Water Relations and Net CO2 Assimilation of Peach Leaves of Different Ages

Net CO 2 assimilation rate (A) and plant water relations of peach [Prunus persica (L.) Batsch cv. Flordaking] leaves were monitored during development under field conditions. Leaf conductance to water vapor (gl) and transpiration rate (E) of unfolding and expanding leaves approached maximum values before maximum A values were achieved. Net CO 2 assimilation rate and water use efficiency (WUE) were greatest for recently expanded leaves and gradually declined with age after full expansion. Leaf water potential (ψ w ) was similar for all leaf ages under field conditions. Leaf dry weight/cm 2 and chlorophyll/cm 2 increased with leaf age after expansion. Diurnal patterns of gl, E, and ψ w were similar for expanded spring- and summer-flush leaves. Midday ψ w of −2.4 MPa (ψ p = about 0.3 MPa) did not reduce gl. Expanding shoots had higher osmotic potentials (ψ π ) and thus maintained lower turgor potentials (ψ p ) than fully expanded shoots. Shoot and leaf elongation rates were related exponentially to ψ p and were reduced drastically below ψ p 1.0 and 0.7 MPa, respectively. The bulk modulus of elasticity (є) increased linearly with ψ p , but there were no significant differences in є of expanding and nonexpanding shoots. As leaf water deficits developed, shoot and leaf expansion were inhibited prior to gl or A. Thus, a moderate level of water stress can reduce the rate of vegetative growth of peach trees without concomitant reductions in carbon assimilation.