Leaf and canopy level photosynthetic responses of French prune (Prunus domestica L. 'French') to stem water potential based deficit irrigation

The effects of full irrigation and two levels of water stress on tree physiology were investigated in a French prune orchard. The control treatment received approximately 100% of the estimated seasonal crop water requirements throughout the season. The moderate and severe water stress treatments were managed to decline from a fully watered status in late spring to a midday stem water potential of -1.5 and -2.0 MPa, respectively, by harvest. Water stress caused a clear reduction in stem water potential throughout the daylight period, as well as reductions in leaf conductance and photosynthesis, and all of these reductions were more severe with increasing water stress. In a moderate and severe stress treatment, the photosynthetic rate of fully exposed leaves was reduced to 90% and 81% respectively, of the control treatment. There was a linear decline in light saturated photosynthesis with a decline in stem water potential. However, the stress treatments also influenced leaf orientation and the resulting incident light distribution on leaves within the canopy, such that light was more uniformly distributed in canopies under water deprivation than it was under full irrigation. The overall effect of water stress was that the moderate and severe stress treatments had 101% and 93% respectively of calculated canopy photosynthesis compared with the control. This may account for the observed tolerance of prune production to deficit irrigation. A midday depression in assimilation was observed on most days in all treatments, but was not associated with feedback inhibition resulting from carbohydrate accumulation in leaves.