Physiological and morphological responses of several genotypes of tropical and sub-tropical maize (Zea mays L.) to water and nitrogen deficit during the early vegetative phase

Intermittent drought combined with nitrogen stress during early growth of maize crops in the tropics limits maize production in developing countries. Plant morphological and physiological responses of different maize genotypes to two aspects of stress, water deficit and nitrogen stress, were examined during the growth of several maize genotypes. Eight maize genotypes including three inbred lines (Gn 160, Gn 106, Gn 175), three single crosses (Gn 160 x Gn 175, Gn 106 x Gn 175, Gn 106 x Gn 160) and two open pollinated cultivars (Tuxpeno Sequla Cycle 0, drought sensitive; Cycle 8, drought resistant) were examined for drought tolerance during early growth. Variables used for assessing tolerance were dry weight of shoot and root, leaf area, relative water content, water potential and stomatal conductance which were measured in both well-watered and the respective droughted treatments for each genotype. Under growth room conditions, the level of severe drought imposed caused a substantial reduction in growth of eight maize genotypes, relative to the respective well-watered conditions. The single cross Gn106 x Gn160 revealed drought tolerance traits in shoot and total biomass, which were higher than the other single crosses. It is concluded that this single cross is more drought-tolerant than the other single crosses during early growth stages. Tuxpeno Sequla Cycle 0 and Cycle 8 had similar shoot and total biomass under both water regimes, and did not show differences in drought tolerance during early growth. There was no clear indication of transmissability of drought tolerance from parent lines to their respective single crosses. The combined effects of drought and timing of N stress during early growth on morphological and physiological traits, which were altered during later growth and yield components, were investigated in two related tropical maize cultivars, Tuxpeno Sequla Cycle 0 (Tux. CO) and Cycle 8 (Tux. C8), which were selected for drought sensitivity (Tux. CO) and tolerance (Tux. C8), during flowering under glasshouse conditions. Total biomass at maturity and green leaf number below the ear at grain filling were similar between Tux. CO and C. 8 under different water regimes. Increased grain yield and harvest index in Tux. C8, compared with Tux. CO, resulted mainly from increased kernel number per row and rows per ear. Tux. C8 was not only higher in yield but also higher in N use efficiency than Tux. CO when experiencing moisture and N stress during early growth. Shoot biomass under moderate drought versus well-watered conditions was found to be more sensitive than root biomass in both Tux. CO and C8. Under drought compared with well-watered conditions, stem biomass was more sensitive than leaf biomass. Leaf area in the high-N treatments was more sensitive to water deficit than in low N treatments, as indicated by the fact that droughted plants in the early N application (NE) and split (NS) treatment produced lower leaf areas than well-watered plants, while in the control (NO) and late N (NL) treatments there was no significant difference in leaf area between drought and well-watered conditions. Early N application resulted in a shorter time from sowing to anthesis than later N application. Clearly N stress is the main limiting factor for growth and yield of maize under moderate level of water deficit. This study revealed that maize plants require N for growth and dry matter accumulation as early as immediately after seedling emergence even under low N input conditions (75 kg N per hectare). Increased grain N content and nitrogen harvest index for a split N application resulted from increased N content in the stem and leaf Even under low N input conditions i.e. the application equivalent to 75 kg N/ha in this experiment, N still needed to be supplied as a split application, in order to increase N use efficiency. Nitrogen harvest index under well-watered conditions was higher than that under drought, indicating that water deficit also had an effect on nitrogen harvest index of both cultivars (CO and C8) under different level of N treatments