Selecting rice genotypes tolerant to zinc deficiency and sodicity stresses. I. Differences in zinc, iron, manganese, copper, phosphorus concentrations, and phosphorus/zinc ratio in their leaves

Zinc (Zn) deficiency is often reported on rice crops growing in sodic soils. Thirty rice genotypes were tested for their tolerance to Zn deficiency and sodicty stresses by transplanting 35-days old seedlings in pots having sodic soil of pH(2) (1:2 soil water ratio) 9.8, and DTPA-extractable Zn 1.8 ppm. Ten genotypes (Group A) showed potential to tolerate both the stresses with better survival (73-100%), tillering and were free from any visible symptoms of either Zn deficiency or sodium (Na) toxicity on their leaves. Sixteen genotypes (Group B) including CSR10, a tolerant check to sodicity, were found sensitive to Zn deficiency with less survival (35-80%), poor tillering and with visible symptoms of Zn deficiency. However, some of the seedlings of these genotypes were free from visible symptoms of Zn deficiency. Four genotypes (Group C) were sensitive to sodicity (survival 3-13%) and exhibited symptoms of Na toxicity within a week of transplanting. Analysis of the top three leaves and their leaf sheaths at 33 days after transplanting showed that Zn deficiency affected plants of group B genotypes (CSR10, CSR23, CSR-89IR1, 89H1-931098, and IR47538-3B-9-3B-1) invariably had higher Zn concentration compared to those plants of the same genotype which were free from these symptoms and also of group A (CSR-88IR15, CSR-89IR14, IR4630-22-2-5-1-2 and Trichi). There was no effect of Zn deficiency on phosphor-us (P) concentration, and P/Zn ratio was rather lower in affected plants. The concentrations of iron (Fe) and manganese (Mn) in the leaves and leaf sheaths were influenced differently. Manganese was more than Fe in Zn deficiency tolerant genotypes/plants of group A and B respectively, but it was the reverse in those susceptible to it. Whether alteration in Fe and Mn concentrations is the cause or effect of Zn deficiency requires more study. This showed that Fe and Mn concentrations should also be considered while studying Zn deficiency problems in rice. The survival of rice seedlings was best, but negatively correlated (r) with the Fe/Mn ratio of any of the top three leaves and leaf sheaths. This was equally good with Mn/Zn and Fe + Mn/Zn ratios. Appearance of Zn deficiency symptoms in plants of group B genotypes in spite of having higher Zn concentration than group A is most likely due to differences in physiological availability of Zn at the cellular level. This needs to be confirmed with further study.