suggests that photosynthetic enzymes could play a increasing role under 450 PPFD. Plant quality, however, was not necessarily correlated with the Pn because only those grown under 250 PPFD had appropriate heights, large and dark green leaves, and well-spread branches, and thus were graded higher than plants grown under the other PPFDs. This study shows that fine-tuning production light level is important for high quality Ficus benjamina production.