Physiological mechanisms underlying heterosis for stress tolerance in maize
Yield improvement in maize ('Zea mays' L.) has been associated with heterosis and increased stress tolerance. It is generally believed that heterosis confers stress tolerance, however, the physiological mechanisms underlying heterosis for stress tolerance have remained obscure. The objective of this research was to quantify the response of a maize hybrid (CG60*MBS1236) and that of its parental inbred lines to a series of environmental stresses. The abiotic stresses used in the studies included shading, drought, low N, low temperature, and high plant density. The stresses were implemented during different phases of plant development. The results showed that the hybrid was more tolerant than its parental inbred lines to abiotic stresses in terms of grain yield and harvest index, but not necessarily in terms of dry matter accumulation. Heterosis for stress tolerance in grain yield was apparent when plants were subjected to stress before and during the critical silking period, but was absent when stresses occurred during the grain-filling period (2 to 5 wk after silking). Results of this study indicated that grain yield is particularly sensitive to stress during the silking period regardless of whether the stress is induced by shading, water deficit, or low N supply. The main causes underlying improved stress tolerance in the hybrid are the higher stability in dry matter partitioning to the grain and increased 'stay-green' compared to its parental inbred lines. For the genotypes used in this study, one of the two parental inbred lines performed similarly to the hybrid under cold stress as determined by leaf photosynthesis and chlorophyll fluorescence. The relative response of leaf photosynthesis to clod stress in the hybrid and its parental inbred lines did not vary between the seedling phase and the grain-filling period.