On the Expression of Plant Age of Soybean by Means of Plastochron Index

[摘要] 1. Six plants of soybean were grown in a green house in 1972. The length of every compound leaf from the 1st to 13th, which was the last one, was measured every other day. 2. When the leaf length was expressed logarithmically, the growth curve was linear against the days after germination during the most part of the growing period. The regression equation of log L (L is referred to leaf length) on the number of days was calculated for each leaf. The regression coefficients of all leaves were not the same. Some of the differences between two successive leaves were significant. In the range from the 1st to the 7th leaf, upper leaves tended to show larger coefficient than the lower leaves ; in the leaves higher than 7th, however, the coefficient tended to decrease gradually. While higher temperature may be involved in part in higher regression coefficients of higher leaves, it may be true that the coefficients depend to some extent on the nodal location of leaves. 3. At first the standard length for calculating plastochron index (referred to as P.I.) was tentatively defined as 30 mm for leaf blades of central leaflets and as 45 mm for whole compound leaves. Based on these standard lengths, the errors of calculated P.I. derived from following causes were estimated; (a) the bending of regression line or the nth leaf before the P.I. reaches n+1, (b) the difference of regression coefficients between two successive leaves and (c) combination of (a) and (b). As to the maximum errors during plastochron, the largest ones out of 65 plastochrons observed were 0.07 by (a), 0.09 by (b) and by (c) in the case of leaflets, and 0.03 by (a), 0.14 by (b) and by (C) in compound leaves, while the frequencies of errors of those sizes seemed very low. After all, the errors brought about by those causes, even in cases those occurred together, did not seem so large for practical use. 4. The P.I. calculated with the standard length defined tentatively as 30 mm or 45 mm as above were almost linear against days in the case of either leaflets or compound leaves. The correction of above three kinds of errors were too small to be distinguished on the graph. 5. Central leaflets appeared more suitable than whole compound leaves to be measured for estimation of P.I., from the viewpoints of size of errors and easiness of measuring leaf length. In order to use P.I. as an index of plant age of soybean, a certain standard length must be defined suitably. The comparison of size of errors between 10 mm and 2O mm as the standard length suggested that 10 mm may be more suitable. It should be noticed in 10 mm standard, however, that the length of the (n+1)th leaf to be measured is always shorter than 10 mm, sometimes as short as 2 mm, which may make the measurements fairly inaccurate, furthermore the errors of measuring length cause larger errors of P.I. in the case of 10 mm than in 20 mm. 6. The errors of calculated P.I. caused by errors in measuring length are larger when either nth or (n+1)th leaves are nearer the standard length. Therefore, the length should be measured as accuretely as possible, especially for the leaves near the standard length.

[学科分类] 农业科学（综合）