Partial rootzone drying (PRD) and regulated deficit irrigation (RDI) effects on stomatal conductance, growth, photosynthetic capacity, and water-use efficiency of papaya

- Split-root papaya had 4 irrigation regimes in the greenhouse and field: PRD, RDI, none, full.
- Both partial rootzone drying and regulated deficit irrigation reduced dry matter in the greenhouse but not in field conditions.
- In the field study, a 30% water deficit in both PRD70 and RDI treatments did not significantly reduce vegetative growth or yield components, compared to FI.
- It appears that papaya can tolerate moderate water deficits without a significant reduction in yield components.

Papaya (Carica papaya, L.) is an important economic crop in tropical and subtropical countries. It is a giant herbaceous species; maintaining adequate tissue turgidity and water availability is necessary to maintain the rigidity of the stem as well as increase productivity. Water-saving irrigation techniques for papaya will be needed in the future due to the possibility of water shortages related to climate change. The morphological and physiological responses of 'Grand Golden' papaya to partial root drying (PRD), regulated deficit irrigation (RDI) and no irrigation followed by re-hydration (NI-gh) in a greenhouse study or no irrigation (NI-field) in a field study were compared with full irrigation (FI). In the greenhouse study, plants were grown in pot with roots split equally between two soil columns. In the field study, drip emitters on opposite sides of the plant delivered irrigation water. In FI, the whole root system was irrigated at 100% of water use; in RDI 50% or 70% of FI water use was supplied to the whole root system (greenhouse and field study, respectively); in PRD 100% of FI was supplied to half the root zone while the other was allowed to dry to 50% or 70% of FI (greenhouse, and field study, respectively). In the field study, PRD was achieved by applying different amounts of irrigation water to alternate sides of the root system within the plant row. The application of 50% water use in PRD and RDI in the greenhouse study decreased shoot and root dry weight production, with a more pronounced effect on root dry weight compared to FI. This decrease in biomass was associated with a decrease in the net photosynthetic rate in the day of most intense water stress for the plants in NI-gh. In the field study, a 30% water deficit in both PRD and RDI treatments did not significantly reduce vegetative growth or yield components (number fruit plant−1, average weight (g) fruit−1, kg fruit ha−1, kg fruit plant−1), compared to FI. In greenhouse conditions there was evidence of a non-hydraulic signal in the PRD treatments decreasing Gs compared to RDI at comparable soil water tension but it was insufficient to affect shoot growth or yield components in field conditions. There was no difference in the instantaneous leaf water use efficiency (WUE, A/E) of PRD or RDI treatments in the greenhouse or the agronomic water use efficiency (AWUE) (kg fruit L−1 and number fruit L−1) in the PRD and RDI treatments in the field but both treatments improved AWUE compared to FI. It appears that papaya can tolerate some water deficits without a significant reduction in yield components indicating that <100% ET0 irrigation replacement can be scheduled but there is little or no benefit to PRD.