Water use by the desert cucurbit Citrullus colocynthis (L.) Schrad.

The rates of water use and leaf surface conductance of Citrullus colocynthis (Cucurbitacea) were evaluated from measurements of the surface temperature and microenvironment of leaves. At desert sites in Saudi Arabia the transpiration rates reached 0.13-0.17 g m-2 s-1 and the leaf temperatures were always close to air temperature. Leaf models (dry) placed in the canopy were considerably warmer than the air. To investigate responses over a wider range of conditions, plants were grown in a controlled environment room. It was found that when conditions were made hotter than those that occurred in the desert, the stomatal conductance increased greatly. Transpiration rate attained 0.6 g m-2 s-1 and the leaves were up to seven degrees cooler than the air. The results suggest a finely-tuned control mechanism working like a switch when the leaves experience extreme conditions, and enabling the plant to avoid lethal temperatures.