Hot Temperature Conditions Research Articles

Low Temperature Eliminates Heterosis for Growth and Gibberellin Content in Maize1

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.

Perceptual responses to exercise

The general purpose of the investigation was to identify some of those physiological parameters, taken individunlly and collectively, which account for the greatest variability in perceived exertion (RPE) responses during thirty minutes of bicycle ergometer exercise. The data from eight independent variables recorded at 5, 15 and 30 minutes during two conditions were submitted to forward selection multiple regression analyses. Six male students between the ages of 18 and 22 years rode during five trials. Three trials were performed under a neutral temperature condition (24°C) at 48, 60 and 68% of maxV̇O2. Two trials were performed under hot temperature conditions (44° and 54°C) at 48% of maxV̇O2. Ventilation (V̇E) accounted for the greatest variance in RPE at 5 and 15 minutes and respiratory rate (RR) made the greatest contribution at 30 minutes in both conditions. The independent variables accounted for more total variance in the heat at each time point (84, 84 and 77%) than in the neutral (66, 80 and 53%), Rectal and skin temperature (Tr and Ts) appear sooner and account for more variance in RPE in the heat than in the neutral environment. It was hypothesized that man does not directly attend to physiological processes, per se, as a basis for perceived exertion ratings but does attend to the externalization of these processes, i.e., increases in metabolic rate result in increases in V̇E, RR and Ts which can be directly perceived.