Does abscisic acid and xylem sap pH regulate stomatal responses in papaya plants submitted to partial root-zone drying?

Papaya ( Carica papaya L.) is a fruit plant that requires high water availability, and therefore irrigation is required throughout its cropping cycle. Some irrigation techniques can be used to improve water use efficiency; Partial root-zone drying fixed (PRD Fixed ) is one of these techniques where water is applied only to one side of the root. Thus, water applied to one side of the root maintains leaf water status and plant growth, and uses root signaling (abscisic acid, ABA) to reduce stomatal conductance and transpiration, without affecting CO 2 assimilation. Here we checked whether papaya plants submitted to PRD Fixed technique changes shoot ABA and proline concentrations, and if these responses would be associated to alteration in xylem sap pH, gas exchange rates and plant growth. The root system of young papaya plants were divided into two square pots and three treatments: fully irrigated (FI) (soil kept at field capacity in both pots), DS (drought stress, irrigation was suspended in both pots) and PRD Fixed (irrigation was suspended in one pot and in the other pot the soil was kept at field capacity). No difference was observed in proline concentration between fully-irrigated and PRD Fixed plants. Drying one side of the root system was not sufficient to promote alkalinization of the xylem sap pH or consistently stimulate foliar ABA accumulation. Although higher rates of CO 2 assimilation and transpiration were noted in PRD Fixed plants compared to drought-stressed plants, this decrease would continue if water limitation was longer than 11 days. Consequently, this decrease would affect biomass over time. Thus, in the first 11 days of water limitation, the stomatal closure of PRD Fixed plants was not affected either by increase in ABA or by alkalinization of the xylem sap.