Studies on Matter Production of Rape Plant (Brassica napus L.) : I. Changes with growth in rates of photosynthesis and respiration of rape plant population

Capacities of photosynthesis and respiration of rape plant population were investigated at various stages of development. The rape plants, var. Norin No. 16, were grown under ordinary cultivating condition. CO2 exchange of the population was observed under field conditions at two weeks intervals during the period from early stage of growth to full maturation. After the beginning of flowering, CO2 exchange of the defoliated plants was also observed in order to estimate the extent of contribution of pods and stems to total CO2 exchange in this period. Dry weight of each part of plant and surface area of leaves and pods were also measured at each occasion of measurment of CO2 exchange. The results obtained were as follows: 1. Dry weight of whole plant increased very slowly till the beginning of bolting and rapidly thereafter. The rapid increase of dry weight of whole plant was continued also in ripening period in spite of marked defoliation due to senescence. 2. Leaf area (per unit area of land) was small and almost constant during the winter season because the withering due to severe cold offset the leaf formation. Leaf area started to increase gradually after the end of February which is just before the beginning of bolting and then rapidly at the beginning of flowering. Leaf area attained to its maximum value (LAI 3.31) 30 days after the beginning of flowering and decreased thereafer because of withering and defoliation. Surface area of pods increased markedly after the middle period of flowering. 3. During the winter season apparent photosytnhetic rate of the plant population increased gradually whereas leaf area was unchanged. The photosynthetic rate increased rapidly after the end of February and attained to its maximum value (5.26 g CO2/m2/h) 44 days after the beginning of flowering, and then it decreased. 4. The respiration rate of plant population increased with growth and attained to its maximum value (0.67 g CO2/m2/h) 44 days after the beginning of flowering, and then it decreased slightly. Depression of respiration in later period of ripening was less than of photosynthesis, consequently respiration/photosynthesis ratio increased as the maturation Proceeded. 5. The proportion of gross Photosynthesis performed by pods+stems to total gross photosynthesis (gross photosynthesis performed by undefoliated plant population) began to increase at the middle period of flowering and reached a value of 76% at the middle period of ripening. Thereafter, the proportion continued to increase further till the end of ripening. In ripening period, the pods possess relatively high photosynthetic activity and extensive surface area. Moreover, the pods were located at upper level in space of the population receiving plentiful sunlight. Therefore, most of the photosynthesis of pods+stems might be constituted by that of pods. From the above-mentioned reasons, it was concluded that pods contributed considerably to matter production in the ripening period.