Effect of temperature on time to panicle initiation and leaf appearance in sorghum

Leaf-tip appearance and duration from planting to panicle initiation (PI) are important developmental processes in cereals. This study was conducted to determine the effect of air temperature on rates of leaf-tip appearance and 1/PI for sorghum [Sorghum bicolor (L.) Moench]. Nine genotypes were grown in pots at mean temperatures of 17 to 33 degrees C and photoperiods of 11.0 to 12.3 h d(-1) in glasshouses. Leaf-tip appearance was recorded every 3 to 5 d and PI determined by apical dissection. The optimum temperature (T-o) for 1/PI was between 26 and 27 degrees C and supra optimal temperatures delayed PI in all genotypes. The base temperature (T-b) ranged from 6 to 10 degrees C but only one genotype had a T-b significantly different from a common value of 8.5 degrees C. Growing degree days (GDD) for PI among genotypes at sub-optimal temperatures varied from 411 to 592 (P < 0.001) above a T-b of 8.5 degrees C. Warmer temperatures (>27 degrees C) reduced the rate of leaf-tip appearance in five genotypes; in the remaining genotypes the rate of leaf appearance was effectively constant at mean temperatures between 23 and 33 degrees C. There was no significant difference (P > 0.20) among genotypes in the phyllochron, which was 57 GDD above a T-b of 8.5 degrees C. The final number of leaf-tips was constant within genotypes at mean temperatures of 18 to 30 degrees C, but varied among genotypes from 11.8 to 17.3. In conclusion, the T-b and T-o for 1/PI were approximately 8.5 and 27 degrees C and there was variation in 1/PI but not in the rate of leaf-tip appearance among the genotypes studied.