Introduction

It is well known that during the seed development of oil-bearing plants fat formation proceeds after sugar accumulation.In the seed，when the process of accumulation of one substance has shifted to that of another，there must be some changes in the intermediary metabolism.During rapid fat formation，what changes occur in the degradative pathways of glucose，and how these changes are regulated and controlled，are questions still open at present.

In recent years，following the works of Langdon［1］，Wakil［2］and Abraham et al.［3］in animals，Lynen［4］in yeast，Stumpf et al.［5］in plants，it is generally considered that in the biosynthesis of fatty acids，NADPH2 is a specific electron donor，and the addition of NADPH2 can noticeably stimulate the incorporation of labelled acetate into fatty acids in livers［1］and lipids in rape seeds（Zhu et al.，unpublished）.Since the synthesis of one molecule of C［16］-fatty acid requires 14 molecules of NADPH2，it appears likely that NADPH2 is one of the important rate-controlling factors in the fat synthesis in vivo.The HMP pathway is the primary source of NADPH2 in plant，therefore，the synthesis of fatty acids is probably related to this pathway and the shift of sugar accumulation to fat accumulation of oil seed is likely accompanied by relative increase of the activity of the HMP pathway.(https://www.daowen.com)

In a previous article［6］we had already shown that in the siliques of Brassica napus glucose can be degraded either via the EMP or the HMP pathways.In the present study，the patterns of the enzyme activities of these two pathways during the development of the siliques and the effect of respiratory inhibitors（iodoacetamide and sodium fluoride）on the respiration of seeds of different degrees of maturation have been investigated.The data obtained confirm the above assumption.In the mean time，a natural inhibitor of G-6-P dehydrogenase and 6-PG dehydrogenase has been found in the buds and young siliques.Since its presence varies with the age of siliques，it seems to play a role in the regulation of transition of the respiratory pathway and fat synthesis.