Leaf phytohormone levels and stomatal control in an evergreen woody species under semiarid environment in a Brazilian seasonally dry tropical forest

Abstract

Phytohormones are essential for controlling abilities of plant species to overcome stress conditions, and influence some aspects of stomatal control, preventing excessive water loss. This study investigates the correlation between foliar phytohormones levels, water status and stomatal conductance in an evergreen woody species (Cynophalla flexuosa) throughout dry and rainy seasons, and the transition between them. We measured stomatal conductance (gs), xylem branch water potential (Ѱx), and leaf concentration of abscisic acid (ABA), jasmonic acid (JA), salicylic acid (SA) and trans-zeatin (tZ). Stomatal conductance was more sensitive to atmospheric conditions, such as VPD than to soil water balance. However, we found correlation between gs and Ѱx, suggesting that these direct water availability measures were a good proxy to explain gs in C. flexuosa. Moreover, ABA leaf concentration had no effect on gs, but ABA and tZ interaction was important to the phenological behaviour of this species. Cytokinins act in delaying leaf senescence, which is crucial to evergreen species, and it is opposite to ABA’s action. JA also showed a significant interaction to ABA, and kept high foliar level during wet season. ABA levels varied throughout the year, and its concentration itself was less important to gs than the interaction to other phytohormones, such as tZ and JA. In conclusion, although ABA did not directly affect stomatal conductance in C. flexuosa, the interaction between ABA, tZ and JA likely played a role in the regulation of stomatal behavior in this species.