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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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J.D. Mcafee and C.R. Rom

Alternative fruit thinners and pesticides are needed for certified organic fruit growers. A transient reduction in photosynthesis has proven to be an effective technique used for fruit thinning. Conversely, pesticides, which reduce Pn may be detrimental to plant growth. This study was developed to measure plant response to foliar applications of various organic acids as potential horticultural chemicals Treatments were applied to vegetative apple trees under controlled environmental conditions to study photosynthetic effects. CO2 assimilation significantly decreased temporarily 3 days after treatment with citric acid. Decreased trends of evapotranspiration were observed for all treatments 1 day after foliar application; however, not significant. Salicylic acid significantly decreased stomatal conductance 1 and 15 days after treatment. Average leaf area was not significantly affected but oxalic acid increased plant stem growth while acetic acid application reduced growth. This model system for screening new and alternative compounds will be a basis to study agents that may have potential to be used as certified pesticides or fruit thinning agents.

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B.E. Maust, J.G. Williamson, and R.L. Darnell

Two southern highbush blueberry cultivars, `Sharpblue' and `Misty', were used to investigate the influence of varying flower bud density and fruit load on vegetative development, whole-plant canopy CO2 exchange rate (CER), and leaf CER. Plants were grown in pots and flower buds were removed so that initial flower bud density (fl ower bud number/total cane length) on a whole-plant basis ranged from 0.05–0.35 flower buds/cm. Vegetative budbreak number, leaf area, and leaf area: fruit ratio decreased with increasing flower bud density. In `Sharpblue', whole-plant canopy CER measured at fruit ripening decreased with increasing flower and fruit load and decreasing leaf area:fruit ratio, while leaf CER increased with increasing fruit load and decreasing leaf area:fruit ratio. In `Misty', whole-plant canopy CER measured 4 weeks after full bloom decreased with increasing flower and fruit load, but whole-plant canopy and leaf CER at fruit ripening were similar among the different fruit loads. Average fruit fresh and dry weights increased and the fruit development period decreased with increased leaf area:fruit ratio in both cultivars. These data suggest that carbohydrate source limitations from reduced leaf area development and whole-plant canopy CER lead to decreased fruit fresh and dry weights and delayed ripening in some southern highbush blueberry cultivars.

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James R. Gorny and Adel A. Kader

Ethylene biosynthesis of Golden Delicious apple fruit at 20°C is rapidly inhibited by a controlled atmosphere of air + 20% CO2. However, in vitro ACC oxidase activity and ACC content were not significantly different between air and air + 20% CO2 treated fruit, To determine the in vivo effects of CO2 treatment, both in vivo and in vitro enzyme activity essays were performed in en atmosphere of air or air + 20% CO2. Western blots were also performed to quantify the amount of ACC oxidase protein present in the air and air + 20% CO2 treated fruit.

We believe that in vivo cytosolic pH changes, induced by CO2, may reduce the in vivo catalytic capacity of ACC oxidase, end hence significantly reduce ethylene biosynthesis in climacteric tissue,

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Abdullah Al-Solaiman and Fouad M. Basiouny

Mango fruits (Mangifera indica L. cv. Tommy Atkins) were harvested at early physiological maturity to study the effects of postharvest treatments on storage and fruit shelf-life. The fruits were subjected to control atmosphere (20 CO2 +3% O2, and 30% CO2 + 3% O2), liquid coating (NatureSeal and Polyamine), and ethanol vapor. The fruits were kept for 4 weeks at 50 + 3°F then removed from the cold storage and maintained at room temperature. Mango fruits stored at high level of CO2 or dipped in NatureSeal had better shelf-life than fruits stored at a low level of CO2 or with ethanol vapor.

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Hirofumi Terai, Hironobu Tsuchida, Masashi Mizuno, and Noriyoshi Matsui

Tomato fruit were given a short-term (24 h) high CO2 (80%) or N2 (100%) treatment and then transferred to air storage at 20 °C. The CO2 treatment stimulated ACC oxidase activity and ethylene production, whereas the N2 treatment increased ACC content but did not increase ethylene production. Both CO2, and N2 treatments delayed ripening for one day, but fruit ripened normally. Although short-term 80% CO2, had a stimulating effect, and 100 % N2 had no effect on ethylene production, ripening was delayed slightly by both treatments. Chemical name used: 1-aminocyclopropane-1-carboxylic acid (ACC).

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J.C. Melgar, J. Dunlop, J.P. Syvertsen*, and F. García-Sánchez

Physiological responses of olive cuttings of `Koroneiki' and `Picual' (Olea europaea L.) to zero or high salinity (NaCl 100 mmol·L-1) and to ambient CO2 (380 ppm) or elevated (700 ppm) CO2 concentration were studied in sand culture in greenhouses. Growth parameters, net gas exchange of leaves and leaf chloride concentration were measured after two months of treatment. `Koroneiki' had significantly greater shoot growth and net assimilation of CO2 (Ac) at elevated CO2 than at ambient CO2 but this difference disappeared under salt stress. Growth and Ac of `Picual' did not respond to elevated CO2 regardless of salinity treatment. Stomatal conductance and leaf transpiration were lower at elevated CO2 such that leaf water use efficiency increased at elevated CO2 in both cultivars regardless of saline treatment. The saline treatment increased leaf chloride (Cl) concentration and reduced growth and net gas exchange responses in both cultivars. There was no difference in leaf Cl accumulation between the two varieties. At high salinity, elevated CO2 had little effect on leaf Cl implying that at least in `Koroneiki', Cl accumulation was not closely linked to water uptake.

