Chapter 23 Mechanisms of Drought and High Light Stress Tolerance Studied in a Xerophyte, Citrullus lanatus (Wild Watermelon)

The majority of higher plants are unable to survive extreme drought in the presence of strong solar radiation. However, a small group of vascular plants termed ‘‘xerophytes’’ have evolved drought and high light stress tolerance, and successfully thrives in the arid areas. This chapter will focus on the physiological, biochemical and molecular responses of wild watermelon (Citrullus lanatus), a xerophyte which is indigenous to the Kalahari Desert despite carrying out C3-type photosynthesis. The electrochromic shift of carotenoids in the thylakoid membranes was analyzed in vivo, which revealed that the proton efflux through chloroplast ATP synthase was strongly suppressed under drought and high light stresses. In addition, cyclic electron flow around photosystem I was significantly activated under the stress, suggesting the functional relevance of these processes to the build-up of large ΔpH across thylakoid membranes, for sustaining high qE quenching under excess light conditions. Biochemical analyses showed that key components for ROS metabolism, such as chloroplastic ascorbate peroxidase and monodehydroascorbate reductase, were markedly fortified in this plant. Moreover, unique responses of wild watermelon under the stress were described like metabolism and function of citrulline, a novel compatible solute with potent activity for scavenging hydroxyl radicals. Furthermore, characteristic gene expression patterns were observed in this plant under drought, which are exemplified by the induction of cytochrome b 561, a trans-plasma membrane protein for transferring reducing equivalents from cytosol to the apoplasts. Interestingly, unprecedentedly high activity of ascorbate oxidase was observed in the leaf apoplasts, suggesting the electron flux from cytosol to this terminal oxidase may be activated under drought. Taken together, these findings offer intriguing implications on how terrestrial plants can achieve effective adaptation to the harsh environmental conditions.