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

Abstract

Land plants encountering low water potentials (low Ψw) 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 Ψw. 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 Ψw was below -1.1 MPa. CO2 entered readily at all Ψw, as demonstrated by varying the CO2 concentration. Results were the same whether the epidermis was removed before or after low Ψw was imposed. Due to the lack of an epidermis and ready movement of CO2 through the mesophyll, thc 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 conccntration of 5%, which supported maximum O2 evolution at low Ψw. In 1% CO2, however, O2 evolution at low Ψw was below the maximum, presumably because stomatal closure restricted CO2 uptake. The inhibition was larger than in peeled discs at how between -1 and -1.5 MPa but became the same as in peeled discs at lower Ψw. Therefore, as photosynthesis began to be inhibited by metabolism at low Ψw, stomatal closure added to the inhibition. As Ψw became more negative, the inhibition became entirely metabolic. © 2002 Annals of Botany Company.