Soil moisture heterogeneity during deficit irrigation alters root-to-shoot signalling of abscisic acid.

The effects of different irrigation techniques on leaf xylem ABA concentration ([X-ABA]) were compared in tomato (Lycopersicon esculentum Mill.). During partial rootzone drying (PRD), water was distributed unevenly to the root system such that part was irrigated while the remainder was allowed to dry the soil. During conventional deficit irrigation (DI), plants received the same volume of water as PRD plants, but water was distributed evenly to the entire root system. When the plant root system was allowed to explore two separate soil compartments, DI plants had a higher [X-ABA] than PRD plants with moderate soil drying, but PRD plants had a higher [X-ABA] than DI plants as the soil dried further. The difference in [X-ABA] between the two sets of plants was not because of differences in either whole pot soil water content (θ) or leaf water potential (Ψ). To investigate the contribution of different parts of the root system to [X-ABA], individual shoots were grafted onto the root systems of two plants grown in two separate pots, so that the graft union had the appearance of an inverted 'Y'. After sap collection from detached leaves, removal of the shoot below the graft union allowed sap collection from each root system. Again, DI plants had a higher [X-ABA] than PRD plants when the soil was relatively wet, but the opposite occurred as the soil dried. Root xylem ABA concentration ([X-ABA]) increased exponentially as soil water content (θ) declined. In DI plants, [X-ABA] from either pot (or the arithmetic mean of [X-ABA]) accounted for a similar amount of the variation in [X-ABA]. In PRD plants, [X-ABA] from the watered side underestimated [X-ABA], whereas [X-ABA] from the dry side overestimated [X-ABA]. The arithmetic mean of [X-ABA] best explained the variation in [X-ABA], implying continued sap flow from the dry part of the root system (J) at soil water potentials (Ψ) at which J had ceased in previous studies of PRD plants (Yao et al. 2001). Evaluating the relationship between J and Ψ may assist in maintaining export of ABA (and other growth regulators) from the drying part of the root system, to achieve desirable horticultural outcomes during PRD.