Hardening and Dehardening of Lolium perenne in Response to Fluctuating Temperatures

Hardening and dehardening responses of two contrasting varietiesof Lolium perenne, measured as LT50 estimates, were followedin fluctuating temperature environments. Unhardened seedlingswere exposed to hardening environments for 7, 14, and 21 d inall combinations of 2, 4, 6, 8 and 10 C with either high dayand low night temperatures or low day and high night temperatures.Seedlings hardened for 28 d at 2 C were exposed to dehardeningenvironments in all combinations of 4, 6, 8, 10, and 12 C withhigh day and low night temperatures. A low day, or night, temperature of 2 C in combination withany other temperature increased hardening compared with theconstant higher temperature. For Premo, a hardy variety, thisincrease was 3 C when night temperature was reduced from 10to 2 C in combination with a day temperature of 10 C. Similarly,a low night temperature reduced the dehardening response ofPremo to higher day temperatures. At 12 C this effect on LT60was greater than 2 C. Much smaller responses to daily periodsof low temperature were found for the less hardy variety, GrasslandsRuanui. During each 24-h period, exposures to 2 C of longer than 4h were required to achieve greater hardening than that achievedin continuous 10 C treatments. Hardiness was not improved furtherby exposures longer than 8 h. Responses to diurnal temperature fluctuations were discussedin relation to possible mechanisms and to changes in hardinessduring the winter under different weather systems. Lolium perenne, cold hardening, cold dehardening, diurnal temperature fluctuations, varieties