Effects of deficit irrigation frequency on plant growth, water use and physiology of Pelargonium x hortorum and tomato (Solanum lycopersicum L. cv. Ailsa Craig)

• Main Author: Boyle, Richard
• Other Authors: Dodd, Ian ; McAinsh, Martin
• Published: Lancaster University 2015
• Subjects: 631.7
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.677257

In horticultural sectors where water is a threatened resource, altering irrigation frequency may present a viable approach to reduce water use, without any negative effect on crop yield and/or quality. However, our understanding of the physiological impact of this approach in containers in a peat based substrate is limited. Pelargonium x hortorum Bullseye plants were grown in glasshouse conditions under well-watered (WW; daily replacement of 100% of evapotranspiration (ET)), frequent (FDI), or infrequent (IDI) deficit irrigation regimes (50% of ET supplied daily or cumulatively every 4 days, respectively) for four weeks. Both FDI and IDI resulted in short-term increases in water use efficiency, and longer term increases in plant quality (canopy compactness) compared to WW plants. From a physiological perspective, stomatal conductance (gs) decreased similarly under both FDI and IDI, but there were treatment differences in leaf water potential (Ψleaf). FDI resulted in a more positive Ψleaf compared to WW plants, whilst Ψleaf under IDI was typically the lowest. Given the lack of a consistent response for Ψleaf, this suggested another mechanism was regulating stomata in P.hortorum. Under a single drying cycle, different components of the xylem sap were measured. Xylem sap pH, Ca2+ and NO3 - did not change, but the plant hormone abscisic acid (ABA) increased in the xylem sap ([X-ABA]leaf) under both irrigation treatments as soil moisture decreased, and showed a strong relationship with gs both in vivo and in a detached leaf transpiration bioassay. However, when plants were irrigated daily at a percentage of daily ET (adapted from FDI), plants showed an attenuated ABA response compared to when irrigation was withheld (adapted from IDI). It was hypothesised that this may have been a root-derived response due to spatial variation in soil moisture distribution, which was investigated in tomato (Solanum lycopersicum). Similar results were found where gs decreased as [X-ABA]leaf increased, but again the ABA response was attenuated. Furthermore, stomata showed similar sensitivity to ABA under both irrigation treatments. However, similar results were found for root tissue ([ABA]root) and xylem ([X-ABA]root) ABA, and modelling revealed that both localised root water uptake and soil moisture content are important for explaining the variation in [X-ABA]root between irrigation treatments. This research furthers the fundamental understanding of ABA signalling and suggests that irrigation frequency can be altered for a short period of the growing cycle to deliver specific grower objectives.