Water loss regulation in mature Hevea brasiliensis: effects of intermittent drought in the rainy season and hydraulic regulation.

Effects of soil and atmospheric drought on whole-tree transpiration (E(T)), leaf water potential (Ψ(L)) and whole-tree hydraulic conductance (K(T)) were investigated in mature rubber trees (Hevea brasiliensis, clone RRIM 600) during the full canopy stage in the rainy season in a drought-prone area of northeast Thailand. Under well-watered soil conditions, transpiration was tightly regulated in response to high evaporative demand, i.e., above reference evapotranspiration (ET(0)) ~2.2 mm day(-1) or maximum vapor pressure deficit ~1.8 kPa. When the trees experienced intermittent soil drought E(T) decreased sharply when relative extractable water in the top soil was < 0.4. The midday leaf water potential (Ψ(md)) on sunny days did not change as a function of soil drought and remained stable at approximately - 1.95 MPa, i.e., displaying isohydric behavior. The decrease in E(T) was mainly due to the change in K(T). K(T) remained constant over a wide range of environmental conditions and decreased sharply at low soil water availability. A simple hydraulic model incorporating critical minimum water potential and the response of whole-tree hydraulic conductance to relative extractable water correctly simulated patterns of transpiration over 6 months. We conclude that an explicit and simplified framework of hydraulic limitation hypothesis was sufficient to describe water use regulation of a mature rubber tree stand in water-limited conditions. Given the complexity of constraints in the soil-plant-atmosphere pathway, our results confirm the relevance of this approach to synthesize the overall behavior of trees under drought.