The Cellular Pathway of Short-distance Transfer of Photosynthates and Potassium in the Elongating Stem of Phaseolus vulgaris L. Stem Anatomy, Solute Transport and Pool Sizes

Abstract

Based on an uniform elongation growth pattern and cellular structure, the apical 0·5-2·5 cm elongation zone of internode 2 of Phaseolus vulgaris L. seedlings was selected as an experimental system to study the radial pathway of photosynthate and potassium transfer from the phloem. An histological examination of the phloem within the elongation zone of internode 2 showed that both proto- and meta- phloem sieve elements were present. The former were fully differentiated at the commencement of elongation and became crushed as elongation proceeded. In contrast, about 50% of the final number of metaphloem sieve element-companion cell complexes differentiated during the same period. The phloem delivered some 99% of the sucrose and 72-82% of the potassium accumulated by the elongation zone. Solute budgets showed that, of the photosynthates and potassium entering the elongation zone, approximately 40% were retained and 60% transferred to the shoot apex. Thus, the elongating stem acts not only as a significant sink for photosynthates and potassium, but also as an axial phloem transport system to supply the shoot apex. Within the elongation zone, the principal tissue sinks were determined by the cellular localisation of [14C] photosynthates and potassium by microautoradiography and ion electron microprobe analysis respectively. About 80% of the photosynthates and potassium were located outside the phloem. The cortex and pith exhibited the greatest accumulation for photosynthates and the pith for potassium.