Hormonal dynamics contributes to divergence in seasonal stomatal behaviour in a monsoonal plant community

The plant hormone abscisic acid (ABA) is a primary regulator of plant transpiration, but its influence in determining seasonal stomatal behaviour in natural plant communities is poorly understood. We examined distantly related vascular plants growing together in a seasonally dry, monsoonal environment to determine whether ABA dynamics contributed to contrasting water use patterns in this natural setting. Regular sampling of angiosperm, cycad, conifer and fern species revealed characteristic seasonal patterns in ABA production, but these were highly distinct among species. Although no general relationship was observed between ABA levels, plant hydration or stomatal conductance among species, the seasonal dynamics in stomatal behaviour within species were predictable functions of either ABA or leaf water potential. Strong divergence in the seasonal role of ABA among species suggests that modification in ABA-stomatal interactions represents an important evolutionary pathway for adaptation in plant water use. Here we test the hypothesis that species specific biosynthesis, metabolism and stomatal sensitivity to ABA can direct contrasting stomatal behaviours among coexisting species in a natural setting. In the natural community we studied four highly-divergent strategies of water use were associated with different patterns of ABA sensing, synthesis and metabolism by the leaf. These findings provide a novel avenue for future stomatal and phytohormone research through the characterisation of ecological strategies regulated by ABA and stomatal behaviour.