Introduction

Sucrose is the most important low-molecular weight carbohydrate in most (if not all) higher plants. It is together with starch the dominant assimilation product in leaves and it is in most plaints the major organic carbon compound, which is translocated in the phloem to the non-photosynthetic and storage tissues. As a storage compound sucrose is common in many plants, too, however, usually at low concentration compared to starch. (The sugars in fruits are not considered as storage compounds here, because they do not serve the plant as carbon source to be resumed but serve as attractant for seed dispersal). Sugarcane in its commercial, varieties is (together with sugarbeet) unusual because of the high sucrose concentrations in the stem and the virtual absence of starch. This fact was the reason for the economic value of sugarcane since more than two thousand years, but also for the interest of agronomists and plant physiologists in that plant in more "modern" times as a most suitable object for the study of mechanism and regulation of sucrose storage (or sugar storage in general). As a consequence sugarcane research institutes had been founded in many tropical and subtropical countries (the oldest being the Hawaiian Sugar Planters'Association Experiment Station from 1896). The knowledge about sugarcane elaborated in the first half of the 20th century was excellently summarized in the book of van Dillewijn "The botany of sugarcane" (1). More recently obtained agronomic data on sugarcane are given by Clements (2). The landmark in enzymology of sucrose storage in sugarcane was set by Australian researchers in the 1960s and early 1970s. Their data were condensed to a scheme of sucrose metabolism in storage parenchyma by Glasziou and Gayler (3), which stayed valid in many aspects up to now. Since then much progress has been made in understanding the mechanisms and regulation of transport processes, of solute compartmentation, of phloem loading and long distance transport, of apoplastic barriers and symplastic cell continuity and of water relations in plants in general and of sugarcane in special. The state of the art is excellently represented in a review by Moore (4). In meanwhile some more information about metabolic control of sucrose storage is available, a few sugarcane genes have been cloned and transformed sugarcane plants have been generated. These data will be incorporated in the framework of physiological aspects of sucrose storage, which were already dealt with previously. This review will not consider photosynthesis and sugar production in the leaf, not consider phloem loading and long distance translocation of sucrose, neither the interaction of sucrose yield with nutrients like N, P or K or else, nor the correlations to growth parameters. It will focus only on the fate of assimilated carbon delivered by the phloem into the stem parenchyma of sugarcane. Justification for that strict limitation may be given by the fact that photosynthetic activity in wild sugarcane species with less than 4% fresh weight as sucrose is the same as in high yield varieties, in which sucrose is 20% of fresh weight (5). Therefore the major differences should be based on sugar transport and metabolism in the storage tissue (6).

0 0

Post a comment