Effects of Irrigation Patterns during Grain Filling on Grain Quality and Concentration and Distribution of Cadmium in Different Organs of Rice

The objective of this study was to investigate whether grain quality could be improved and the proportion of cadmium (Cd) be reduced in grains through a proper irrigation management when rice (Oryza sativa L.) was planted in Cd-contaminated soil. Two rice cultivars, Yangjing 9538 (japonica) and Yangdao 6 (indica), were pot-grown. A total of 150 mg kg−1 Cd was added into pots before seedling transplantation (Cd treatment), and Cd-free treatment was used as the control. Three irrigation patterns, well-watered (WW), moderate dry-wet alternate irrigation (MD, soil was re-watered when soil water potential reached −20 kPa), and severe dry-wet alternate irrigation (SD, soil was re-watered when soil water potential reached at −40 kPa), were carried out from 7 d after heading to maturity. Under the same Cd concentration in soil and compared to WW, MD significantly increased, but SD significantly reduced seed-setting rate, 1000-grain weight, grain yield, brown rice rate, milled rice rate, and head rice rate. Moderate dry-wet alternate irrigation markedly reduced, whereas SD increased chalkiness. There were no significant differences in seed-setting rate, 1000-grain weight, and quality traits of rice between the Cd treatment and the control. Under the Cd treatment and compared to WW, MD and SD significantly increased the concentration and proportion of Cd in roots, but significantly reduced Cd concentration in stems and leaves and reduced Cd proportion in grains. Cadmium concentrations in grains and milled rice were significantly higher under SD than under WW, and exhibited no significant difference between MD and WW. Severe dry-wet alternate irrigation resulted in lower proportion of Cd in milled rice compared to MD or WW. The results from this study indicate that moderate dry-wet alternate irrigation during grain filling increases grain yield and improves milling and appearance quality, but does not increase Cd concentration or even reduce Cd proportion in grains. The reasons for variations in grain yield, quality, and the concentration and proportion of Cd under different irrigation patterns were analyzed in terms of root activity, photosynthetic characters, and Cd translocation.