Some aspects of drought resistance in Citrullus colocynthis L. in the Egyptian deserts

Samples of Citrullus colocynthis shoots were collected from nine stands during two seasons (winter and summer 2015) in two tributaries of Wadi Qena (W. El-Atrash and W. El-Ghuza) in the Eastern Desert of Egypt. Soil of these stands was characterized by slight alkalinity and low soil moisture and organic matter percentages. Soil macronutrients were arranged as Ca>K> Na>Cl>SO4>Mg>PO4. However, in C. colocynthis tissues, these nutrients accumulated in the following order: SO4 >K > Ca > Mg > Na> Cl > PO4. The plant chlorophyll contents (Chl. a and b), their ratio and their stability index (CSI) increased drastically during summer. Shoot accumulated SO4, PO4 and all cations during summer more than winter, except Cl that was higher during winter. Soluble sugars (SS) and soluble proteins (SP) increased during summer while total free amino acids were higher during winter. Data revealed that there were close relationships between the high SO4, K and Ca contents and the hot-dry conditions and the accumulation of SS and SP on the relatively hot-dry season  However, this may be primarily related to metabolism of drought resistance in such desert plants. Correlations between plant and soil variables showed important positive relations between soil Ca, Mg and plant Mg and between soil SO4 with Ca and Mg of plant. Spatial distribution of C. colocynthis was the most important factor that controls WC, Na, K, Ca, Mg, Cl, PO4, CSI a, CSI b , Chl. a and b, Chl. a/b ratio, TAA, , SS and SP. While, the interaction between both factors, spatial and temporal, could be the promoter.