Photochemical energy use in Sorghum plants grown under drought and elevated CO2.

Abstract

Effects of Free Air Carbon-Dioxide Enrichment (FACE) and drought on PSII photochemistry, carbon assimilation (A) and photoprotection in leaves of Sorghum bicolor has been examined. In the drought-stressed plants (drys) growth under elevated CO2 increased the quantum yield of PSII (fPSII) throughout the day. This enhancement of fPSII is attributed to increases in both photochemical quenching (qp) and the effective quantum yield of PSII (Fv¢ /Fm¢ ). CO2 enhancement of fPSII in the well-watered plants (wets) occurred only during midday measurements and was entirely attributed to changes in qp. Non-photochemical quenching (NPQ) was higher in the drys but was reduced by elevated CO2 in both water treatments. Drought increased the total xanthophyll cycle pigments relative to total chlorophyll but was unaffected by growth [CO2]. The de-epoxidation state of the xanthophyll pool, DPS (A+Z/V+A+Z), was high under field conditions but showed no treatment differences. To further characterize the relationships between NPQ and DPS measurements were made on excised leaves in a controlled laboratory environment. There was no water treatment effect on NPQ, however, NPQ rates were reduced under elevated [CO2]. Conversely, growth [CO2] had no effect on DPS, however, the drys had a 57% higher DPS then the wets. Growth under elevated CO2 enhances C4 photosynthesis when A is limited by drought and/or midday conditions. Additionally, elevated CO2 and water-stress differently affected NPQ and pigment composition. Low NPQ under high CO2 did not correlate with DPS which maybe attributed to a reduced thylakoid D pH sufficient to alleviate NPQ but not DPS.