Individual and Interactive Effects of Nitrogen and Phosphorus on Drought Stress Response and Recovery in Maize Seedlings

Plants have an inherent mechanism for perceiving drought stress and respond through a series of physiological, cellular and molecular changes for maintaining physiological water balance. It has been shown that nitrogen (N) and phosphate (P) can help to improve plant tolerance to water limitation by increasing the activities of the photosynthetic machinery and antioxidant enzymes. Maize is highly sensitive to drought stress, especially at the seedling stage. In this study, we used four maize genotypes (HKI-161, HKI-193-1, HQPM-1 and HQPM-7) and studied the effect of N and P application on response to drought stress and recovery at germination and seedling stage. We show that application of N and P had no effect on rate of germination but increased the seedling growth, chlorophyll content, malondialdehyde levels, proline, anthocyanin content, gas exchange parameters and antioxidant enzymes (APX, CAT and GR) during drought stress. The variation in the effect was visible across genotypes, but the observed changes indicate improved drought stress tolerance in the maize seedlings. During drought recovery, seedlings of HKI-161 and HKI-193-1 genotype that did not receive N and/or P treatment or that were pre-supplemented with only P showed rapid transition to flowering stages. Seedlings pretreated with N showed comparatively late transition to flowering. The HQPM-1 seedlings, which received N treatment moved to flowering stage while HQPM-7 seedlings showed only normal vegetative growth under all treatment conditions. Molecular analysis identified 2016 transcripts that are differentially expressed in the drought tolerant and susceptible genotypes. About 947 transcripts showed >3-fold change in expression and were expressed during stress tolerant genotype. Transcripts coding for proteins in P and N metabolism were identified within the drought regulated transcripts. The analysis showed that transcripts related to P metabolism were expressed during stress and recovery phases in the susceptible genotype while transcripts related to N metabolism were down regulated during drought stress and recovery stages in all the genotypes.