Effects of elevated CO2 and water stress on physiological responses of Perilla frutescens var. japonica HARA

The aim of the present study was to investigate the interactive effects of expected environmental constraints, specifically elevated CO₂and drought conditions, on the physiological responses of Perilla frutescens var. japonica Arum. Perilla frutescens var. japonica ‘Arum’ was exposed to 700 μmol mol⁻¹of CO₂under both well-watered and water-stressed conditions. Photosynthetic rate was higher under elevated CO₂conditions. Stomatal resistance increased while transpiration rates declined, which suggests that water-use efficiency rose under elevated CO₂conditions. Under water-stressed conditions, elevated CO₂concentrations induced much higher stomatal resistance than ambient CO₂levels. This result implies that elevated CO₂concentrations might increase plant sensitivity to water stress, thereby providing plants with increased protection against drought. Furthermore, elevated CO₂concentrations alleviated drought-induced photosynthetic decline in the early stages of drought, although this effect was not sustainable. Water stress also elevated ascorbate peroxidase (APX) activity but had little effect on glutathione reductase activity. Water stress-induced APX activity was much lower under elevated CO₂conditions compared to ambient CO₂levels. Therefore, elevated CO₂concentrations might increase plant resistance to water stress. Moreover, there is a possibility that increased ethylene evolution under elevated CO₂conditions could improve the capacity of plants to scavenge reactive oxygen species by enhancing APX activity.