Photosynthetic Activity of the Pods of Rape Plants (Brassica napus L.) and the Contribution of the Pods to the Ripening of Rape-seeds

Abstract

The present study was conducted to clarify the photosynthetic activity of the pods of rape plants and the effect of the pods on the ripening of rape-seeds. In making the present study, the productive characters of the plants and the net photosynthetic rates of the pods were investigated. Furthermore, the processes of accumulation of ¹⁴C-photosynthates were also traced by means of a ¹⁴C-tracer method. The study results are summarized as follows: (1) At the early ripening stage, the surface areas of pods showed on about eight-fold areas, as compared with those of the leaf blades. Marked increases in the pod surface areas were observed during the period ranging from the early ripening stage to the late ripening stage, while the leaf blade areas showed decreases during the same period. (2) The dry weights of the pods or seeds were becoming heavier according as they have grown and the higher distribution ratios of dry matters were observed in the pods, as compared with those in other organs. The pods and leaf blades showed almost the same net photosynthetic activity at the early ripening stage, but at the late ripening stage, the pods came to show higher rate than that of leaf blades. (3) Chlorophyll-a and -b contents in the pods were low, as compared with those in leaf blades. Chlorophyll contents in the pods were equivalent to about one-fifth of those in the leaf blades. (4) From the results obtained through the ¹⁴C-tracer experiments in photosynthates, higher specific radioactivities were observed in the tissues of pods and peduncles, while lower ones were observed in the stems and roots. ¹⁴C-photosynthates in the seeds observed one hour after the ¹⁴CO₂ gas exposure accounted for about sixty percent of those of the seeds observed seven days after the ¹⁴CO₂ exposure. On the seventh day after ¹⁴CO₂ feeding, a markedly rich accumulation of ¹⁴C-photosynthates was found only in the case of seeds, and the decreases in ¹⁴C-photosynthates were observed in the cases of other tissues. Therefore, it may safely be said that ¹⁴C-photosynthates in the pods may translocate mainly to the seeds. (5) Viewed from the histochemical point, the assimilating parenchyma and vascular bundles were developed in pods as well as the leaf blades. The starch granules were found far more abundantly in the palisade parenchyma of leaf blades than in the assimilating parenchyma of the pods. (6) From the above mentioned results, it might be concluded that the pods of rape plants have the photosynthetic function and contribute very actively to the accumulation of photosynthates in the rape-seeds.