PHYTOCHROME EFFECTS ON STEM CARBON GAIN IN LIGHT-GROWN MUSTARD SEEDLINGS ARE NOT SIMPLY THE RESULT OF STEM EXTENSION-GROWTH RESPONSES

Carbon allocation and partitioning were investigated in the first inte rnode of light-grown Sinapis alba L. seedlings exposed to white light (WL) with or without supplementary far-red light (FR). In the internod e, supplementary FR increased the rates of extension-growth and the ac cumulation of radiolabeled carbon (fed through the leaves), reducing s ugars (even per unit volume), starch, hemicellulose and cellulose, but had no effect on the levels of sucrose and ammonium oxalate-solubilis ed cell wall carbohydrates, on invertase activity or on the use of add itional sucrose fed through the leaves. In source leaves, supplementar y FR had no effect on photosynthesis rates and reduced the accumulatio n of radiolabeled carbon. Mechanical reduction of stem extension-growt h responses to supplementary FR did not affect internode carbohydrate or carbon accumulation responses. Supplementary FR provided only to on e leaf had no effect on internode extension growth but increased carbo n accumulation in the internode, provided that supplementary FR and ra diolabeled carbon were both given to the same leaf. Phytochrome-mediat ed effects on carbon partitioning are not the mere consequence of inte rnode extension-growth responses. Some additional control point(s) (e. g. leaf-source strength) must be under the direct influence of phytoch rome.