Physiological and morphological response to drought stress in seedlings of ten citrus

Under drought stress, growth seedlings decreased, but proline and soluble sugar increased in shoots and roots of tolerance citrus. There are correlations between these parameters under water stress. Citrus fruit production is confronted worldwide with increasing biotic and abiotic constraints. In this study, ten citrus fruits (Bael, Sour orange, Sweet orange, Lemon, Rough lemon, Shell Mahalleh, Alemow, Trivestrange, Red blush and Pineapple) were compared for drought stress tolerance. Drought stress was induced by polyethylene glycol 6000 to produce water potentials of 0, -0.25, -0.5, -0.75, -1 and -1.5 MPa. Under stress conditions, shoot and root length, shoot and root dry weight, shoot and root fresh weight, tissue water content and germination percentage decreased markedly. There were variations in degree among the cultivars. Plants exposed to drought stress had a higher amount of soluble sugar in shoots and roots of tolerant cultivars (Bael, Rough lemon, Sour orange and Alemow) and a lower amount of starch in their tissues. Also, the levels of proline increased in parallel with the severity of drought stress in both shoots and roots in tolerant cultivars. The accumulation of soluble sugar and proline indicates a possible role of these osmolytes in drought tolerance. We assume that there would be correlations between these parameters under different levels of water stress. It is possible to use proline and soluble sugar as an index of drought stress in citrus trees. Cluster analysis produced a dendrogram with three groups differing in their tolerance to osmotic stress.