Abstract
The seasonal changes of leaf growth from the onset of leaf expansion in early spring to heading and its adaptive significance were studied in natural populations of timothy (Phleum pratense L.). Seeds were collected from 12 locations in northern area (northern group) and 16 locations in southern area (southern group) of Hokkaido (Fig.1) and sown in field at Sapporo. In the second year after seeding, the most vigorous tiller on each plant was selected on May 2, and the number, length and width of successively expanded leaves on the tiller were measured on May 16 (early growing stage), June 5 (middle growing stage) and after heading (late growing stage), respectively. Based on these measurements leaf area expansion per tiller was determined as the sum of sizes (length×width) of fully expanded leaves in each growing stage. The southern group had larger leaf area expansion than the northern group in the early growing stage (Table 1, Fig.2). It was true for leaf length in early and middle growing stages and leaf width in the early growing stage. In the late growing stage the northern group had larger leaf number than the southern group. However, there was no difference in leaf area expansion summed over three growing stages. The variance ratio σ_B^2/(σ_B^2+σ_W^2) for leaf area expansion was higher in the early growing stage than in the middle or late one (Fig.4), where σ_B^2 and σ_W^2 are the variances due to between and within groups, respectively. Heritability of leaf area expansion in the early growing stage was also high (Fig.3). It was suggested by this result that these two groups differed from each other in the leaf growth type well adapted under low temperature conditions in early spring. Leaf area expansion in the early growing stage was positively correlated with monthly mean temperature in winter at the collection site (Fig.5). It was pointed out from these results that northern populations inhibited leaf growth under relatively low temperature conditions and increased cold tolerance, and thereby adapted to the unpredictable climatic fluctuations in early spring peculiar to the northern area of Hokkaido.
