On 17 August at 12:00 Ott Kangur will defend her doctoral thesis "Nocturnal water relations and predawn water potential disequilibrium in temperate deciduous tree species".
Prof. Arne Sellin, University of Tartu, Estonia
Prof. Sandra J. Bucci, National University of Patagonia San Juan Bosco and Institute of Biosciences of Patagonia (CONICET), Argentina
Plants are usually studied during daytime when photosynthesis and intensive water exchange between the plant and the environment takes place. However, nocturnal and predawn water relations of plants have also gained considerable attention during recent decades. Nevertheless, there are still uncertainties what factors and how may affect characteristics like nocturnal sap flow, nocturnal and predawn stomatal conductance, and predawn leaf water potential – a characteristic commonly used for estimating soil water status. The aim of this thesis was to compare nocturnal stomatal conductance and predawn water potential disequilibrium between leaf and soil (PDD) in temperate deciduous tree species differing in ecological demands and life strategies; to elucidate the environmental factors that govern nocturnal transpiration and PDD; and to test whether increasing daytime atmospheric humidity – a climate trend predicted for high northern latitudes – affects nocturnal water use in broadleaved trees. An experiment in nine temperate tree species in a greenhouse demonstrated that under ample soil water availability, fast-growing pioneer species exhibited higher night-time stomatal conductance compared to slow-growing late-successional species. That was true about the species originating from more humid and cooler regions (e.g. Populus tremula, Betula pendula, Alnus glutinosa). In drought conditions the differences in stomatal conductance between the species disappeared. Field studies carried out in Estonia proved the occurrence of significant PDD and nocturnal sap flow in hybrid aspen (Populus tremula × P. tremuloides). Positive effect of nocturnal atmospheric evaporative demand on both characteristics is a solid proof of open stomata at night. This suggests that under drier and warmer atmospheric conditions predawn leaf water status cannot be used as a reliable estimate of soil water status. Our experiments proved that hybrid aspen trees are able to absorb dew water from leaf surface, exhibiting foliar water uptake, which might also bias predawn leaf water status. In a growth chamber experiment, hybrid aspen saplings grown under elevated air humidity responded with increased sap flux density to a rise in atmospheric evaporative demand at night, which evidences weakened stomatal control over water loss, although the field studies did not support that finding. Taking into account the regional and global climate trends, it is very likely that the proportion of nocturnal water losses in total daily transpiration of trees as well as the necessity for considering nocturnal transpiration in calculations of landscape or ecosystem water balance will increase in the future.