Water availability effects on plant growth, seed yield, seed quality in Cassia obtusifolia L., a medicinal plant

The seeds of Cassia obtusifolia L. are widely used in traditional Chinese medicine and health tea. The objective of this study was to investigate the effect of water availability on plant growth, seed yield, and seed quality in C. obtusifolia for improving its cultivation. Pot and field irrigation experiments were conducted in Qingdao of China in 2013 and 2014, respectively. Seven water irrigation treatments including 100%, 90%, 80%, 70%, 60%, 50% and 40% of field capacity (FC) in pot experiments while five water irrigation treatments including 100%, 85%, 70%, 55% and 40% of field capacity (FC) in field experiments were applied on the plant of C. obtusifolia, respectively. In both experiments, the results showed that different drought stress significantly resulted in different reduction in plant growth and seed yield attributes of C. obtusifolia, and more pronounced with the severity of drought. However, as for the seed quality attributes, both results showed that weak and moderate drought stress significantly increased the content of anthraquinones (plant secondary metabolites) while significantly decreased the content of protein (plant primary metabolites) in the seeds of C. obtusifolia, except that strong drought stress significantly decreased both of them. Notably, the weak drought stress (70% FC in pot and in field) did not decrease the seed yield significantly which resulted in a biggest harvest index among other treatments and thus resulted in the highest anthraquinones yields per plant. In addition, results showed that strong and moderate drought stress affected the proportion between hydrophilic and lipophilic anthraquinones in seed of C. obtusifolia significantly. Our study suggested that a certain of deficit irrigation has the potential to compromise seed yield and seed quality of C. obtusifolia, which can be used as a practical irrigation strategy for cultivation improving of C. obtusifolia as a medicinal plant.