Phytohormonal Regulation of Gases Exchange and Water Use Efficiency in Beta Vulgaris L. and Zea Mays L. During Water Stress.

The effect of abscisic acid (ABA) and synthetic cytokinin benzylaminopurine (BAP) on photosynthesis rate (Pn), transpiration rate (E) and water use efficiency (WUE) of sugar beet, cv. Takt, and maize, cv. Anjou 245, was studied. Young plants were grown in the pots under managed conditions in a greenhouse and were exposed to gradual drying-up of soil. Growth regulators were applied on leaves 3 days after interruption of irrigation of plants in the pots. 100 µM solution of ABA and 1 µM and 10 µM solution of BAP were used. Plants exposed to gradual drying-up of soil for 12 days after interruption of irrigation retained a high RWC, virtually at the level of non-stressed plants. However, the gradual drying-up of soil reduced the photosynthesis rate and transpiration rate in the sugar beet and maize. The experimental results proved the effect of exogenously applied abscisic acid and benzylaminopurine on the control of gas exchange in the leaves of sugar beet and maize under water stress. Shortly after it is applied, ABA reduces Pn and E and slows down the progression of development of water stress in the treated plants. BAP significantly increases the parameters of gas exchange in the treated plants. However, the experiment confirmed the importance of choosing the right concentration of the substance. The used concentration of 1 µM BAP was better alternative as concerned water use efficiency. With a higher concentration of BAP (10 µM), the high photosynthesis rate was accompanied with high transpiration. This significantly deteriorated water use efficiency. During a longer-term drought, the use of too high a concentration of BAP can result in faster drainage of water and, ultimately, in deterioration of the water stress.