Climatic Variation in Forage Grasses. I. Leaf Development in Climatic Races of Lolium and Dactylis

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Abstract
1. A survey has been made of the pattern of leaf expansion in ten local populations of Lolium perenne and seven of Dactylis glomerata covering the climatic range from the Mediterranean region along the maritime Atlantic coast to northern and central Europe. 2. The populations were sown in an unheated glasshouse at intervals throughout the year and at a range of controlled temperatures in growth rooms. For each treatment the rate of expansion of leaf surface per tiller was recorded, and analysed into leaf length, leaf width and rate of leaf appearance. 3. The seasonal sowings revealed a regular cline in relative rate of leaf expansion in the winter from the Mediterranean region, along the Atlantic coast to north Europe and Scandinavia, associated with the winter temperatures of the places of origin. 4. In the experiments in growth rooms the leaf surface expanded most rapidly in all populations at 25⚬/12⚬ C, but at 10⚬ and even more at 5⚬ C leaf expansion was most rapid in the Mediterranean populations, least in the northern, and intermediate in the maritime material. There is a strong negative correlation between the rate of leaf expansion at low temperature and cold hardiness. The relative performance of populations at 5⚬ C in the growth rooms was highly correlated with their performance in the winter in the glasshouse, indicating that the differences between varieties in rate of leaf expansion are determined mainly by temperature, and that populations can be effectively screened for winter growth in controlled environments. 5. The developmental bases of these differences are discussed in terms of rates of cell division and extension, and it is suggested that the basic difference between these climatic races lies not in any difference in photosynthetic activity, but in differences in the use of assimilates in the expansion of new leaf surface. 6. These results emphasize the value of Mediterranean populations as a source of winter production in maritime western Europe, but the effective use of this material in a breeding programme is limited by the inverse association between winter growth and winter hardiness. Possible ways of overcoming this undesirable correlation are suggested.

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