PHYSIOL. PLANT. 58: 295-301. Copenhagen 1983 Stomatal and non-stomatal contribution in the decline in leaf net CO2 uptake during rapid water stress Gabriel Cornic, Jean-Louis Prioul and Genevieve Louason Cornic, G., Prioul, J. L. and Louason, G. 1983. Stomatal and non-stomatal contribu- tion in the decline in leaf net CO2 uptake during rapid water stress. - Physiol. Plant. 58;295-301. A simple method is proposed for quantitative evaluation of stomatal and non-stomatal components of the decline in leaf CO, uptake during rapid water stress. The changes in leaf conductance were measured during the stress and were used to calculate the photosynthetie rate which would be observed if stomatal closure were the only cause of the decline in photosynthesis. Photosynthesis-CO, 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. 1 his analysis was tested on Sittapis 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 CO^ 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 87o, respectively, at high, normal and low COj. 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. Additional key words — Excised leaves, Sinapis alba, G, Cornic and G, Louason (reprint request), Le Phytotron, CNRS, F 91190 Gif-sur- Yvette, France; J, L, Priotil, Structure et Metabolisme des Plantes, associe au CNRS (LA 40), Bat, 490, Univ, de Paris-Stid, 91405 Orsay Cedex, France, Introduction It is evident from the literature that non-stomatal com- ponents of photosynthesis are directly inhibited by wa- ter stress (see reviews of Boyer 1976, Osmond et al, 1980, Hanson and Hitz 1982). Leaf desiccation is also accompanied by substantial stomatal closure (Boyer 1976). This dual effect of water stress makes it difficult to ascertain the degree of inhibition of leaf photo- synthetic gas exchange which is caused by lowered stomatal conductance (which could change the internal COj pressure, Pj) and the direct effect of the stress on the photosynthetie reaction. As pointed out by Boyer (1976), to demonstrate the role of stomata in the water stress induced decline of photosynthesis occuring under natural conditions, it is necessary to show that internal CO2 pressure is limiting net photosynthesis during clo- sure. Gas-exchange studies have established that direct in- hibition of photosynthesis in water-stressed leaves can be observed when measurements are made under con- ditions which minimise the effect of reduced stomatal aperture. Measurements of leaf CO2 uptake rate in leaves of water stressed plants were made at low light levels and it was concluded that the observed inhibition was not attributable to stomatal effects (Wardlaw 1967, Boyer 1971), In another study, photosynthesis was measured at constant intercellular CO2 pressure (by forcing air through the leaf), and yet the rate of CO^ uptake declined when the plants were desiccated Received 26 October, 1982; revised 1 March, 1983 Physiol. Plant. 58. 1983 295