Partial root zone drying: regulation of photosynthetic limitations and antioxidant enzymatic activities in young olive (Olea europaea) saplings

The effect of partial root drying (PRD) irrigation on split-root olive (Olea europaea L. cv Picholine marocaine) saplings was investigated. An irrigated control and two PRD regimes were applied (control: irrigation applied on both sides of the root system to keep the soil water content close to field capacity; PRD50: irrigation applied at 50% of the control amount on one side of the root system and irrigation withheld from the other side, with irrigation regimes switched between the sides of the root system every 2 weeks; and PRD100: irrigation applied at 100% of the control amount on one side and irrigation withheld on the other side, with irrigation regimes switched between the sides of the root system every 2 weeks. Only saplings in the PRD50 regime were subjected to water-deficit irrigation. The PRD treatments significantly affected water relations and vegetative growth throughout the growing season. Predawn leaf water potential and relative water content differed significantly between the PRD50 and PRD100 saplings, leading to reduced stomatal conductance, carbon assimilation, shoot length and leaf number in PRD50 saplings. However, the PRD50 water-deficit treatment did not affect the capacity of the saplings to assimilate CO2. Activities of superoxide dismutase, soluble and insoluble peroxidase (POX) and polyphenol oxidase were up-regulated by the PRD50 and PRD100 treatments compared with control values. The higher activities of both soluble and insoluble POX observed in PRD50 saplings may reflect the greater inhibitory effect of this treatment on vegetative growth. Up-regulation of the detoxifying systems in the PRD100 and PRD50 saplings may have provided protection mechanisms against irreversible damage to the photosynthetic machinery, thereby allowing the photosynthetic apparatus to function and preventing the development of severe water stress. We also measured CO2 assimilation rate/internal leaf CO2 concentration (A/C(i)) after exposing the attached leaves to very low [CO2] (approximately 50 μmol mol-1) to force stomatal opening. These results confirmed that, under conditions of moderate water stress, the sum of the diffusional resistances (i.e., stomatal and mesophyll resistances) sets the limit to photosynthetic rates. Assessing photosynthetic capacity without removing the diffusional limitations may lead to an overestimation of the biochemical limitations to photosynthesis in sclerophyllous plants.