Abstract
Maximum crop growth rate was compared among the four clones of Italian ryegrass of which leaves are different internally in the level of chlorophyll content based on leaf area. The four clones were selected from the variety 'Nakei No.4' and named as the clone Green 2 (G2), Green 1 (G1), Yellow 2 (Y2) and Yellow 1 (Y1) according to their chlorophyll content. The contents were 6.6, 5.8, 3.8 and 3.0mg (a+b)/dm^2 on leaf area basis in the plants grown late in winter before the beginning of the experiment. The tillers of each clone were increased enough to cover the experimental field of 100m^2 per clone and were transplanted at three densities (Low:50×50, Mid.:25×25 and High:12.5×25cm^2 for space per unit stubble, respectively) in middle September on fertile soil with high level of fertilizer application. Plots were arranged by split-plot design with two replicates. Dry matter increase in both the above- and underground organs of the pure clonal sward was measured with six replicates at the intervals of 7-10 days in three times of samplings (sampling I, II and III) during the most prosperous period from middle of April to middle of May, since the period might cover satisfactorily the 'optimal' LAI stage of every sward. The maximum crop growth rate, the optimal LAI and Chlorophyll Index for each of the clonal canopies in the three density plots were determined. The net assimilation rate at the optimal LAI was compared by analizing into the following elements shown as the next equation ; NAR=(a-r)-r_n・SLW-R・(C/F)・SLW…(Equation (3) in OKUBO et al., 1975a) 1. Optimal LAI ranged widely from 6 to 11 and optimal Chlorophyll Index (CI) varied more from 2.4g/m^2 to 8.4g/m^2 (Table 2, Table 3). Much variation in chlorophyll content on leaf area basis (Ch_A) was observed-the maximum 6.6-6.9mg(a+b)/dm^2 in the clone G2 and the minimum 2.5-2.8mg(a+b)/dm^2 in Y1. The range of the optimal LAI showed a wider tendency in the sward of the clone having higher level of Ch_A than in the clone of lower Ch_A (Table 3). 2. The values of the maximum crop growth rate ranged widely among the clones from 27-35g/m^2/day in G2 to 9-18g/m^2/day in Y1 on average for three weeks. Those variations depended fairly on optimal CI but little on optimal LAI (Fig.1). This fact was due to the dependence of the net assimilation rate at the period of the optimal LAI upon the levels of Ch_A of the clones (Fig.2 and Table 4). 3. Net assimilation rate on chlorophyll weight basis (E_c) in sward condition varied with density to some extent. But the E_c showed approximately the same value among the clones i.e. 6-7g/g/day at the lowest level of density and 3-5g/g/day at the highest and middle density. 4. The sward of the clone having a higher Ch_A in the surface leaves on a canopy showed a higher Ch_A also in the lower leaf layer, especially as in G2, compared to the clone with lower level of Ch_A as in Y1, at the stage around optimal LAI (Table 5). 5. It can be safely inferred that the dependence of variation in maximum crop growth rate upon Ch_A would be mainly due to the difference in photosynthetic activities of sward leaves as far as in these swards of the four clones concerned above. The photosynthetic activity would be affected first by chlorophyll content per unit of leaf area on leaf weight relating to 'weak light factor', second by specific leaf weight, cell size or some other factors, and third by some ability to maintain the Ch_A as high as possible even in the shaded leaves situated in the lower layer of a canopy on high level of LAI.
