Growth Studies of the Sugarcane Plant. II. Some Effects of Root Temperature and Gibberellic Acid and Their Interactions on Growth

Three-month-old sugarcane (Saccharum officinarum) plants were raised from single-eye cuttings and grown in aerated nutrient solution for an additional 28 days at an air temperature fluctuating between 21.5 C (night) and 30.5 C (day). The nutrient solutions in which the roots were immersed were kept at eight different temperatures ranging from 12.5 C to 33.5 C. Half the number of plants in each temperature treatment were treated weekly with gibberellic acid (GA). The GA (100 µg) was applied in 0.1 ml of water per plant in the folded leaves above the top visible dewlap. A loss in mean dry weight of the plants at a root temperature of 12-5 C indicated a higher respiratory than photosynthetic activity. Above 15.5 C an increase in root temperature resulted in an almost linear increase in dry matter production up to 30.5 C. The effect of GA on dry matter production was more pronounced at 18.5 C than at 27.5 C to 30.5 C. There was very little response to GA application at intermediate temperatures. Tiller production was reduced by root temperature below 24.5 C, above which there was an increase in the mean number and mean dry weight of tillers with increasing temperature. GA caused a reduction in tiller dry weight at 27.5 C; however, the effect decreased above that temperature. It is suggested that GA production in the roots, or translocation from the roots, or both, are inhibited by root temperatures below 18.5 C, which would explain the high response to applied GA at 18.5 C. At temperatures of 27.5 C and 30.5 C a positive response to exogenous GA might be the result of depletion, through consumption of endogenous GA due to the rapid growth rate of the plants at these temperatures. The growth of the GA-treated plants was maximum at a root temperature of 30.5 C under the conditions of the experiment. for the control plants maximum growth was attained at the root temperature of 33.5 C. However, the maximum response to GA, compared to the controls at the same temperature, occured at 18.5 C.