Sap fluxes from different parts of the rootzone modulate xylem ABA concentration during partial rootzone drying and re-wetting.

J G Pérez-Pérez,I C Dodd

doi:10.1093/jxb/erv029

Abstract

Previous studies with partial rootzone drying (PRD) irrigation demonstrated that alternating the wet and dry parts of the rootzone (PRD-Alternated) increased leaf xylem ABA concentration ([X-ABA]leaf) compared with maintaining the same wet and dry parts of the rootzone (PRD-Fixed). To determine the relative contributions of different parts of the rootzone to this ABA signal, [X-ABA]leaf of potted, split-root tomato (Solanum lycopersicum) plants was modelled by quantifying the proportional water uptake from different soil compartments, and [X-ABA]leaf responses to the entire pot soil-water content (θpot). Continuously measuring soil-moisture depletion by, or sap fluxes from, different parts of the root system revealed that water uptake rapidly declined (within hours) after withholding water from part of the rootzone, but was rapidly restored (within minutes) upon re-watering. Two hours after re-watering part of the rootzone, [X-ABA]leaf was equally well predicted according to θpot alone and by accounting for the proportional water uptake from different parts of the rootzone. Six hours after re-watering part of the rootzone, water uptake by roots in drying soil was minimal and, instead, occurred mainly from the newly irrigated part of the rootzone, thus [X-ABA]leaf was best predicted by accounting for the proportional water uptake from different parts of the rootzone. Contrary to previous results, alternating the wet and dry parts of the rootzone did not enhance [X-ABA]leaf compared with PRD-Fixed irrigation. Further work is required to establish whether altered root-to-shoot ABA signalling contributes to the improved yields of crops grown with alternate, rather than fixed, PRD.

Highlights

Soil moisture varies considerably both temporally and spatially, and both plant fitness and crop productivity depend on the root system capturing sufficient water to sustain growth
The irrigation technique of partial rootzone drying was conceived as a field adaptation of laboratory split-root soil drying experiments, to enhance root-to-shoot chemical signalling to improve crop water use efficiency by causing partial stomatal closure and decreasing excessive vegetative vigour (Dry et al, 1996; Kang and Zhang, 2004)
Many authors have postulated a role for root-sourced ABA in causing these physiological responses, there is considerable variability in the relative response of [X-ABA]leaf when comparing partial rootzone drying (PRD) and deficit irrigation (DI) plants (Dodd, 2007; Wang et al, 2012), perhaps related to the timing of measurements during drying/re-wetting cycles (Dodd et al, 2006) and/or total soil water availability (Romero et al, 2012)

Summary

Introduction

Soil moisture varies considerably both temporally (due to rainfall events or irrigation of crop plants) and spatially (roots usually dry the surface soil layers while considerable moisture may be available at depth), and both plant fitness and crop productivity depend on the root system capturing sufficient water to sustain growth. Even under controlled environment conditions, PRD increased (Dodd, 2007), decreased (Dodd, 2007; Wang et al, 2012) or had no consistent effect (Wang et al, 2010, 2012) on xylem ABA concentration compared with DI plants, perhaps due to the timing of measurements during the drying/ rewetting cycles Taken together, these results suggest that the agronomic promise of PRD is unlikely to be consistently translated into improved crop water use efficiency in the field, unless irrigation managers can better predict its physiological effects

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