Metabolite-level carbon-flow analysis reveals that starch synthesis from hexose is a limiting factor in a high-yielding rice cultivar

Understanding the limiting factors of grain filling is essential for the further improvement of rice (Oryza sativa L.) grain yields. The slower grain growth of Momiroman, a high-yielding rice cultivar, is not improved by increasing the carbon supply. Thus, a low sink activity, which is the metabolic activity of assimilate consumption/storage in sink organs, may be a limiting factor of grain filling. However, there is no metabolic evidence corroborating this hypothesis, partly because there is no consensus on how to define and quantify sink activity. In this study, we investigated the carbon flows, at a metabolite level, from photosynthesis in leaves to starch synthesis in grains of three high-yielding cultivars using a stable isotope of carbon, 13C. The large amount of newly fixed carbon assimilates in Momiroman was stored as hexose instead of being converted to starch. Additionally, the activity of ADP-glucose pyrophosphorylase and the expression of AGPS2b, encoding an ADP-glucose pyrophosphorylase protein, in the superior grains of Momiroman were lower than in the other two rice cultivars. Thus, the slower starch synthesis from hexose, which is partly explained by the low expression level of AGPS2b, may be the primarily metabolic reason for the lower sink activity in Momiroman. (199/200 words)

Reduced Carbon Dioxide Sink and Methane Source under Extreme Drought Condition in an Alpine Peatland

Potential changes in both the intensity and frequency of extreme drought events are vital aspects of regional climate change that can alter the distribution and dynamics of water availability and subsequently affect carbon cycles at the ecosystem level. The effects of extreme drought events on the carbon budget of peatland in the Zoige plateau and its response mechanisms were studied using an in-field controlled experimental method. The results indicated that the peatland ecosystem of the Zoige plateau functioned as a carbon sink while under the control (CK) or extreme drought (D) treatment throughout the entire growing season. Maximum fluxes of methane (CH4) emissions and the weakest carbon sink activity from this ecosystem were in the early growth stage, the most powerful carbon sink activity was during the peak growth stage, while the absorption sink activity of carbon dioxide (CO2) and CH4 was present during the senescence stage. Extreme drought reduced the gross primary productivity (GPP) and ecosystem respiration (Re) of the peatland ecosystem by 14.5% and 12.6%, respectively (p < 0.05) and the net ability to store carbon was reduced by 11.3%. Overall, the GPP was highly sensitive to extreme drought. Moreover, extreme drought significantly reduced the CH4 fluxes of the ecosystem and even changed the peatland from a CH4 emission source to a CH4 sink. Subsequent to drought treatment, extreme drought was also shown to have a carry-over effect on the carbon budget of this ecosystem. Soil water content and soil temperature were the main driving factors of carbon budget change in the peatland of the Zoige plateau, but with the increase in soil depth, these driving forces were decreased. The findings indicated that frequent extreme drought events in the future might reduce the net carbon sink function of peatland areas, with an especially strong influence on CO2.

Thermal acclimation of leaf respiration as a way to reduce source–sink imbalance at low temperatures in Erythronium americanum, a spring ephemeral

Many spring geophytes exhibit greater growth at colder than at warmer temperatures. Previous studies have suggested that there is less disequilibrium between source and sink activity at low temperatures, which delays leaf senescence and leads to higher accumulation of biomass in the perennial organ. We hypothesized that dark respiration acclimates to temperature at both the leaf and bulb levels, mainly via the alternative respiratory pathway, as a way to reduce source–sink imbalance. Erythronium americanum Ker-Gawl. was grown under three temperature regimes: 8/6 °C, 12/8 °C, and 18/14 °C (day/night). Plant respiratory rates were measured at both growth and common temperatures to determine whether differences were due to the direct effects of temperature on respiratory rates or to acclimation. Leaf dark respiration exhibited homeostasis, which together with lower assimilation at low growth temperature, most likely reduced the quantity of C available for translocation to the bulb. No temperature acclimation was visible at the sink level. However, bulb total respiration varied through time, suggesting potential stimulation of bulb respiration as sink limitation builds up. In conclusion, acclimation of respiration at the leaf level could partly explain the better equilibrium between source and sink activity in plants grown in low-temperatures, whereas bulb respiration responds to source–sink imbalance.