The regulation of carbohydrate metabolism and sourceÀsink relationships among organs play a key role in plant adaptation to drought. This study aimed at characterising the dynamics of transpiration, development, growth and carbon metabolism, as well as the expression of invertase genes, in response to drought during a dry-down cycle. Three 1-month experiments were conducted in controlled environment using the rice genotype IR64 (Oryza sativa L., indica). Plant leaf relative transpiration and expansion rates decreased linearly when fraction of transpirable soil water (FTSW) dropped below 0.66 and 0.58, respectively. Hexose and starch concentration responses to FTSW in a given organ were generally linear and opposite: in source leaves, hexose concentration increased and starch decreased, and vice versa in sink leaves and roots. Sucrose remained constant in source leaves and increased slightly in sink leaves. Starch reserves built up during stress in sink organs were rapidly mobilised upon rewatering, indicating its involvement in a mechanism to ensure recovery. Expression of cell-wall and vacuolar invertase genes under stress increased in sink leaves, interpreted as a mechanism to maintain sink activity (cell wall) and osmotic adjustment (vacuolar). It is concluded that carbohydrate metabolism in sink organs under drought is highly regulated, and important for stress adaptation.
Sugar accumulation in sorghum (Sorghum bicolor (L.) Moench) stems is a complex trait that is particularly plastic in response to photoperiod. This study investigated sucrose accumulation in a sterile (no grain filling) and fertile nearisogenic line of the photoperiod-sensitive cultivar IS2848 in two greenhouse experiments. Variable phenology was induced by applying a short (12-h PP) and a long (13-h PP) photoperiod. Dynamics of plant growth, phenology, sugar accumulation and related enzyme activities in internodes were investigated. Under 13-h PP, plants flowered 28 days later and attained threefold higher sucrose concentration at anthesis compared with those under 12-h PP. Sucrose accumulation in individual internodes was driven by organ physiological age, not by plant phenology. Competition with grain filling was marginal but greater under 12-h PP (i.e. when sucrose accumulation in internodes occurred after flowering). Enzyme activities showed marked developmental patterns but contributed little to explaining differences between treatments and genotypes. The study demonstrates that sucrose storage physiology in sweet sorghum resembles that of sugarcane (Saccharum spp.) but is more complex due to photoperiod effects on phenology. It confirms the field results on 14 sorghum genotypes contrasting for phenology and photoperiod sensitivity presented in a companion paper. Perspectives for developing sorghum ideotype concepts for food and fuel crops are discussed. (Résumé d'auteur
Assimilate storage in vegetative organs is an essential buffer for the source–sink imbalances that inevitably occur in perennial plants. In contrast to temperate trees, little information is available on such storage in tropical perennials, and almost none for Cocos nucifera. This paper describes the chemical nature, quantity and distribution of carbohydrate reserves in coconut plants grown in an environment favourable to production. The study was carried out on the island of Santo (Republic of Vanuatu, Southern Pacific) on twelve 17-year-old adult plants, representative of a large population, which were felled and characterized for root, trunk and crown dry matter, and contents of soluble sugars and starch. Roots were divided into three diameter classes and distal/proximal portions, the trunk into three axial and three or four radial zones, and the crown into petiole, rachis and leaflets for various leaf ages. The aggregate reserve pool size was compared with estimates of incremental demand for assimilates for growth and fruit production. Plants contained little starch but large quantities of sucrose were found, mainly located in the trunk. Less sucrose was present in roots and little in leaf blades. Large glucose and fructose pools were found in leaves, near the apex of the trunk and in the terminal portions of large roots. Aggregate soluble and non-soluble sugar pools were about equivalent to six months of copra production or 51 days of crop growth. More studies are needed on the dynamics of these sugars to evaluate their physiological role, particularly with regards to stress periods and fluctuating demand for fruit filling.
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