Studies on the Dry Matter Production of the Sweet Potato : VIII. The internal factors influence on photosynthetic activity of sweet potato leaf.

Abstract

In the previous paper, it was recognized that intimate correlations, positive or negative, were found among factors concerning photosynthetic activity, such as potassium per cent, nitrogen per cent., respiratory rate and carbohydrate content in the leaf, but it was not blear that which one of these factors showed a true correlation with photosynthetic activity. Therefore, the present study has been attempt to elucidate the factors showing a true correlation with that. Various experiments shown in table 2 and table 4 were carried out and photosynthetic rate, three major nutrient elements content and carbohydrate content in the leaf were determined. 1) Partial correlation coefficients were calculated between the factors and photosynthetic rate. A high significant partial correlation was found only between carbohydrate content and photosynthetic rate, all the other correlations proving insignificant. 2) In order to evaluate the influence of carbohydrate accumulation on photosynthetic rate, the diurnal changes of both photosynthesis and carbohydrate content in the leaf were observed. The photosynthetic rate showed no marked diurnal fluctuation through morning to afternoon, while the carbonhydrate content showed higher values in the afternoon than in the morning. It is considered from this that carbohydrates accumulation does not exert, at least, direct inhibitory influence upon photosynthetic rate. On the other hand, in cases such as plants were cultured under differential nutrient conditions or at different growth stages, those leaves highly active in their photosynthesis were shown to be low in carbohydrate content, and those less active leaves were shown to be high in carbohydrate content. From these it was considered that the velocity of removing photosynthates from leaves might be more important factor for photosynthesis rather than carbohydrate accumulation itself. 3) The growth of tuber which is the largest acceptor (sink) of the photosynthates in the sweet potato plant at later growth stage, was inhibited by exposing tubers to light. The treated plant was depressed in the photosynthetic rate and increased in starch content of leaves owing, presumably, to the restricted translocation of photosynthates from leaves. Farthermore, in another experiment, there was a parallel relation found between the net assimilation rate and the dry-weight increase of tubers. 4) All the above mentioned results suggest the following hypothesis : since the accumulation of photosynthates in the leaf does not bring any great change in the diurnal course of photosynthesis of sweet potato leaves, it is not accumulation of photosynthates in the leaf but the rate of movement of photosynthates from leaf (source) that is essential in controlling photosynthetic rate from inside. According to this hypothesis, to promote the growth of tuber means to increase the rate of translocation of photosynthates from leaves, and this, in turn, will give an accelerating influence to photosynthetic activity. Thus, the fact that potassium content in the leaf had a very intimate correlation with photosynthetic rate can be explained primarily by the nutritional effect of potassium for promoting the tuber growth.