Field study of endogenous and exogenous contributions to diurnal stomatal conductance rhythms in Eucalyptus pauciflora saplings

Abstract

Internal circadian rhythms and exogenous factors such as light intensity, vapour pressure deficit (VPD) and wind speed affect diurnal cycles of photosynthetic capacity and gas exchange in plants. Stomatal conductance of Eucalyptus pauciflora Sieber ex Spreng saplings was monitored in leaves both in situ and excised in a darkroom for 24–48 hours. We tested for evidence of endogenous rhythms by examining the change in stomatal conductance with the microclimate variables photosynthetic photon flux density (PPFD), VPD and wind speed. In situ, saplings showed clear diurnal cycles of stomatal conductance and increased stomatal conductance pre-dawn on only one of the two measurement mornings. Leaves placed in continuous darkness took over four hours to decrease stomatal conductance to night-time levels and failed to increase stomatal conductance for the next natural photoperiod. Variation in stomatal conductance was found to have a high correlation with light intensity variation (R² = 0.6736), but low correlation with VPD and wind speed (R² = 0.0116 & R² = 0.1694). Our results conflict on whether stomatal conductance is regulated by circadian rhythms in E. pauciflora, however it is possible that with a longer study and more replicates that circadian regulation would be confirmed.