Photosynthetic oxygen evolution at low water potential in leaf discs lacking an epidermis

Land plants encountering low water potentials (low psi(w)) close their stomata, restricting CO2 entry and potentially photosynthesis. To determine the impact of stomatal closure, photosynthetic O2 evolution was investigated in leaf discs from sunflower (Helianthus annuus L.) plants after removing the lower epidermis at low psi(w). Wounding was minimal as evidenced by O2 evolution nearly as rapid as that in intact discs. O2 evolution was maximal in 1% CO2 in the peeled discs and was markedly inhibited when psi(w) was below -1.1 MPa. CO2 entered readily at all psi(w), as demonstrated by varying the CO2 concentration. Results were the same whether the epidermis was removed before or after low psi(w) was imposed. Due to the lack of an epidermis and ready movement of CO2 through the mesophyll, the loss in O2 evolving activity was attributed entirely to photosynthetic metabolism. Intact leaf discs showed a similar loss in activity when measured at a CO2 concentration of 5%, which supported maximum O2 evolution at low psi(w). In 1% CO2, however, O2 evolution at low psi(w) was below the maximum, presumably because stomatal closure restricted CO2 uptake. The inhibition was larger than in peeled discs at psi(w) between -1 and -1.5 MPa but became the same as in peeled discs at lower psi(w). Therefore, as photosynthesis began to be inhibited by metabolism at low psi(w), stomatal closure added to the inhibition. As psi(w) became more negative, the inhibition became entirely metabolic.