Stress-adaptive changes in tropical maize (Zea mays L.) under excessive soil moisture stress

Abstract

Response of maize plants to excessive soil moisture (EM) has been studied extensively. However, sys- tematic information on the stress-adaptive changes and cascade of events conferring the EM-tolerance is yet to be established. We attempted to assess the stress-adaptive physiological changes associated with EM-induced anoxia stress, and to establish mechanism of EM-tolerance in trop- ical maize. Tropical/sub-tropical elite maize inbred lines with known reaction to EM-stress were used in this study. Germplasm were exposed to EM-stress at knee-high stage (V 7-8 growth stage) by flooding the plots continuously for seven days. EM-induced changes in root geotropism (sur- face rooting) and increased brace roots development were identified as stress-responsive traits; however, the later one was found to be a stress-adaptive trait resulting in im- proved stress tolerance. Anatomical studies showed drastic changes in cortical region of root tissues in tolerant geno- types in terms of development of large aerenchymatous spaces. In terms of stress-induced metabolic adjustments, increased NAD+-alcohol dehydrogenase (ADH) activity was prevalent in all the genotypes under EM-conditions. Though, the enzyme activity was slightly higher in tolerant entries but not high enough to justify the significant geno- typic variability. However, the product of ADH-activity (ethanol) was relatively much higher in root and leaf tis- sues of susceptible genotypes. Analysis of ethanol concen- tration in shoot, root and inundated water showed that the level of ethanol was relatively much higher in the water present in rhizosphere of relatively tolerant genotypes. The finding suggested that EM-tolerant maize genotypes were able to extrude out the toxic level of ethanol from root tis- sues to rhizosphere. Our results suggest that mechanism of EM-tolerance in maize germplasm involves morphological and anatomical adaptation through development of brace roots and aerenchyma formation, and metabolic adjust- ment through regulatory induction of alcohol dehydroge- nase (ADH) and extrusion of ethanol out of root tissues.