Slowly-Developing Drought Stress Increases Photosynthetic Acclimation of Catharanthus Roseus

Plant drought stress studies often neglect the importance of the rate at which the drought stress is imposed. However, the rate of drought stress imposition may affect the physiological responses of the plants to the drought. To test this, the change in the whole-plant photosynthetic rate of vinca (Catharanthus roseus) subjected to different drought imposition rates was monitored. A load cell–based, automated irrigation system controlled the rate at which the substrate dried out by changing the target pot weight gradually throughout the drying period. Fast, intermediate, and slow drying treatments reached their final pot weight [500 g, substrate water content (θ) ≈ 0.1 m³·m^-3] after 4, 8, and 12 days, respectively. When θ was 0.15 m³·m^-3, whole-plant photosynthesis in the fast, intermediate, and slow drought treatments was reduced to 29, 60, and 75% of the pre-drought rate, respectively. Plants exposed to slow drying were able to maintain photosynthetic rates of > 50% of the pre-drought rate at θ of 0.08 m³·m^-3. When pots weight reached 500 g, plants that dried slowly had approximately 0.4 MPa higher turgor than those subjected to fast drying, despite similar substrate moisture levels. Slow drying also resulted in higher whole-plant water use efficiency than fast drying. These results clearly show that the rate at which plants are subjected to drought stress affects the plants response to drought and this should be considered in drought stress physiology studies.