Leaf emergence rates of wheat in a mediterranean environment

Rates of leaf appearance on the main stem were measured for various wheat varieties for five to ten sowing dates in three field experiments in Western Australia. Rate of leaf appearance was constant in relation to thermal time for any given variety and sowing date, and ranged from 0.0064 to 0.0132 leaves ("C day)-'. Most of this variation could be accounted for as a response to sowing date or rate of change of daylength, although the response was complicated by interactions with variety and year. Because successive measurements were made on the same plants, it was possible to estimate directly the effects of temperature on the rate of leaf emergence. In the three years, mean rates of leaf emergence were 0.008, 0.008 and 0.011 leaves day-' OC-' with base temperatures (temperatures at zero rate) of 0.08, - 1.2 and 0.4"C respectively. Contrary to expectation, rate of leaf emergence decreased as temperatures increased in late sowings due probably to depression of leaf emergence as daytime temperatures exceeded 25°C. For Gamenya, the only variety common to the three years, the rate of leaf emergence (RLE) on the main stem was related to the rate of change of daylength (ADL, min day-' negative when daylength shortening) by the equation RLE = 0.00949 + 0.000988 (ADL). For crops emerging in late June (ADL approximately zero) in southern Australia, this implies a constant thermal time for leaf appearance of 105°C day leaf-'.