Heterosis for water status in maize seedlings

Heterosis is one of the greatest practical achievements of plant breeding and has been extensively used in crop improvement in maize. However, the physiological basis of heterosis still remains poorly understood despite its manifestation at early stages of the plant life cycle. Therefore, a better understanding of the physiological mechanisms associated with heterosis may enable further exploitation of this phenomenon. Five maize hybrids and their parental lines were grown in a greenhouse under two water regimes (well watered and water stressed). Plant growth, different root traits, plant water use and water use efficiency and gas-exchange and chlorophyll fluorescence, nitrogen content and stable oxygen, carbon and nitrogen composition and stomatal density of leaves were measured. Plant height, shoot biomass and leaf area were higher in hybrids than in their respective parents in both control and stress conditions. Moreover, hybrids showed a better water use and water use efficiency for biomass than inbred lines. Significant heterosis was also found for photosynthetic rate, stomatal conductance, transpiration rate and the Ci/Ca ratio under water stress conditions, whereas for control conditions the differences were not significant. Likewise, root weight density and root length density were higher in hybrids than parents, especially underwater stress. No heterosis occurred for stable isotope composition under either water regime. The results do not support a constitutively better water status as a cause of heterosis under well watered conditions.