Chlorophyll Amount for Analysis of Matter Production in Forage Crops : I. Changes in leaf area index and chlorophyll amount with the regrowth of Ladino clover sward.

Abstract

Dry matter production was analysed in terms of the leaf area index (LAI) and the total chlorophyll amount of herbage per unit area of land, in the sward of Ladino clover (Trifolium repens L. race giganteum), and an attempt was made to replace the LAI by the total chlorophyll amount (chlorophyll index, g/m2 of land, abbreviated here to CI). It has been pointed out by some workers that the chlorophyll content of higher plant would not be usually limiting photosynthesis and the use of the total chlorophyll amount would not eliminate the complexities associated with leaf display. The CI, however, may account for the probable photosynthesis in petiole or leaf sheath and the differences in light transmissibility among leaves. Besides, it may also decrease errors and save manhours in separating the mixed pasture samples and measuring the leaf area of the herbage with small and slender leaves in field experiments. At several days' intervals over a period of seven weeks from defoliation, measurements were taken of the dry matter yield, the leaf area, the chlorophyll content, the CI (chlorophyll in petiole and stolon was also taken into account), and the relative light intensity penetrating to the ground level. The vertical distributions of leaf area, chlorophyll amount and relative light intensity in the sward were also measured at every 5 cm from the ground level by the stratifying clip method, on the 30th and 51st day of regrowth. The mean chlorophyll content per dry matter weight increased with the regrowth in leaflet, hardly changed in petiole, and decreased in stolon, over the period. The LAI reached to the maximal value of 4.7 on 26 days after cutting, and thereafter it showed an approximately constant value of 4.5, while the total dry matter yield increased throughout the period, and the CI similarly increased even after the LAI attained a plateau (Table 1). The CI correlated significantly to the LAI, the total dry matter yield (TDM, g/m2) and the logarithmic relative light intensity, over the whole period of regrowth. The correlationships, however, did not become so close among these measurements after the maximal LAI was once attained, except the case between CI-TDM (Table 2). The daily dry matter increase per unit value of CI (Ec, g/g/day) was calculated from the equation (2) in this text similarly to the calculation of the net assimilation rate from the equation (1), after Watson, and the Ec declined with the increase in CI (Fig. 3). The growth rate (g/m2/day) attained the maximum at the CI value between 1.2-2.3 g/m2 of land, and this value was regarded as "the optimal CI" of the sward, corresponding to the optimal LAI (Fig. 3). The vertical distribution of relative light intensity (logarithmic value, loge I/I0) showed highly significant correlations to the leaf area of herbage (F) above the light-measured level and to the chlorophyll amount (Ch1) of the herbage (Fig. 2). The relation of I/I0 to the Ch1 can be expressed as the equation (4) in the text, where a constant Kc corresponds to the coefficient of light extinction by a chlorophyll layer in the sward. The value of Kc was determined to be 1.21 per chlorophyll g/m2 of land on the 30th day of the regrowth.