GERMINATION AND ELONGATION OF THE HYPOCOTYL AND RADICLE OF KENAF (HIBISCUS-CANNABINUS) IN RESPONSE TO TEMPERATURE

Temperature controls pre-emergent development of seedlings, and is generally the prime determinant of the duration from sowing to seedling emergence. This study was conducted to examine seed germination and the elongation of the hypocotyl and radicle of kenaf (Hibiscus cannabinus L.) under constant temperature conditions, and to provide the temperature parameters necessary to model the processes of seedling emergence. Seed of kenaf, cv. Guatemala-4, were germinated in incubators at 8 constant temperatures (15, 20, 25, 30, 35, 40, 43 and 45°C). The numbers of germinated seeds and lengths of the hypocotyl and radicle were measured over time in each temperature regime. Final germination percentage was unaffected by temperatures in the range 15-35°C, but declined sharply at the higher temperatures. The optimum (To) and maximum temperatures (Tm) for rate of seed germination, which was defined as the inverse of the half-time of germination, were 35°C and 45°C, respectively. Values of To and Tm for the lag and linear phases of hypocotyl elongation were about 31°C and 43°C, respectively. Similar values for To and Tm were calculated for elongation of the radicle. Above a base temperature (Tb) of 10°C, the thermal time requirement for 50% seed germination was 8.2°C d, the lag phase of hypocotyl elongation required 17.0°C d, and linear hypocotyl elongation required 0.45°C d mm-1. It is proposed that the emergence of kenaf seedlings can be modelled from a specified sowing depth by the summation of the thermal time required for germination and the lag and linear phases of hypocotyl elongation. © 1990.