Effects of CO2 enrichment on soluble amino acids and organic acids in barley primary leaves as a function of age, photoperiod and chlorosis

Responses of soluble amino acids and organic acids to either ambient (36 Pa) or elevated (100 Pa) CO2 treatments were determined using barley primary leaves (Hordeum vulgare L. cv. Brant). Total soluble amino acids were increased 33% by CO2 enrichment 9 days after sowing (DAS), but a decrease relative to the ambient CO2 treatment was observed with increasing leaf age. Marked declines of glutamine and asparagine were observed under CO2 enrichment, both diurnally and with advancing leaf age. Consequently, total soluble amino acids were 59% lower in the elevated compared to the ambient CO2 treatment 17 DAS. It was likely that chlorosis in response to CO2 enrichment negatively impacted soluble amino acid levels in older barley primary leaves. In contrast to the ambient CO2 treatment, glutamine and most other soluble amino acids decreased as much as 60% during the latter half of a 12 h photoperiod in primary leaves of 13-day-old seedlings grown under enhanced CO2. Malate was decreased about 9 percent by CO2 enrichment and citrate and succinate were increased by similar amounts when measured 9 and 13 DAS. Malate accumulation was also decreased about 20% by CO2 enrichment on a diurnal basis. The onset of CO2-dependent leaf yellowing had much less of an effect on organic acids than on soluble amino acids. This above results emphasized the sensitivity of N metabolism to CO2 enrichment in barley. Increased levels of citrate and succinate in response to CO2 enrichment suggested that the tricarboxylic acid cycle was upregulated in barley by CO2 enrichment. In summary, organic and amino acid levels in barley primary leaves were dynamic and were altered by age, diurnally and in response to CO2 enrichment.