PHOTOSYNTHESIS AND EXPORT OF PHOTOASSIMILATES - DEVELOPMENT OF THE TRANSPORT-SYSTEM AND SOURCE-SINK RELATIONS

Development of the structures providing for communication routes in th e course of plant ontogenesis proceeds as functional source-sink relat ions are elaborated. Recent evidence concerning the structure of the t ransport system presumes that radial (parenchymal) transport zones mai ntain control over the development and functioning of the transport sy stem throughout the plant lifespan. The specific domain organization o f parenchyma assimilate transport is discussed in particular using dat a on the apoplast and endoplast structures in leaf parenchyma. These s tructures were shown to correlate with the time-course of photosynthes is and assimilate export under normal conditions and when the photosyn thesis/export rates were severely affected in experiments. The relatio n of photosynthesis to the structural patterns of transport routes in parenchyma of various vascular plant species was analyzed using publis hed data and evidence collected by the author in the Transaltai Gobi. The species composition of the flora of this desert is reduced along t he aridity gradient due to the elimination, first, of the advanced spe cies which developed a specialized system of assimilate translocation from the leaf (type 1), and, next, of the advanced species which emplo y rapid assimilate export via apoplast (type 2). The extremely arid ec otopes are inhabited by species with a primitive, archaic (non-special ized) organization of communication routes in parenchyma (type 0), low photosynthesis and metabolism, slow growth, and long intervals of arr ested development. The distribution of plant species on the basis of t wo classes of traits characteristic of the photosynthetic capacity and the pattern of the export system is discussed in relation to plant li fe strategies in increasingly arid ecotopes.