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Lili Zhou, Maria Eloisa Q. Reyes, and Robert E. Paull

total photosynthetic capacity of papaya plants. Papaya has a characteristic C 3 anatomy ( Campostrini and Glenn, 2007 ; Marler et al., 1994 ). The net photosynthetic carbon assimilation rate at 2000 μmol·m −2 ·s −1 PAR ranges from 25 to 35 μmol·m −2

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R.E. Moran, D.E. Deyton, C.E. Sams, J. Cummins, and C.D. Pless

Soybean oil can be used as an alternative pesticide for fruit trees. Two separate studies were conducted to determine the effects of oil concentration on leaf phytotoxicity and net CO2 assimilation (ACO2). In one study, concentrations of 0%, 2%, 4%, and 6% soybean oil in water were applied to individual shoots with a hand-held mist bottle. In the second study, 0%, 1.0%, and 1.5% were applied to whole trees with an airblast sprayer. Petroleum oil was applied as a separate treatment. Net CO2 assimilation was measured on single leaves. Oil residue was removed from the leaf with chloroform, dried, and weighed. Chlorosis and defoliation occurred with applications of 4% and 6% soybean oil. No visible phytotoxicity occurred with 2% or less oil. Net CO2 assimilation decreased as the rate of soybean oil increased from 0% to 4% oil, but there was no difference between 4% and 6%. Net CO2 assimilation decreased with increasing oil concentration from 0% to 1.5% and recovered to the rate of the control on day 7. Net CO2 assimilation was negatively related to oil residue. At an equivalent oil residue, there was no difference in ACO2 between petroleum and soybean oil. Below a residue of 0.15 mg·cm–2, foliar phytoxicity did not occur. Reductions in ACO2 were small and did not last longer than 7 days if residues were ≤0.10 mg·cm–2.

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Maria Derkacz, Calvin Chong, and John Proctor

Growth of peach fruits is characterized by a double-sigmoid curve; two periods of rapid enlargement (stages I and III) separated by a lag phase (stage II). Seasonal net CO2 assimilation rates (NAR) were compared in leaves from fruiting and non-fruiting (deblossomed) trees of `Harrow Diamond' (early), and `Vivid' (mid-summer) peach (Prunus persica L. Batsch.) and `Fantasia' (late) nectarine (P. persica) to determine 1) the influence of fruits on photosynthesis and 2) the relationship between NAR and fruit growth. Seasonal trends in NAR tended to be qualitatively similar among the three cultivars, despite genotypic and phenotypic differences. There was a distinct increase in NAR at the time of horticultural fruit maturity (stage III) of each cultivar. Shortly after harvest, NAR rates declined. The average seasonal NAR of fruiting `Harrow Diamond', `Vivid', and `Fantasia' trees was 9%, 11%, and 10% higher, respectively, than that of corresponding non-fruiting trees. Parallel data for total chlorophyll was 28%, 20%, and 19% higher, and specific leaf weight (SLW) was 3%, 5%, and 6% lower, respectively. A negative correlation between NAR and SLW may indicate a feedback inhibition of photosynthesis.

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John L. Jifon and Jim Syvertsen

Maximum CO2 assimilation rates (ACO2) in citrus are not realized in environments with high irradiance, high temperatures, and high leaf-to-air vapor pressure differences (D). We hypothesized that moderate shading would reduce leaf temperature and D, thereby increasing stomatal conductance (g s) and ACO2. A 61% reduction in irradiance under aluminum net shade screens reduced midday leaf temperatures by 8 °C and D by 62%. This effect was prominent on clear days when average midday air temperature and vapor pressure deficits exceeded 30 °C and 3 kPa. ACO2 and gs increased 42% and 104%, respectively, in response to shading. Although shaded leaves had higher gs, their transpiration rates were only 7% higher and not significantly different from sunlit leaves. Leaf water use efficiency (WUE) was significantly improved in shaded leaves (39%) compared to sunlit leaves due to the increase in ACO2. Early in the morning and late afternoon when irradiance and air temperatures were low, shading had no beneficial effect on ACO2 or other gas exchange characteristics. On cloudy days or when the maximum daytime temperature and atmospheric vapor pressure deficits were less than 30 °C and 2 kPa, respectively, shading had little effect on leaf gas exchange properties. The results are consistent with the hypothesis that the beneficial effect of radiation load reduction on ACO2 is related to improved stomatal conductance in response to lowered D.

