Kinetics of Leaf Growth in Lolium temulentum at Optimal and Chilling Temperatures

Lolium temulentum plants were grown at 20 °C, under an 8-h daylength, in a controlled-environment chamber, and the kinetics of leaf expansion were observed by measuring the movement of an optical grid attached to the fourth leaf. The leaf emerged 23–24 d after sowing and was fully expanded 9–10 d later. Extension rate was maximal between the second and fifth days after emergence and declined markedly thereafter. During the rapid growth phase the rate of elongation exhibited a distinct diurnal rhythm, fluctuating between 1.9 to 2.3 mm h−1 in the light period, and 1.3 to 1.7 mm h−1 in the dark. A circadian oscillation with a period of about 27 h was observed in leaves elongating in continuous darkness. When plants were transferred to 5 °C soon after emergence of the fourth leaf there was an immediate reduction in rate of growth to about 22 per cent of the rate at 20 °C: the Q10 for the mean elongation rate in the range 20–5 °C was 3.7. When plants were transferred from 20 to 2 °C at fourth leaf emergence, mean extension rate declined to less than 5 per cent, corresponding to a Q10 in the range 5–2 °C of more than 300. Furthermore, growth at 2 °C was confined almost entirely to the dark phase of the photoperiod cycle. The responsive tissue was shown to be a small area of expanding leafless than 1.5 cm above the shoot apex and the possible mechanisms underlying low temperature effects in this region are discussed.