Changes in leaf gas exchange and biomass of Eucalyptus camaldulensis in response to increasing drought stress induced by polyethylene glycol

Abstract

Under increasing short-term drought stress, E. camaldulensis plantlets showed reduced stomatal conductance, then decreased carboxylation efficiency, and finally, reduced quantum efficiency of photosynthesis and chlorophyll content. We evaluated the physiological responses of a drought-tolerant (DT) clone and a drought-sensitive (DS) clone of Eucalyptus camaldulensis to drought stress. We evaluated leaf gas exchange, chlorophyll fluorescence, chlorophyll content, and biomass of 4-month-old eucalyptus plantlets during a graduated series of drought stress treatments over 24 days. The plantlets were grown in half-strength Hoagland’s solution, and drought stress was imposed by adding polyethylene glycol (PEG 6000) to the nutrient solution. The plantlets were subjected to three stepwise levels of drought stress, each 8 days long. The osmotic potential of the nutrient solution was −70, −140, and −280 kPa under mild, moderate, and severe drought stress, respectively. Compared with unstressed plantlets, the drought-stressed plantlets of both clones showed significant decreases in leaf gas exchange, chlorophyll fluorescence parameters, and biomass accumulation. The limitation of photosynthesis under mild drought stress was mainly because of reduced stomatal conductance. The reduction in the photosynthetic rate (P n ) under moderate to severe drought stress was because of both stomatal and non-stomatal limitation; that is stomatal closure, reduced carboxylation efficiency, reduced quantum efficiency, and decreased chlorophyll content. Both clones used stomatal control as an avoidance mechanism under drought stress. The differences between the two clones were already shown under mild drought stress. Compared to its non-stressed condition, the DS clone showed larger decreases in P n , stomatal conductance, and transpiration rate than the DT clone. The stepwise scheme in imposing drought stress can differentiate the drought response traits in E. camaldulensis.