Temporal scaling of physiological responses from gas exchange to tree rings:: a gender-specific study of Acer negundo (Boxelder) growing under different conditions

1. Plant responses to water availability may change over time due to seasonality, interannual variation and developmental changes. Our past investigations demonstrated that male and female Boxelder (Acer negundo) exhibit differences in their responses to water availability. Here we use the genders as a model system to: (1) examine if cultural conditions have an effect on instantaneous gas exchange and time-integrated carbon isotope discrimination (Delta); (2) compare these physiological responses across a range of temporal scales (seasonal, annual, interannual); and (3) describe the responses of the genders at different ages to water availability. 2. Under well watered conditions, differences in instantaneous leaf gas exchange and Delta were observed between the genders at all developmental stages. Within a gender, Delta was similar throughout the growing season and between years, indicating a conserved physiological set-point despite changing growth conditions. When water stress was imposed, females exhibited greater reductions in photosynthetic rate and higher stomatal limitations to photosynthesis than males. 3. For well watered plants, a strong correlation existed between time-integrated intercellular leaf [CO2] calculated from Delta(c(i)) and c(i) obtained from instantaneous gas exchange. However, when drought was imposed this correlation weakened in males and was non-existent in females. 4. This study indicates that instantaneous gas exchange and integrated Delta values can be used to characterize leaf-level responses to water availability when growth conditions are relatively constant. However, when water availability fluctuates, care must be taken in equating these measurements because changing conditions may decouple responses integrating different time-scales, and the degree of this decoupling may vary even at the subspecies (gender) level.