LEAF GAS-EXCHANGE OF 11 SPECIES OF FRUIT CROPS WITH REFERENCE TO SUN-TRACKING NON-SUN-TRACKING RESPONSES

Leaf gas exchange of 11 species of fruit crops supplied with nonlimiting soil moisture was measured under varying levels of irradiance. In full sunlight net CO, assimilation (A) and stomatal conductance to water vapor (gs) were highest (A = 16.4-mu-mol m-2 s-1, gs = 455 mmol m-2 s-1) for pecan, intermediate to high (A = 12.1 to 14.6-mu-mol m-2 s-1, gs = 230 to 370 mmol m-2 s-1) for peach, apple, pear, grape, blackberry and fig, and low (A = 5.7 to 10.2-mu-mol m-2 s-1, gs < 220 mmol m-2 s-1) for Satsuma, persimmon, blueberry and kiwi. Shadecloth was used to apply 80-s reductions in irradiance in the sequence 100, 66, 33, and 10% full sun. For all species A declined rapidly, yet gs was not altered. Transpiration rate (E) declined slightly, and water use efficiency (WUE) was often near zero at 10% sun. Intercellular CO2 concentration (Ci) was 199 to 237-mu-mol mol-1 in full sun, but rose to near ambient levels with increased shading. Upon re-exposure to full sun A, gs, E, WUE, and Ci returned to pretreatment levels within 1 min for all species. Interior canopy leaves of pecan, peach, apple, and grape exposed to prolonged periods of low irradiance manifested greater reductions in A than gs or E, hence, WUE was lower and Ci was higher than for sun-exposed leaves. In addition, pecan leaves shaded (33% full sun) for 54 min manifested 50 and 25% reductions in A and gs, respectively. These data indicate that the 11 species under well-watered conditions may be classified as "non-sun tracking".