Impact of light intensities and low temperature on the composition and diurnal use of transient carbon stores in barley (Hordeum vulgare)

Abstract

The carbon partitioning and accumulation are critical for biomass formation in plants and can be affected by environmental changes and regulated by circadian clock. Thus, in this study, barley was grown under three light intensities and under three temperature regimes to evaluate growth, photosynthesis and carbon metabolism. We also established a high throughput enzymatic protocol that allows determination of fructans and glucose, fructose and sucrose from small tissue samples. Barley photosynthesis and carbon compounds accumulation increase accordingly to increments of light intensity, mainly in blades. Surprisingly, malate is accumulated in high quantities and is the major pool of carbon in barley, especially under low light, when sucrose accumulation is lower. Turnover rates of metabolites were very similar between light intensities and the participation of circadian clock on the regulation was investigated. Introgression lines with mutation in the ELF3, an element of the circadian core clock, presented increased content of hexoses and lower starch content at end of day with similar turnover rates to wild-type. Impaired elf3 function did not lead to inhibition of growth in barley, which might be explained by the low contribution of starch and hexoses to the pool of C used at night. Growth in barley is dependent of sucrose and malate levels, which are not under strong influence of elf3. Plants grown under cold condition presented inhibition of photosynthesis and growth. Surprisingly, they also presented inhibition of starch accumulation during the day under cold nights. However, mobilization rates of starch were similar in warm days and nights. Malate and sucrose compose the majority of the carbon pool available for the night growth in all temperature regimes, accumulating in blades under cold days. Although fructan accumulation was expected under cold, it was negligible for all tissues. The growth in barley is impaired under cold nights, even though there is carbon available for use at night. Hence, the data suggests that barley growth machinery is sensitive to cold night, leading to a negative feedback on photosynthesis, starch accumulation, and sucrose and malate mobilization at night. Glucose, starch and sucrose accumulation was reduced in elf3 mutants grown under cold nights, suggesting a clock control on the synthesis of these compounds, despite the absence of an effect of ELF3 mutation on growth and photosynthesis. Thus, further investigation targeting other clock components would clarify the extent of the growth regulation and carbon partitioning in sucrose accumulating plants, as well as the growth response to cold.