1975 Volume 21 Issue 2 Pages 136-145
To ascertain the dependence of maximum crop growth rate (_<max>CGR) and net assimilation rate (NAR) upon chlorophyll content of leaf surface in the canopy having a nealy optimal LAI, as seen in the previous investigation, crop growth rate was measured in the alfalfa swards (Medicago sativa L., Du Puits) with four plant densities. The plots with four different level of density were coded as D1>D2>D3>D4 (see the explanation of fig.1). Several times of the measurements of dry matter growth both for the top and underground organ were set out when the canopy attained to 80% light interception at the ground level and continued at one-week intervals for each of the densities. Around the optimal LAI, two times of the measurements with stratified clip method were also carried at the intervals of a week at each density plot in order to determine the light extinction coefficient (K). The vertical changes in chlorophyll content and specific leaf area within a canopy, which was divided into 4〜5 leaf layers, were also measured. Other details in the method for CGR and chlorophyll measurements were the same as the previous papers (OKUBO et al, 1969, 1975a). Changes in photosynthetic rate under dim light (55, 100, and 150cal/dm^2/hr, 400〜700mμ) with leaf age were also determined for alfalfa leaves (Du Puits). The leaf materials were taken from various positions of the plants grown in artificial community, which were planted individually on a peat soil pot and arranged with space of 25×25cm in a glasshouse, and they had been marked on the day of leaf unfolding as leaf age zero. Initial slope of light-photosynthesis curve was calculated as the ratio of gross photosynthesis to incident visible light on energy basis ; the maximum energy efficiency (φ0). Apparatus used was the same as those in the work by GABRIELSEN et al (1959). Chlorophyll determination method was seen in the previous papers (OKUBO et al, 1964, 1975a) Symbols used were given in the explanation of fig.1. 1. Variation in _<max>CGR was observed among the different levels of plant density of alfalfa sward, and it obviously depended more on CI than on _<opt>L (fig.2). Since the CI at this stage can be shown as Cl=_<opt>L×ChA+stalk's chlorophyll and there was not so much difference among the values of _<opt>L, this dependence on CI means that the _<max>CGR and the NAR had close relations with chlorophyll content on leaf area basis (ChA per LAI) at the stage of _<opt>L for each canopy at each density, although solar radiation and C/F ratio showed relations more or less to the _<max>CGR or the NAR (fig.3). 2. ChA of a canopy leaves increased with growth and attained once to the highest level at the growth stage of _<opt>L (at 90〜95% light interception, exactly) followed by a decrease again at excess LAI. ChA measured for each leaf layer with different depth in a canopy showed at most 4.0〜5.0mg/dm^2 at the period of _<opt>L and were mostly below 4.0mg/dm^2 in the canopies before the _<opt>L (fig.4). 3. Photosynthetic rate under dim light increased with leaf age to the highest on 15-20th day from unfolding and decreased. ChA of the leaf showed approximately the same time course as the photosynthesis. The initial slope of the light-photosynthesis curve on energy basis (φ0) showed a close correlation with ChA in the range of 1.0〜5.0mg/dm^2 (fig.5, fig.6). 4. The dependence of _<max>CGR and NAR on ChA shown in the previous paper (OKUBO et al, 1975a) would be caused by the facts that the variation of photosynthetic activity occurred with aging and mutual shading of canopy leaves followed by changes in microenvironmental condition, especially in light, and that the change of the activity was apparently observed in terms of ChA in the canopy at _<opt>L. But the ChA as a weak light factor also must have contribute partly to the close correlation among _
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