Measured Responses to CO2 x Water & Temperature.

The CO₂ concentration of the atmosphere is increasing, Earth is getting warmer, and precipitation patterns may change in the future. Elevated CO₂ alone increased photosynthesis, biomass, and yield in all C₃ species, an average 21% for shoot biomass for enrichment to 550 µmol mol^-1, but less in C₄. Elevated CO₂ generally also decreased stomatal conductance and transpiration per leaf area while increasing canopy temperatures and water use efficiency in all plants. Therefore the reduction of ET following rain or irrigation event enables a CO₂-enriched crop to sustain photosynthesis and growth more days into a growth cycle and the degree of CO₂ growth stimulation is greatly dependent on dynamics of drought cycles. For the most part, however, when water is limited, growth stimulations are as large or larger under water-stressed compared to well-watered conditions. Elevated temperature alone increases growth and yield when normal temperature is below optimum for a particular plant. If warmer temperature prevents frost damage, positive responses can be dramatic. If normal temperature is above optimum, decreases in growth and yield result. Moreover, if warmer temperature damages pollen and seed-set, negative response can be dramatic. Generally, warmer temperatures accelerate plant development time so that for determinant cereal crops like wheat, shortened grain-filling periods can decrease yield. The temperature optimum for photosynthesis shifts to higher temperatures at elevated CO₂. During the vegetative stage of plant growth and below the temperature optimum, the interaction appears mostly strong and positive. Above the temperature optimum, partial stomatal closure and associated canopy temperature rise can exacerbate crop damage, especially with regard to seed-set issues. At the same time, higher photosynthetic rates at elevated CO₂ enable plants to better withstand damaging high temperatures. In mixtures of C₃ and C₄ grasses, both warming alone and combined warming plus elevated CO₂ favored C₄ grasses.