Photosynthetic Oxygen Evolution at Low Water Potential in Leaf Discs Lacking an Epidermis

A. C. Tang; Y. Kawamitsu; M. Kanechi & John S. Boyer

Land plants encountering low water potentials (low 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 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 w was below –1·1 MPa. CO2 entered readily at all w, as demonstrated by varying the CO2 concentration. Results were the same whether the epidermis was removed before or after low 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 w. In 1 % CO2, however, O2 evolution at low w was below the maximum, presumably because stomatal closure restricted CO2 uptake. The inhibition was larger than in peeled discs at w between –1 and –1·5 MPa but became the same as in peeled discs at lower w. Therefore, as photosynthesis began to be inhibited by metabolism at low w, stomatal closure added to the inhibition. As w became more negative, the inhibition became entirely metabolic.

Key words: Photosynthesis, water potential, epidermis, oxygen evolution, stomata, dehydration, carbon dioxide, Helianthus annuus L., sunflower.

Annals of Botany: http://aob.oxfordjournals.org/cgi/reprint/89/7/861 Marcos Vinicius