The time course of carbohydrate transport and storage in the leaves of the plant species with symplastic and apoplastic phloem loaded under the normal and experimentally modified conditions.

摘要

Transport networks and carbohydrate pools were studied by electron microscopy in two groups of plant species, three species with symplastic phloem loading (Cucurbita pepo, Fuchsia hybrida and Thymus hybridum) and three species with apoplastic phloemloading (Zinnia elegans, Pisum sativum and Trifolium repens) diurnally under normal conditions and following the experimental restriction of photosynthesis, water supply and photosynthate export. Photosynthesis was inhibited by darkening particular leaves or the whole plant, water supply was interrupted by detaching leaves, and assimilate export was arrested by cold-girdling of leaf petioles. The first group of plant species, with a joint symplast in the mesophyll and phloem, manifested systemic responses to the experimental restriction of photosynthesis, water uptake, and metabolite export, and these responses were propagated towards the vascular bundles. Fluctuations in starch content, changes in the configuration and volume of vacuoles and endoplasmic reticulum, and osmotic effects of matrix condensation and decondensation in plastids and mitochondria were among the most characteristic structural changes resulting from the restriction. Reparation proceeded only after the restriction was lifted. Inthe second group of plant species, with mesophyll and phloem symplasts separated, the parenchyma cells of two tissues responded locally and in different ways. When affected by similar agents, the responses were usually contrasting in mesophyll and phloem domains. The structure of the endoplasmic labyrinth was highly sensitive toward the changes in the photosynthesis/export ratio, whereas the structure of the apoplastic labyrinth withstood most experimental treatments. The complicated system of intermediate pools produced the fluctuating pattern of responses to the experimental treatments. The fluctuation of sugar concentration in the apoplast resulted in the osmotic changes in the cytosol comparable to the condensation/decondensation of the organellematrix in the first group of plant species. Reparation was observed while the blockade lasted. Under similar experimental treatments, the gradients of starch accumulation, cell vacuolation, and osmotic responses diverged in the leaves of two plant groups. Thus, the data on plant structure can be used to diagnose the functional patterns of carbohydrate turnover in leaves.