Ceusters Johan, Borland Anne M, Londers Elsje, Verdoodt Veerle, Godts Christof, De Proft Maurice P
Faculty of Bioscience Engineering, Department of Biosystems, Division of Crop Biotechnics, Katholieke Universiteit Leuven, Willem De Croylaan 42, Heverlee, Belgium.
Physiol Plant. 2009 Feb;135(2):174-84. doi: 10.1111/j.1399-3054.2008.01186.x. Epub 2008 Dec 5.
CAM requires a substantial investment of resources into storage carbohydrates to account for nocturnal CO(2) uptake, thereby restricting carbohydrate partitioning to other metabolic activities, including dark respiration, growth and acclimation to abiotic stress. Flexible modulation of carbon flow to the different competing sinks under changing environmental conditions is considered a key determinant for the growth, productivity and ecological success of the CAM pathway. The aim of the present study was to examine how shifts in carbohydrate partitioning could assure maintenance of photosynthetic integrity and a positive carbon balance under conditions of increasing water deprivation in CAM species. Measurements of gas exchange, leaf water relations, malate, starch and soluble sugar (glucose, fructose and sucrose) contents were made in leaves of the CAM bromeliad Aechmea 'Maya' over a 6-month period of drought and subsequently over a 2-month period of recovery from drought. Results indicated that short-term influences of water stress were minimized by elevating the level of respiratory recycling, and carbohydrate pools were maintained at the expense of export for growth while providing a comparable nocturnal carbon gain to that in well-watered control plants. Longer term drought resulted in a disproportionate depletion of key carbohydrate reserves. Sucrose, which was of minor importance for providing substrate for the dark reactions under well-watered conditions, became the major source of carbohydrate for nocturnal carboxylation as drought progressed. Flexibility in terms of the major carbohydrate source used to sustain dark CO(2) uptake is therefore considered a crucial factor in meeting the carbon and energy demands under limiting environmental conditions. Recovery from CAM-idling was found to be dependent on the restoration of the starch pool, which was used predominantly for provision of substrate for nocturnal carboxylation, while net carbon export was limited. The conservation of starch for the nocturnal reactions might be adaptive with regard to responding efficiently to a return of water stress.
景天酸代谢（CAM）植物需要大量资源用于储存碳水化合物，以应对夜间二氧化碳吸收，从而限制了碳水化合物向其他代谢活动的分配，包括暗呼吸、生长和对非生物胁迫的适应。在不断变化的环境条件下，灵活调节碳流向不同竞争库被认为是景天酸代谢途径生长、生产力和生态成功的关键决定因素。本研究的目的是研究在景天酸代谢植物水分亏缺增加的条件下，碳水化合物分配的变化如何确保光合完整性的维持和正碳平衡。在6个月的干旱期以及随后2个月的干旱恢复期内，对CAM凤梨科植物美叶光萼荷‘玛雅’叶片进行了气体交换、叶片水分关系、苹果酸、淀粉和可溶性糖（葡萄糖、果糖和蔗糖）含量的测定。结果表明，通过提高呼吸循环水平，水分胁迫的短期影响最小化，碳水化合物库得以维持，但以牺牲生长所需的输出为代价，同时夜间碳增益与水分充足的对照植物相当。长期干旱导致关键碳水化合物储备不成比例地消耗。在水分充足的条件下，蔗糖对为暗反应提供底物的重要性较小，但随着干旱的进展，蔗糖成为夜间羧化作用的主要碳水化合物来源。因此，在限制环境条件下，用于维持夜间二氧化碳吸收的主要碳水化合物来源的灵活性被认为是满足碳和能量需求的关键因素。发现从景天酸代谢闲置状态恢复取决于淀粉库的恢复，淀粉主要用于为夜间羧化作用提供底物，而净碳输出有限。为夜间反应保留淀粉可能有利于有效应对水分胁迫的再次出现。
