Regulation of assimilate partitioning in leaves

Abstract

Concepts of the regulation of assimilate partitioning in leaves frequently consider only the allocation of carbon between sucrose and starch synthesis, storage and export. While carbohydrate metabolism accounts for a large proportion of assimilated carbon, such analyses provide only a restricted view of carbon metabolism and partitioning in leaf cells since photosynthetic carbon fixation provides precursors for all other biosynthetic pathways in the plant. Most of these precursors are required for biosynthesis of amino acids that form the building blocks for many compounds in plants. We have used leaf carbon : nitrogen ratios to calculate the allocation of photosynthetic electrons to the assimilation of nitrogen necessary for amino acid formation, and conclude that this allocation is variable but may be higher than values often quoted in the literature. Respiration is a significant fate of fixed carbon. In addition to supplying biosynthetic precursors, respiration is required for energy production and may also act, in both light and dark, to balance cellular energy budgets. We have used growth CO2 concentration and irradiance to modify source activity in Lolium temulentum in order to explore the interactions between photosynthetic carbon and nitrogen assimilation, assimilate production, respiration and export. It is demonstrated that there is a robust correlation between source activity and foliar respiration rates. Under some conditions concomitant increases in source activity and respiration may be necessary to support faster growth. In other conditions, increases in respiration appear to result from internal homeostatic mechanisms that may be candidate targets for increasing yield.

Effects of nitrogen form on growth, CO2 assimilation, chlorophyll fluorescence, and photosynthetic electron allocation in cucumber and rice plants

“…Significant differences (P<0.05) between treatments within the same species are indicated by different letters in the light reaction, which is not completely used during CO 2 assimilation, is either exported from the chloroplasts or consumed by the reductant sinks (e.g., leaf 3 NO − assimilation or water-water cycle) (Brück and Guo, 2006). Previous studies have indicated that a substantial portion of photosynthesis or respiration electron transport generates reducing equivalents for 3 NO − reduction rather than for carbon fixation (Bloom et al, 1989;Noctor and Foyer, 1998;Lewis et al, 2000), and thus, 3 NO − reduction may represent a considerable reductant sink (Brück and Guo, 2006 (2000), we divided the total J PSII into J c , J o , J a (O 2 -dependent), and J a (O 2 -independent). In this model, J a (O 2 -dependent) and J a (O 2 -independent) were most driven by the water-water cycle and 3 NO − reduction, respectively (Miyake and Yokota, 2000).…”

Functional Mitochondrial Complex I Is Required by Tobacco Leaves for Optimal Photosynthetic Performance in Photorespiratory Conditions and during Transients

“…Therefore, manipulation of respiration is a potential approach to improving agricultural yields (Lewis et al, 2000). One current difficulty is that, quite apart from the roles played by respiration in heterotrophic tissues, the importance of leaf respiration for the process of carbon fixation itself is not yet fully established.…”

“…Although the importance of these enzymes in photorespiratory carbon recycling is well established (Somerville and Ogren, 1981; Blackwell et al, 1990; Heineke et al, 2001), and although isolated mitochondria show high rates of Gly-linked O 2 consumption (e.g. Krö mer and Heldt, 1991), the role of the mitochondrial electron transport chain during whole-leaf photorespiration remains unclear (Leegood et al, 1995;Foyer and Noctor, 2000).Data on mitochondrial roles in photosynthesis have largely been produced either by analysis of isolated organelles or by using inhibitors. Evaluation of the physiological roles of mitochondrial electron transport is complicated by the existence in plants of several NAD(P) H dehydrogenases and an alternative terminal oxidase, in addition to cytochrome oxidase (Møller, 2001).…”

“…Although the importance of these enzymes in photorespiratory carbon recycling is well established (Somerville and Ogren, 1981;Blackwell et al, 1990;Heineke et al, 2001), and although isolated mitochondria show high rates of Gly-linked O 2 consumption (e.g. Krö mer and Heldt, 1991), the role of the mitochondrial electron transport chain during whole-leaf photorespiration remains unclear (Leegood et al, 1995;Foyer and Noctor, 2000).…”

Generalized Münch Coupling between Sugar and Water Fluxes for Modelling Carbon Allocation as Affected by Water Status

“…Starch storage can be considered as an active competitor to growth in a number of cases (Lacointe et al, 1993;Cannel & Dewar, 1994). In leaves, starch synthesis and hydrolysis are directly involved in the regulation of carbohydrate export (Komor, 2000;Lewis et al, 2000).…”