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
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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.