Comparative effects of partial root drying (PRD) and regulated deficit irrigation (RDI) on water relations and water use efficiency in common bean (Phaseolus vulgaris L.)

In order to investigate the hypothesis that plant responses to altered water relations was related to hormonal signals in water-restricted plants, the comparative effects of partial root drying (PRD) and regulated deficit irrigation (RDI) were investigated by measuring plant growth, water relations, leaf abscissic acid (ABA) concentrations and xylem sap pH in common bean (Phaseolus vulgaris L.). In a first experiment, plants were grown in a split-root system, with roots divided between two compartments and subjected to one of three irrigation treatments: well-watered (WW) receiving 100% of plant transpiration (T), PRD with half of the root system exposed to soil drying and other half kept well-watered with 50% T, and RDI with 50% T supplied, half to each side of the root system. The results showed a significant decrease in leaf water potential for both PRD and RDI treatments compared to the WW, but without any significant differences between PRD and RDI. Transpiration did not show any significant effect of irrigation treatments during the first 10 days of drought stress application, and thereafter decreased in PRD and RDI to a level around 50% of WW. Both RDI and PRD treatments resulted in a significant reduction of shoot and pod biomass compared to the control. However, since the total water used by transpiration was reduced by half in both water deficits treatments, this has resulted in a substantial increase in plant water use efficiency. Leaf ABA concentrations did not change under PRD and RDI throughout the experiment, until day 25 of water deficit application, where plants under RDI treatment accumulated higher amounts of ABA compared to WW and PRD plants. It was therefore concluded that ABA might not be directly involved in the initial stages of stomatal control under PRD or RDI. A second experiment showed that the relationship between relative transpiration rate and the fraction of transpirable soil water (FTSW) was well described by a linear plateau function that allowed the calculation of the FTSW thresholds at which transpiration started to decrease for RDI and PRD compared to WW. These thresholds were similar for RDI and PRD treatments. These results confirmed the conclusion of the first experiment, and did not support the hypothesis of a specific root-to-shoot signaling mechanism under PRD triggering stomatal closure, since no earlier decline in transpiration rate was observed in PRD compared to RDI.