Increasing photosynthetic benefit with decreasing irrigation frequency in an Australian temperate grassland exposed to elevated carbon dioxide

Elevated atmospheric CO2 (e[CO2]) often enhances plant photosynthesis and improves water status. However, the effects of e[CO2] vary significantly and are believed to be influenced by water availability. With the future warmer climate expected to increase the frequency and severity of extreme rainfall, the response of plants to e[CO2] under changing precipitation patterns remains uncertain. We examined the effects of e[CO2] and different irrigation regimes on perennial ryegrass in a free-air CO2 enrichment (FACE) experiment. Immediately after irrigation, the mean net photosynthetic rate was 21.2% higher under e[CO2] compared with ambient conditions. This benefit increased over time, reaching a 31.3% higher rate as days since watering increased, indicating a substantial increase in photosynthetic benefit with longer intervals between watering. Mean stomatal conductance was 21% lower in ryegrass under e[CO2] immediately after irrigation compared with ambient plots. However, the reduction in stomatal conductance under e[CO2] decreased as the interval between irrigation events increased, showing no difference 7–10 d after an irrigation event. These results imply that plants benefit most from carbon fertilization, assimilating relatively more carbon and losing less water, during periods with less frequent rainfall. These findings have significant implications for understanding leaf-level responses to climate change.

Methods
In a Free-Air Carbon Enrichment site (FACE) in Tasmania, we investigated pasture grass (perennial ryegrass) responses to elevated CO2 and irrigation frequency. We assessed the impacts of treatments on gas exchange, water relations, leaf area index and soil water content on perennial ryegrass over one year.