Analysis of the response of leaf extension to chilling temperatures in Lolium temulentum seedlings

Lolium temulentum L. plants were grown at 20°C and transferred to 2°C or 5°C at 21, 28 or 35 days after sowing, when leaves 3, 4 and 5, respectively, were at mid‐expansion and leaves 4, 5 and 6 were just emerging. Leaves of plants exposed to 2°C for 7 or 14 days before their date of emergence at 20°C failed to appear at all during the course of the experiment. Transfer to 2°C at emergence resulted in a delay of about 40 days before expansion was detected and subsequent growth was extremely slow. By contrast, although leaves of seedlings exposed to 5°C at or prior to emergence were significantly smaller and slower‐growing than the same leaves of plants maintained at 20°C, the difference in vegetative development and tillering between 2°C and 5°C was much less marked than between 5°C and 2°C, implying the existence of a rather sharp threshold for growth between the latter temperatures. Leaves transferred to 2°C at mid‐expansion attained a final size not very different from leaves exposed to 5°C at the same time, but expension rates were only 20–30% of those at 5°C, and the time taken to achieve full expansion a corresponding 3 to 5 times longer. These responses were quantified by fitting Richards functions to measurements of leaf extension and determining, from the parameters of the curves, asymptotic maximal lengths, mean relative and absolute extension rates, inflexion points and durations of growth. The potential usefulness of Lolium temulentum as a model species for studying the relationship between temperature and growth in the Granmineae is discussed.