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R. Romero-Aranda and J.P. Syvertsen

The penetration of foliar-applied urea and salt solutions into citrus leaves was investigated using `Duncan' grapefruit and `Valencia' orange seedlings in a greenhouse, and 8-year-old `Ruby Red' grapefruit trees in field tests during the summer and fall. Net gas exchange rates, Cl, nitrogen, and chlorophyll concentrations of singles leaves were measured during or after the period of foliar applications. Foliar-applied salt treatments increased leaf Cl, and visible burn symptoms were observed when Cl levels reached ≈0.4% of leaf dry weight. After 11 weeks, green areas from salt-treated leaves had similar rates of net CO2 assimilation as control plants. Leaf nitrogen and total chlorophyll increased with repeated sprays. Urea sprayed at 15% caused foliar burn symptoms after two to three applications and increased the amount of leaf abscission. Urea sprayed at 6% increased CO2 assimilation rate ≈50% after three foliar applications in 3 weeks. Nitrogen content and net CO2 assimilation of urea and urea + salt leaves were similar.

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Renae E. Moran, Dennis E. Deyton, Carl E. Sams, Charles D. Pless, and John C. Cummins

Soybean [Glycine max (L.) Merrill] oil was applied to apple trees [Malus sylvestris (L.) Mill var. domestica (Borkh.) Mansf.] as a summer spray in six studies to determine if it controls European red mites [Panonychus ulmi (Koch.)], how it affects net CO2 assimilation (A), and if it causes phytotoxicity. Sprays of 0.5%, 1.0%, and 1.5% soybean oil {TNsoy1 formulation [soybean oil premixed with Latron B-1956 (LAT) spreader-sticker at 10 oil: 1 LAT (v/v)]} reduced mite populations by 94%. Sprays of 1% and 2% soybean oil reduced mite populations to three and four mites per leaf, respectively, compared to 25 per leaf on water-sprayed plants. Soybean oil concentrations of 1.0% and 1.5% applied to whole trees reduced A for less than 7 days. Phytotoxicity did not occur when soybean oil was applied with an airblast sprayer at concentrations of 1.0% and 1.5% or with a mist bottle at 2%. Phytotoxicity occurred when soybean oil was applied with a mist bottle at 4% and 6%, which left soybean oil leaf residues of 0.22 to 0.50 mg·cm-2. No phytotoxicity occurred with 4% SunSpray, which resulted in a mean leaf residue of only 0.13 mg·cm-2. Spraying 1% soybean oil tended to give better mite control than 1% SunSpray Ultra-Fine oil, but caused greater oil residues and a greater reduction in A.

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Gary W. Stutte, Neil C. Yorio, and Raymond M. Wheeler

The effect of photoperiod (PP) on net carbon assimilation rate (Anet) and starch accumulation in newly mature canopy leaves of `Norland' potato (Solanum tuberosum L.) was determined under high (412 ∝mol·m-2·s-1) and low (263 ∝mol·m-2·s-1) photosynthetic photon flux (PPF) conditions. The Anet decreased from 13.9 to 11.6 and 9.3 μmol·m-2·s-1, and leaf starch increased from 70 to 129 and 118 mg·g-1 drymass (DM) as photoperiod (PP) was increased from 12/12 to 18/6, and 24/0, respectively. Longer PP had a greater effect with high PPF conditions than with low PPF treatments, with high PPF showing greater decline in Anet. Photoperiod did not affect either the CO2 compensation point (50 μmol·mol-1) or CO2 saturation point (1100-1200 μmol·mol-1) for Anet. These results show an apparent limit to the amount of starch that can be stored (≈15% DM) in potato leaves. An apparent feedback mechanism exists for regulating Anet under high PPF, high CO2, and long PP, but there was no correlation between Anet and starch concentration in individual leaves. This suggests that maximum Anet cannot be sustained with elevated CO2 conditions under long PP (≥12 hours) and high PPF conditions. If a physiological limit exists for the fixation and transport of carbon, then increasing photoperiod and light intensity under high CO2 conditions is not the most appropriate means to maximize the yield of potatoes.

