Elevated CO2 accelerates flag leaf senescence in wheat due to ear photosynthesis which causes greater ear nitrogen sink capacity and ear carbon sink limitation

Abstract

It was anticipated that wheat net photosynthesis would rise under elevated CO 2 , and that this would alter the progress of senescence due to the unbalance of carbohydrates and nitrogen. Our study showed that ear carbon sink was limited, and sugar was accumulated, hexokinase activities and levels of phosphorylated sugar were increased within the flag leaves, grain nitrogen sink capacity was enhanced, and flag leaf senescence was accelerated under elevated CO 2 . However, if the ear of the main stem was covered, these responses to elevated CO 2 were absent, and the senescence of flag leaf was not accelerated by elevated CO 2 . Thus, it appeared that elevated CO 2 accelerated the rate of flag leaf senescence, depending on ear photosynthesis. The ears have far higher enhancement of net photosynthesis than flag leaves, and the role of the flag leaf relative to the ear was declined in supplying C assimilation to grain under elevated CO 2 . This indicates that as CO 2 rises, the grain sink needs the N more than C assimilate from flag leaf, so the declining rates of N% and soluble proteins concentration were markedly accelerated under elevated CO 2 conditions. This suggests that, the large increase in ear net photosynthesis accelerated grain filling, accelerated remobilising N within flag leaf as the result of the greater grain nitrogen sink capacity. In addition, as the result of grain carbon sink limitation, it limited the export of flag leaf sucrose and enhanced sugar cycling, which was the signal to accelerate leaf senescence. Hence, elevated CO 2 subsequently accelerates senescence of flag leaf.

Response of Rice Nitrogen Physiology to High Nighttime Temperature during Vegetative Stage

“…Amino acid concentration in the leaf was determined by the Ninhydrin method [33] using L-leucine as the standard. Soluble carbohydrates and starch were determined following the method of Zhu et al [34] using sucrose as the standard. …”

Expression patterns of C- and N-metabolism related genes in wheat are changed during senescence under elevated CO2 in dry-land agriculture

“…For example, Ommen et al [25] used chlorophyll as a marker to conclude accelerated senescence in wheat, and García et al [66] interpreted a lower than expected CO 2 -stimulation of carbon assimilation during later developmental stages as accelerated senescence. Zhu et al [23] and Fangmeier et al [26] suggested that leaf senescence was accelerated as a result of changes in the relative demand of the developing spikes for N and C (relatively greater N sink strength of the spike). In contrast to some of these suggestions, our chlorophyll measurements (using SPAD values as a proxy) give no evidence that elevated [CO 2 ] accelerated leaf senescence ( Fig.…”

“…As carbon assimilation increases and N concentration decreases under elevated [CO 2 ], tissues can have greater C/N ratios, and altered metabolite concentrations [22][23][24]. Elevated [CO 2 ] may also change leaf senescence: In cereals such as wheat and barley, changes in leaf functioning under elevated [CO 2 ] were ascribed to accelerated senescence processes, marked as faster chlorophyll decline [25].…”

Maintenance of C sinks sustains enhanced C assimilation during long-term exposure to elevated [CO2] in Mojave Desert shrubs

“…For example, plants photosynthetically acclimate (i.e., downregulate) to growth in elevated [CO 2 ] through changes in the photosynthetic apparatus, including lower light-and CO 2 -saturated photosynthesis (A max ) as well as lower V cmax (maximum Rubisco carboxylation) and J max (maximum electron transport) (Tissue et al 1993(Tissue et al , 2001. A second process that may impact the long-term enhancement of photosynthesis under elevated [CO 2 ] is reduced N availability, such as through re-allocation of N within the plant to meet other growth needs (Theobald et al 1998;Zhu et al 2009) or reduced N cycling in the ecosystem (Zak et al 2000). Support for this process comes from meta-analyses of both FACE (free-air carbon dioxide enrichment) and OTC (open-top chamber) studies: some plants, but particularly woody species, that exhibit photosynthetic acclimation also exhibit declines in leaf N and Rubisco content Ainsworth and Long 2005).…”