Browsing by Author "Campbell, Richard J."

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    Canopy light environment influences apple leaf physiology and fruit quality
    Campbell, Richard J. (Virginia Tech, 1991-04-11)
    Several experiments were conducted to determine: the influence of canopy position, girdling, and defoliation on nectar production; whether instantaneous light measurements yield reliable estimates of cumulative seasonal light levels within the canopy; and the effect of the canopy light environment on spur leaf physiology and fruit quality. Defoliation of nongirdled flowering spurs had no effect on nectar production or composition, while defoliation of girdled spurs induced nectar sugar concentration by 24%. Canopy position had no influence on nectar production or composition. At full bloom there were differences in photosynthetic potential of spur leaves from different canopy positions. Exterior leaves had a greater maximum photosynthetic rate and an unique photosynthetic light response curve compared to the intermediate and interior leaves. Differences among positions persisted throughout the season. Stomatal conductance, specific leaf weight, dark respiration, and light levels were greater for the exterior leaves throughout the season. Instantaneous light measurements made on a single uniformly overcast day after the canopy was fully-developed (average of four times during the day) provided reliable estimates (predictive R2 > 0.90, n = 30) of total cumulative seasonal photosynthetic photon density (PPD). There was a I-to-l relationship between instantaneous and cumulative PPD after canopy development was complete providing both measures were expressed as a percentage. The relationships were equal over multiple dates for two consecutive years. Cloudless conditions provided poor estimates (predictive R2 = 0.49 to 0.80, n = 30). Light environment and harvest date influenced fruit quality characteristics within the canopy. Fruit red color, intensity of red color, and soluble solids concentration were all positively related to light level, with the highest R 2 on the early harvest dates. Fruit weight, firmness, length/ diameter ratio, starch index, and seed number were not consistently influenced by the light environment. The number of hours above an average photosynthetic photon flux density threshold of 250 I-£mol. m-2• sec'! explained slightly more of the variation in fruit quality characteristics than any other expressions of light.
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    Effect of nitrogen fertilization on the physiological damage and subsequent recovery of mite-infested apple leaves
    Campbell, Richard J. (Virginia Tech, 1988-12-10)
    Two experiments were conducted with field- and container-grown 'Delicious' apple trees to evaluate the effect of nitrogen fertilization on the physiology of apple leaves subjected to European red mite feeding, and the subsequent recovery of leaf function following mite removal. Nitrogen (N) fertilization affected the visual damage, and the photosynthetic response of 'Imperial Delicious' apple leaves to mite feeding. Visual damage occurred first, and remained greatest on the low N treatment throughout the experiment. High N trees exhibited a linear decline in Pn with increasing MD, in contrast to the quadratic decline found under low N treatment. With equal mite populations, high N trees retained higher Pn rates. Transpiration, dark respiration, relative water content, leaf nitrogen, and total chlorophyll were negatively related to MD, but specific leaf weight was positively related to MD. No interaction was found between N treatment and mite feeding for these parameters. Following removal of ERM populations, 'Redchief Delicious' leaves subjected to greater than 4000 MD exhibited recovery in Pn by 28 days after mite removal. Leaves receiving less MD (1500-3000) demonstrated an inconsistent response over time. Transpiration and diffusive resistance showed no recovery. Relative water content, specific leaf weight, leaf nitrogen, and total chlorophyll linearly declined with mite feeding, and were unaffected by foliar urea treatment.