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J.P. Syvertsen, M.L. Smith, J. Lloyd, and G.D. Farquhar

Five- to six-year-old `Redblush' grapefruit (Citrus paradisi Macf.) trees on `Volkamer' lemon [VL = C. volkameriana (Ten. & Pasq.)] or sour orange (SO = C. aurantium L.) rootstock, were grown individually in 7.9-m3 lysimeters for 2.5 years using low to high rates of fertilizer N. Net CO2 assimilation (ACO2) of leaves and leaf dry mass per area (DM/a) increased with leaf N concentration, whereas leaf tissue C isotope discrimination (Δ) decreased. Leaf tissue Δ was negatively related to ACO2 and DM/a. Transient effects of rootstock on leaf N were reflected by similar effects on Δ. There was no effect of leaf N on water-use efficiency (WUE) of leaves (WUEL = ACO2/transpiration); WUEL was not correlated with Δ. Although photosynthetic N use efficiency (ACO2/N) consistently decreased with increased leaf N, Δ was not consistently related to ACO2/N. Annual canopy growth, tree evapotranspiration (ET), and fruit yield increased with whole tree N uptake. Leaf tissue Δ was negatively related to all of these tree measurements at the end of the second year. By that time, whole-tree WUE (WUET, annual canopy growth per ET) also was negatively related to Δ. Larger trees on VL had higher ET than trees on SO, but there were no rootstock effects on WUET or on Δ. Leaf tissue Δ was consistently higher than Δ values of trunk and woody root tissues. Citrus leaf tissue Δ can be a useful indicator of leaf N, characteristics of leaf gas exchange, tree growth, yield, and WUET in response to N availability.

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G.W. Stutte, N.C. Yorio, and R.M. Wheeler

Photoperiod treatments were imposed on potato (Solanum tuberosum L. cv. Norland) grown in the Biomass Production Chamber (BPC) at Kennedy Space Center under HPS lamps (670 μmol m-2s-1 PPF) at 1200 μmol mol-1 CO2. Stand A decreased with dark cycle length, which correlated with the change in leaf starch concentration during the dark cycle, but not absolute starch concentration. A series of growth chamber experiments were performed to characterize the effect of photoperiod and PPF on CO2 assimilation and starch mobilization in single leaves. Plants grown on a 12/12 photoperiod at either low (300 μmol m-2s-1) or high (600 μmol m-2s-1) PPF were subjected to short-term photoperiod treatments of 8/16, 16/8, and 24/0 and diurnal CO2 assimilation rates, CO2 response curves, and leaf starch content were determined. CO2 compensation point was not affected by either photoperiod or PPF. However, Amax (when normalized for PPF) decreased with increasing photoperiod. Anet correlated with the changes in specific leaf weight and starch content during the dark cycle.

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Timothy L. Righetti, Carmo Vasconcelos, David R. Sandrock, Samuel Ortega-Farias, Yerko Moreno, and Francisco J. Meza

; Righetti et al., 2007 ; Tanner, 1949 ) were evaluated to demonstrate how ratio-based assessments of CO 2 assimilation are strongly dependent on leaf size. Net photosynthetic rates (μmol·m −2 ·s −1 ) are usually expressed on a leaf-area basis. When we