A simple method is proposed for quantitative evaluation of Stomatal and non-stomatal components of the decline in leaf CO2 uptake during rapid water stress. The changes in leaf conductance were measured during the stress and were used to calculate the photosynthetic rate which would be observed if Stomatal closure were the only cause of the decline in photosynthesis. Photosynthesis-CO2 response curves, determined just before the stress, were used for this calculation. The difference between the calculated and the actual rate is a measure of the non-stomatal effect of water stress.
This analysis was tested on Sinapis alba submitted to rapid and severe water stress by excising leaves or roots. Experiments were performed at saturating light conditions under high (61 Pa), normal (34 Pa) or low (11 Pa) ambient CO2 pressure. The non-stomatal effect on de-rooted plants reaches a maximum at the beginning of the stress and is dependent on the CO2 pressure: after 45 min its influence is still about 100°, 70° and 8°, respectively, at high, normal and low CO2. In the excised leaf system in which desiccation was more rapid, the non-stomatal effect accounted for nearly 100° of the assimilation decline whatever the CO2 pressure.