HARDENING AND DEHARDENING OF LOLIUM-PERENNE IN RESPONSE TO FLUCTUATING TEMPERATURES

Hardening and dehardening responses of two contrasting varieties of Lolium perenne, measured as LT50 estimates, were followed in fluctuating temperature environments. Unhardened seedlings were exposed to hardening environments for 7, 14, and 21 d in all combinations of 2, 4, 6, 8 and 10 °C with either high day and low night temperatures or low day and high night temperatures. Seedlings hardened for 28 d at 2 °C were exposed to dehardening environments in all combinations of 4, 6, 8, 10, and 12 °C with high day and low night temperatures. A low day, or night, temperature of 2 °C in combination with any other temperature increased hardening compared with the constant higher temperature. For Premo, a hardy variety, this increase was 3 °C when night temperature was reduced from 10 to 2 °C in combination with a day temperature of 10 °C. Similarly, a low night temperature reduced the dehardening response of Premo to higher day temperatures. At 12 °C this effect on LT50was greater than 2 °C. Much smaller responses to daily periods of low temperature were found for the less hardy variety, Grasslands Ruanui. During each 24-h period, exposures to 2 °C of longer than 4 h were required to achieve greater hardening than that achieved in continuous 10 °C treatments. Hardiness was not improved further by exposures longer than 8 h. Responses to diurnal temperature fluctuations were discussed in relation to possible mechanisms and to changes in hardiness during the winter under different weather systems. © 1992 Annals of Botany Company.