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Marc W. van Iersel and Orville M. Lindstrom

Temperature-response curves for photosynthesis and respiration are useful in predicting the ability of plants to perform under different environmental conditions. Whole crop CO2 exchange rates of three magnolia (Magnolia grandiflora L.) cultivars (`MGTIG', `Little Gem', and `Claudia Wannamaker') were measured over a 25 °C temperature range. Plants were exposed to cool temperatures (13 °C day, 3 °C night) temperatures before the measurements. Net photosynthesis (Pnet) of all three cultivars increased from 3 to 15 °C and decreased again at higher temperatures. `MGTIG' had the highest and `Little Gem' the lowest Pnet, irrespective of temperature. The Q10 (relative increase in the rate of a process with a 10 °C increase in temperature) for Pnet of all three cultivars decreased over the entire temperature range. `MGTIG' had the lowest Q10 at low temperatures (1.4 at 8 °C), while `Little Gem' had the lowest Q10 for Pnet at temperatures >17 °C and a negative Q10 > 23 °C. This indicates a rapid decline in Pnet of `Little Gem' at high temperatures. The decrease in Pnet of all three cultivars at temperatures >15 °C was caused mainly by an exponential increase in dark respiration (Rdark) with increasing temperature. `Little Gem' had a lower Rdark (per unit fresh mass) than `MGTIG' or `Claudia Wannamaker', but all three cultivars had a similar Q10 (2.46). Gross photosynthesis (Pgross) was less sensitive to temperature than Pnet and Rdark. The optimal temperature for Pgross of `MGTIG' was lower (19 °C) than those of `Little Gem' (21 °C) and `Claudia Wannamaker' (22 °C). The Q10 for Pgross decreased with increasing temperature, and was lower for `MGTIG' than for `Little Gem' and `Claudia Wannamaker'. All three cultivars had the same optimal temperature (11 °C) for net assimilation rate (NAR), and NAR was not very sensitive to temperature changes from 3 to 17 °C. This indicates that the plants were well-adapted to their environmental conditions. The results suggest that respiration rate may limit magnolia growth when temperatures get high in winter time.

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John L. Jifon and David W. Wolfe

The widely observed reduction in photosynthetic (Pn) capacity following long-term exposure to elevated CO2 is believed to result from an imbalance in source–sink status. We hypothesized that nitrogen fixation in root nodules would provide a strong sink for photosynthate and lead to a sustained positive photosynthetic response to elevated CO2. Bean plants (Phaseolus vulgaris L., cv Redkloud) were grown in poly chambers at one of four combinations of temperature (35/21 or 26/15°C day/night), and CO2 (350 or 700 ppm). Half the plants in each chamber were inoculated with Rhizobium and fertilized with a complete nutrient solution lacking nitrogen; control plants received a similar solution with nitrogen. Total nitrogenase activity (acetylene reduction assay; 8 weeks after planting) of excised whole root systems was stimulated (up to 4-fold) by elevated CO2, but this response was only significant for 26/15°C-grown plants. Inoculated plants also accumulated more biomass (10%) than control plants. Nodule abundance and size were significantly higher in high CO2-grown plants than ambient CO2 plants, but the Pn capacity of inoculated plants was only slightly greater than that of control plants. Averaged across other treatments, high CO2-grown plants accumulated more biomass (42%) and had higher Pn rates (50%) than ambient CO2 plants. Treatment effects on leaf carbohydrate levels and Pn acclimation to CO2 were not consistent. The results suggest that the higher total nodule activity was due to increased nodule number and size in proportion with increased plant size under high CO2, rather than an increase in nitrogenase activity per nodule. It is also evident that plants with symbiotic nitrogen fixation capability can benefit from elevated CO2, even with reduced input of inorganic nitrogen.

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

The effects of elevated CO2 on stomatal density and index were investigated for five crop species currently being studied for NASA's Advanced Life Support program. Lettuce (cv. Waldmann's Green) and radish (cv. Giant White Globe) were grown at 400, 1000, 5000, or 10,000 μmol·mol–1 CO2, tomato (cvs. Red Robin and Reimann Philip 75/59) were grown at 400, 1200, 5000, or 10,000 μmol·mol–1 CO2, and wheat (cv. Yecora Rojo) and potato (cv. Denali) were grown at 400, 1000, or 10,000 μmol·mol–1 CO2 within controlled-environment growth chambers using nutrient film technique hydroponics. Leaf impressions were made by applying clear silicone-based RTV coating to the adaxial and abaxial leaf surfaces of three canopy leaves of each crop at each CO2 treatment. Impressions were examined using a light microscope, whereby the number of stomatal complexes and epidermal cells were counted to calculate stomatal density and stomatal index. Results indicate that stomatal density increased for lettuce and radish at 10,000 μmol·mol–1 CO2, whereas tomato density was highest at 1200 μmol·mol–1 CO2. Potato had the lowest density at 1000 μmol·mol–1 CO2, and there was no effect of CO2 on density for wheat. Stomatal index correlated with density for lettuce and tomato; however, stomatal index for radish, potato, and wheat was not influenced by CO2. This suggests that there may be a species-specific CO2 response to epidermal cell size that influences stomatal density and stomatal index.