Directorate of Groundnut Research, Junagadh 362001, Gujarat, India.
J Plant Physiol. 2014 Feb 15;171(3-4):319-28. doi: 10.1016/j.jplph.2013.09.008. Epub 2013 Oct 21.
Photorespiration is generally considered to be an essentially dissipative process, although it performs some protective and essential functions. A theoretical appraisal indicates that the loss of freshly assimilated CO2 due to photorespiration in well-watered plants may not be as high as generally believed. Even under moderately adverse conditions, these losses may not exceed 10%. The photorespiratory metabolism of the source leaves of well-watered and well-nourished crop plants ought to be different from that of other leaves because the fluxes of the export of both carbohydrates and organic N-transport compounds in source leaves is quite high. With a heuristic approach that involved the dovetailing of certain metabolic steps with the photorespiratory cycle (PR-cycle), a novel network is proposed to operate in the source-leaves of well-watered and well-nourished plants. This network allows for the diversion of metabolites from their cyclic-routes in sizeable quantities. With the removal of considerable quantities of glycine and serine from the cyclic route, the number of RuBP oxygenation events would be several times those of the formation of hydroxypyruvate. Thus, to an extreme extent, photorespiratory metabolism would become open-ended and involve much less futile recycling of glycine and serine. Conversion of glyoxylate to glycine has been proposed to be a crucial step in the determination of the relative rates of the futile (cyclic) and anabolic (open-ended) routes. Thus, in the source leaves of well-watered and well-nourished plants, the importance of the cyclic route is limited to the salvaging of photorespiratory intermediates for the regeneration of RuBP. The proposed network is resilient enough to coordinate the rates of the assimilation of carbon and nitrogen in accordance with the moisture and N-fertility statuses of the soil.
光合作用通常被认为是一种本质上耗散的过程，尽管它具有一些保护和基本的功能。理论评估表明，在水分充足的植物中，由于光合作用而损失的新同化的 CO2 可能并不像通常认为的那样高。即使在中等不利的条件下，这些损失也可能不会超过 10%。水分充足和营养良好的作物源叶的光合作用代谢应该与其他叶片不同，因为源叶中碳水化合物和有机 N 运输化合物的输出通量相当高。通过一种涉及某些代谢步骤与光合作用循环（PR 循环）吻合的启发式方法，提出了一种新的网络，该网络在水分充足和营养良好的植物的源叶中运行。该网络允许大量将代谢物从其循环途径中转移。通过从循环途径中去除相当数量的甘氨酸和丝氨酸，RuBP 氧化事件的数量将是形成羟丙酮酸的数倍。因此，在极端程度上，光合作用代谢将变得无限制，并涉及甘氨酸和丝氨酸的大量无效循环回收。已经提出将乙醛酸转化为甘氨酸是确定无效（循环）和合成（无限制）途径相对速率的关键步骤。因此，在水分充足和营养良好的植物的源叶中，循环途径的重要性仅限于回收光合作用中间体以再生 RuBP。所提出的网络具有足够的弹性，可以根据土壤的水分和氮肥状况协调碳和氮同化的速率。
