Time Course Changes in Metabolite Accumulation and their Implications for Heat Stress Tolerance in Maize (Zea mays L.) Seedlings

Abstract

Maize shows reduced crop stand under relatively suboptimal conditions, the main reason for this is sensitivity of bud tissue to temperature fluctuations and salinity at sowing time. The aim of these studies was to explore the physiological and biochemical occurring at seedling stage in a time course manner. The determinations were made on growth, photosynthetic pigments, accumulation of osmolytes, oxidative damage and antioxidants, secondary metabolites and nutritional quality characteristics in shoot and root of two differentially heat stress tolerant varieties of maize. Results revealed that reduction in length, fresh and dry weight of shoot and root and loss of chlorophylls and carotenoids in the leaves were the explicit determinants of heat damage. The heat tolerance of maize was associated to greater accumulation of glycinebetaine followed by free proline. As evident from H 2 O 2 and MDA data, heat stress produced oxidative stress on both the varieties, although lowly in the tolerant one. Enhanced or steady state levels of niacin, ascorbic acid and riboflavin and enhanced ability of reducing powers led to the alleviation of oxidative damage on the maize plant. Among the secondary metabolites, enhanced synthesis of flavonoids and anthocyanins and saponin was closely related to the heat tolerance of maize. As for nutritional quality characteristics, heat stress reduced the crude fiber, starch, oil and proteins contents and led to an overall reduction in the accumulation of nitrate, phosphate and K, and led to a hampered nutritional value and metabolizable energy in both the varieties, but tolerant maize was superior for these attributes. In short, heat tolerance in maize was associated greater photosynthetic pigments levels and better metabolic adjustment, which resulted in better nutritional quality of the vegetative parts for use as fodder for the animals. All these changes were more precise in the shoot.