Low Temperature Eliminates Heterosis for Growth and Gibberellin Content in Maize1

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Abstract
Two Canadian maize (Zea mays L.) inbreds, CM7 and CM49, and their F1 single‐cross hybrid were grown in controlled‐environment conditions in which air and soil temperatures were independently regulated to simulate field conditions at planting during a warm (25/15°C (day/night) air; 17°C soil) or cool (20/10°C air; 12°C soil) spring. Heterosis (overdominance) for height, dry weight, and for concentration of gibberellin (GA)‐like substances occurs with this hybrid when grown under hot temperature conditions [25/20°C (day/night) — air and soil]. Under the decreasing temperature regimes of the present study, growth rate in all genotypes was decreased. While heterosis for increasing plant height was previously observed in the hot conditions, incomplete dominance for increasing plant height was observed in the warm or cool conditions. Potence ratios, indicating the overall degree of dominance, were decreased under the cool temperature regime for height, dry weight, and content of GA‐like substances in apical meristem shoot cylinders. Thus, lowering temperature progressively eliminated heterosis for both growth rate and level of GA‐like substances in the maize hybrid CM7 ✕ CM49. This observed elimination of heterosis at low temperature is inconsistent with one theory of the physiological basis for heterosis which indicates that the hybrid's biochemical polymorphism provides it with superior metabolic buffering of environmental stress. Thus, although the hybrid CM7 ✕ CM49 has a superior growth potential relative to its inbred parents, this potential for hybrid vigor can be suppressed under environmentally unfavorable conditions.