Humidity Pretreatment Affects the Responses of Stomata and CO<sub>2</sub> Assimilation to Vapor Pressure Difference in C<sub>3</sub> and C<sub>4</sub> Plants

Abstract

Experiments were carried out to investigate the long-term influence of humidity on the short-term responses of stomata and CO2 assimilation to vapor pressure difference in Oryza sativa (rice, C3 species) and Panicum maximum (green panic, C4 species). Plants were grown for four weeks in growth chambers set at 35% and 85% relative humidity at 25°C air temperature, 38+2 Pa CO2 partial pressure and 1,700μmol m-2s-1 photon flux density. Soil was saturated with water in both humidity treatments. Low humidity pretreatments caused low leaf conductance and low rates of transpiration and CO2 assimilation in O. sativa, but small changes in stomatal responses to humidity and in CO2 assimilation were found in P. maximum. From the short-term gas exchange experiments, it was noted that the responsiveness of leaf conductance to vapor pressure difference were affected by humidity pretreatments in O. sativa, whereas unaffected in P. maximum. In O. sativa measurements of CO2 assimilation as a function of internal CO2 partial pressure (A-Ci curve) indicated that low humidity pretreatments reduced the CO2 assimilation at high internal CO2 partial pressure, but the initial slope of the A-Ci curve was unaffected. Furthermore, plant characteristics such as total dry weight and leaf area of plants subjected to low umidity were lower than plants subjected to high humidity. The reductions in O. sativa, however, were larger than in P. maximum. Stomatal frequency from low humidity grown plant was higher than that from high humidity grown plants in both species although there is no significant difference. The data indicated that if the short term inhibition of net CO2 assimilation at a high vapor pressure difference was imposed during vegetative growth, the photosynthetic biochemistry and the resultant plant growth were largely depressed in O. sativa, a C3 species.