Premature senescence of cotton under drip irrigation below mulch film (DI) is much more widespread than under flooding irrigation (FI), and the change of plant nutrients under DI has been regarded as a possible influencing factor. In this study, a field experiment was conducted to investigate the cotton growth and nutrient status under DI in comparison to FI. Severe symptoms of premature senescence occurred at the later growth stages of cotton under DI, while no such symptoms were observed under FI. At 125 DAS (late bolling stage), shoot biomass under DI was significantly higher than that under FI, while root biomasses showed an opposite trend, and the shoot/root ratio under DI was significantly higher than that under FI. The shoot dry weight of plants grown under DI dropped abruptly from 125 DAS (late bolling stage) to 160 DAS (boll opening stage). Field observations indicated that the main reason for the decrease was the shedding of a large number of leaves, squares and bolls. The shoot to root ratio under DI also decreased to the same level as that of the FI treatment. An analysis of leaf nutrition revealed that the nitrogen and phosphorus concentrations of old leaves were lower than those of young leaves under both of DI and FI at the later growth stages, whereas the potassium concentration presented an opposite picture. The K concentration of young leaves under DI was lower than that under FI, and the K concentration of cotton shoots showed a similar trend. The above results suggested that premature senescence of cotton under DI may result from the fact that the requirement of the vigorously growing shoots for K exceeds the K amount that the poorly developed roots are able to supply.
