Initial observations of increased requirements for light-energy dissipation in ryegrass (Lolium perenne) when source / sink ratios become high at a naturally grazed free air CO2 enrichment (FACE) site

Although photosynthetic response to long-term elevated CO2 has been extensively studied, little attention has yet been directed at coordinated adjustments between the use of absorbed light for CO2 fixation, and the dissipation of potentially harmful excess light. In this study, we have performed an initial analysis of photosynthetic light use and excess light dissipation in response to grazing-induced variation in the source / sink ratio in ryegrass (Lolium perenne L.) after 6 years’ exposure to Free Air CO2 Enrichment (FACE). Before grazing, when the source / sink ratio was relatively large, significant down-regulation of photosynthetic capacity (Amax) was observed in the FACE leaves compared with control leaves at the same stage of maturity. The decrease in Amax partly offset the direct stimulation of elevated CO2 on light-saturated photosynthesis, and was accompanied by a reduction in photochemical electron flow that was accompanied by a large increase in susceptibility to photoinhibition. This was indicated by large increases in both non-photochemical quenching (NPQ) and the de-epoxidised state of xanthophyll cycle (DEPS), and also by changes in the photochemical reflectance index (PRI). However, no significant increase in the xanthophyll pool size in FACE leaves was observed, despite the apparent large increase in requirements for photodissipation in FACE leaves. After grazing, when the source / sink ratio was relatively small, the CO2 fixation rates in both the FACE and control leaves were, as expected, significantly higher compared with those before grazing, and there was no down-regulation of photosynthetic capacity in the leaves under FACE conditions. In addition, the extent of photodissipation in the FACE and control leaves was not significantly different. Overall, the profile of leaf physiological and biochemical responses supports the hypothesis that the effect of long-term elevated CO2 can be significantly influenced by short-term variation in the source / sink ratio. As the xanthophyll pool size does not change significantly, this poses the question of whether the increased photodissipative demand observed here under even moderately elevated CO2 concentrations may lead to increased plant susceptibility to photoinhibition, and thus to an increased risk of damage to plant function, under conditions of low sink demand. This question clearly deserves further study.