Changes in photosynthetic capacity, carboxylation efficiency, and CO 2 compensation point associated with midday stomatal closure and midday depression of net CO 2 exchange of leaves of Quercus suber

The carbon-dioxide response of photosynthesis of leaves of Quercus suber , a sclerophyllous species of the European Mediterranean region, was studied as a function of time of day at the end of the summer dry season in the natural habitat. To examine the response experimentally, a “standard” time course for temperature and humidity, which resembled natural conditions, was imposed on the leaves, and the CO 2 pressure external to the leaves on subsequent days was varied. The particular temperature and humidity conditions chosen were those which elicited a strong stomatal closure at midday and the simultaneous depression of net CO 2 uptake. Midday depression of CO 2 uptake is the result of i) a decrease in CO 2 -saturated photosynthetic capacity after light saturation is reached in the early morning, ii) a decrease in the initial slope of the CO 2 response curve (carboxylation efficiency), and iii) a substantial increase in the CO 2 compensation point caused by an increase in leaf temperature and a decrease in humidity. As a consequence of the changes in photosynthesis, the internal leaf CO 2 pressure remained essentially constant despite stomatal closure. The effects on capacity, slope, and compensation point were reversed by lowering the temperature and increasing the humidity in the afternoon. Constant internal CO 2 may aid in minimizing photoinhibition during stomatal closure at midday. The results are discussed in terms of possible temperature, humidity, and hormonal effects on photosynthesis.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47468/1/425_2004_Article_BF00397440.pd