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 2 assimilation (A) and stomatal conductance to water vapor (gs) were highest (A = 16.4 μmol m −2 s −1 , gs = 455 mmol m −2 s −1 ) for pecan, intermediate to high (A = 12.1 to 14.6 μmol 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 μmol 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 CO 2 concentration (Ci) was 199 to 237 μmol 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".