Studies on matter production of rape plants. VIII. Effects of shading, leaf cutting and pod thinning treatments on yield and its components.

Abstract

Using rape plants (Brassica napus L., cv. Norin No.16), the following two experiments were conducted. In Exp. I, leaf cutting (cutting of all leaves) and leaf shading (80% of natural condition) were carried out at various stages during the period from the beginning of flowering to near maturity. In Exp. II, just after the end of the flowering period, pod thinning (25 and 50% of non-treated plant), leaf cutting (cutting of all leaves) and shading (whole plant, 50% of natural condition) were carried out. From the data obtained, times and factors for determination of yield components were examined mainly from the view point of assimilate supply. The results are summarized as follows : 1. Number of pods per plant kept increasing upto the end of the flowering period and then decreased rapidly due to abortion of young pods located at upper positions of inflorescences. The abortion ceased a week after the end of the flowering period and pod number kept a constant level thereafter (Fig. 1). 2. Number of pods decreased significantly by shading and cutting of leaves conducted at and before the middle of the flowering period. However, the treatments carried out later than that stage did not show such an effect. From this fact, it was suggested that the development of individual pods had been potentially decided at the end of the flowering period (Taable 1). 3. Number of seeds per pod was significantly affected by shading and cutting of leaves carried out at and earlier than the end of the flowering period and was not affected so by later ones (Table 1). 4. Dry weight per seed was affected significantly by the treatments carried out at and earlier than a week after the end of the flowering period (Table 1). 5. From the above-mentioned results, it was concluded that final magnitude of yield components was fixed in the order of number of pods per plant, number of seeds per pod and dry weight per seed . 6. Number of pods per plant decreased by shading and cutting of leaves at early stages in the flowering period. The extent of decrease in pod number that resulted from a treatment, corresponded to the decrease in dry matter production of plant due to the treatment. Since there wils no substantial difference in the relationships between dry matter production of plant and pod number in the both treatments, pod number was considered to be determined by assimilate supply (Table 1, Fig. 2). 7. The effects of whole plant shading and leaf cutting on number of seeds per pod were quite different. The shading affected little or only slightly seed number though the treatment resulted in considerable reduction in dry matter production of plant per pod. Pod thinning (without leaf cutting) resulted in a large increase in dry matter production of plant per pod, nevertheless it did not bring about an increase in seed number. Contrastingly, leaf cutting caused a large reduction in the seed number even when dry matter production of plant per pod was similar to that for control. From these facts, it was considered that the process of seed set was regulated rather by some unknown substances from leaves than assimilate (Table 2, Fig. 3). 8. Dry weight per seed was changed by pod thinning, leaf cutting and whole plant shading closely correlating with dry matter production of plant per seed. From this, it was shown that dry weight per seed was primarily determined by assimilate supply (Table 2, Fig.4). 9. Percentage of oil in seed was changed by treatments and increased with increasse in dry matter production of plant per seed. The contents of protein and other constituents without oil and protein (most of these are considered to be polysaccharides) decreased with increase in dry matter production of plant per seed with a much larger rate in the latter. Since energy cost for biosynthesis is oil > protein > polysaccharides, it was suggested that the higher the energy cost for biosynthesis of a substance, the synthesis is acceralated at the higher rate as assim