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 CO₂ and drought conditions, on the physiological responses of Perilla frutescens var. japonica Arum. Perilla frutescens var. japonica ‘Arum’ was exposed to 700 μmol mol⁻¹ of CO₂ under both well-watered and water-stressed conditions. Photosynthetic rate was higher under elevated CO₂ conditions. Stomatal resistance increased while transpiration rates declined, which suggests that water-use efficiency rose under elevated CO₂ conditions. Under water-stressed conditions, elevated CO₂ concentrations induced much higher stomatal resistance than ambient CO₂ levels. This result implies that elevated CO₂ concentrations might increase plant sensitivity to water stress, thereby providing plants with increased protection against drought. Furthermore, elevated CO₂ concentrations 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 CO₂ conditions compared to ambient CO₂ levels. Therefore, elevated CO₂ concentrations might increase plant resistance to water stress. Moreover, there is a possibility that increased ethylene evolution under elevated CO₂ conditions could improve the capacity of plants to scavenge reactive oxygen species by enhancing APX activity.