Mechanisms contributing to seasonal homeostasis of minimum leaf water potential and predawn disequilibrium between soil and plant water potential in Neotropical savanna trees

Seasonal regulation of leaf water potential (psi L) was studied in eight dominant woody savanna species growing in Brazilian savanna (Cerrado) sites that experience a 5-month dry season. Despite marked seasonal variation in precipitation and air saturation deficit (D), seasonal differences in midday minimum psi L were small in all of the study species. Water use and water status were regulated by a combination of plant physiological and architectural traits. Despite a nearly 3-fold increase in mean D between the wet and dry season, a sharp decline in stomatal conductance with increasing D constrained seasonal variation in minimum psi L by limiting transpiration per unit leaf area (E). The leaf surface area per unit of sapwood area (LA/SA), a plant architectural index of potential constraints on water supply in relation to transpirational demand, was about 1.5-8 times greater in the wet season compared to the dry season for most of the species. The changes in LA/SA from the wet to the dry season resulted from a reduction in total leaf surface area per plant, which maintained or increased total leaf-specific hydraulic conductance (Gt) during the dry season. The isohydric behavior of Cerrado tree species with respect to minimum psi L throughout the year thus was the result of strong stomatal control of evaporative losses, a decrease in total leaf surface area per tree during the dry season, an increase in total leaf-specific hydraulic conductance, and a tight coordination between gas and liquid phase conductance. In contrast with the seasonal isohydric behavior of minimum psi L, predawn psi L in all species was substantially lower during the dry season compared to the wet season. During the dry season, predawn psi L was more negative than bulk soil psi estimated by extrapolating plots of E versus psi L to E=0. Predawn disequilibrium between plant and soil psi was attributable largely to nocturnal transpiration, which ranged from 15 to 22% of the daily total. High nocturnal water loss may also have prevented internal water storage compartments from being completely refilled at night before the onset of transpiration early in the day.