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  • Author or Editor: Mary M. Peet x
  • Journal of the American Society for Horticultural Science x
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Leaf gas-exchange and chlorophyll fluorescence measurements were used as indexes for evaluating heat tolerance among five taxa of birch: paper (Betula papyrifera Marsh.), European (B. pendula Roth.), Japanese (B. platyphylla var. japonica Hara. cv. Whitespire), Himalayan (B. jacquemontii Spach.), and river (B. nigra L. cv. Heritage). Gas-exchange measurements were conducted on individual leaves at temperatures ranging from 25 to 40C. River birch maintained the highest net photosynthetic rates (Pn) at high temperatures, while Pn of paper birch was reduced the most. Further study of river and paper birch indicated that the reduced Pn at high temperatures and the differential sensitivity between taxa resulted from several factors. Inhibition of Pn at higher temperatures was due largely to nonstomatal limitations for both taxa. Increases in respiration rates, decreases in maximal photochemical efficiency of photosystem (PS) II (F V/F M), and possible reductions in light energy directed to PS II (F 0 quenching) were apparent for both taxa. The capacity of river birch to maintain greater Pn at higher temperatures seemed to result from a lower Q10 for dark respiration and possibly greater thermotolerance of the Calvin cycle as indicated by a lack of nonphotochemical fluorescence quenching with increasing temperatures. Thermal injury, as indicated by a rapid increase in minimal, dark-acclimated (F 0) fluorescence, was not evident for either paper or river birch until temperatures reached ≈49C and was similar for both taxa.

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Lycopersicon esculentum Mill. `Laura' plants were grown in the North Carolina State Univ. phytotron at 26C day temperature and 18, 22, 24, or 26C night temperatures to determine the effects of night temperature on pollen characteristics, growth, fruit set, and early fruit growth. Total and percentage normal pollen grains were higher in plants grown at night temperatures of 18 and 22C than at 24 and 26C, but germination was highest in pollen produced at 26C. Seed content was rated higher on the plants grown at 18C night temperatures than in any of the other treatments. Numbers of flowers and fruit on the first cluster were lower in the 26C night treatment than in the other night temperature treatments. Plant height was greatest but total shoot dry mass was lowest in the 22C night temperature treatments. Fruit fresh mass increased with night temperature, reflecting more rapid development, but the experiment was not continued to fruit maturity, so the effect of night temperature on final fruit size and total plant production could not be determined. Night temperatures of 26C reduced fruit number and percentage fruit set only slightly at a day temperature of 26C, even though these temperatures were above optimal for pollen production and seed formation. To separate temperature effects on pollen from direct or developmental effects on female reproductive structures, pollen was collected from plants in the four night temperature treatments and applied to stigmas of a male-sterile cultivar kept at 24-18C minimum temperatures in adjacent greenhouses. In the greenhouse-grown male sterile plants, no consistent effects of night temperature treatment given the pollen could be seen in fruit set, fruit mass, seed content (either on a rating or seed count basis), seedling germination, or seedling dry mass.

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Commercial recommendations exist for using short-term salt-shocks on tomato (Lycopersicon esculentum Mill.) to improve fruit quality. Six experiments were conducted to 1) assess the influence of nutrient concentration and short-term salt-shocks on fruit quality and yield and 2) identify a vegetative predictor of subsequent fruit quality. The first objective was addressed in three nutrient film technique (NFT) experiments (Expts. 1-3). Four treatments were applied: two maintained constant at two baseline concentrations (0.25X and 1X-commercial level) and two provided salt-shock periods of 30 min, twice daily. There were no effects of baseline concentration or salt-shocks on total number and weight of marketable fruit. Fruit quality was better at the 1X baseline concentration as observed by higher titratable acidity (Expt. 2), higher percent dry matter (Expts. 2 and 3), higher soluble solids concentration (Expt. 2), and lower pH (Expts. 2 and 3), however, weight per marketable fruit was lower (Expt. 2). Salt-shocks had little effect on fruit quality, refuting its commercial potential. Salt-shocks decreased fruit pH (Expts. 1 and 3). However, titratable acidity increased at the 0.25X level and decreased at the 1X level (Expt. 3). In Expt. 2, but not in Expt. 3, citrate concentration in the fifth leaf from the apex of young vegetative plants was correlated with subsequent fruit quality. Three additional experiments in static hydroponics with vegetative plants showed no significant differences in leaf citrate levels due to a single, short-term salt-shock. Thus, citrate is not a good predictor of fruit quality.

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