Effects of partial rootzone drying (PRD) on adult olive tree (Olea europaea) in field conditions under arid climate II. Photosynthetic responses

The great challenge for agriculture in the coming decades will be to increase crop production with less water, particularly in countries with limited water resources. The partial rootzone drying (PRD) irrigation technique has been recently proposed as an agronomic practice to improve crop water productivity. However, the physiological regulation of plant water relations under the PRD is still unknown. This study was conducted to determine the photosynthetic limitations of adult olive (Olea europaea L.) trees (cv. Picholine marocaine) caused by water deficits under PRD irrigation in an experimental orchard planted in 1989 in Marrakech (Morocco). Three irrigation treatments were used: Control (irrigated with 100% of the crop evapotranspiration, on the two sides), PRD50 (irrigated with 50% of the control on one side switching every two irrigations) and PRD100 (irrigated with 100% of the control on one side, switching every irrigation). There were no significant differences (P > 0.05) in the diurnal evolution of leaf water potential of the olive plants between Control and PRD100. Whereas decreasing by 50% the water supply to half of the rootzone area (PRD50), caused significant decrease in time-course leaf water potential. However, this decrease was marginal despite large water saving, and resulted in similar leaf relative water content (RWC) to that of Control plants and the maintenance of the photosynthetic capacity. In fact, values of the photosynthetic parameters Amax, Jmax, Vcmax, Rd, and Jmax/ Vcmax ratio, i.e. a parameter that gives an indication of the nitrogen partitioning between thylakoid (photochemistry) and soluble (Calvin cycle) proteins, were not significantly influenced by the irrigation treatments. Finally, our results showed that leaf gas exchange studies of stomatal versus non-stomatal limitations to photosynthesis based on conventional measurements made at ambient [CO2], could lead to significant underestimations of diffusional limitations.