Gas Exchange of Mitchell Grass (Astrebla lappacea (Lindl.) Domin) in Relation to Irradiance, Carbon Dioxide Supply, Leaf Temperature and Temperature History

Abstract

Relationships were established between photosynthesis in A. lappacea and photosynthetic quantum flux, ambient CO₂ concentration and leaf temperature. There were no substantial differences between plants raised in a glasshouse and under three temperature regimes in growth cabinets in their responses to light level and CO₂ concentration. Large variations in the rate of photosynthesis were observed in plants raised under any environmental regime, this being due in part to the effects of leaf age. It was concluded that the greatest rate of photosynthesis observed in a series of experiments represented the best approximation to the rate attainable in the absence of limitations imposed by factors extraneous to the experiment concerned. The temperature responses of net photosynthesis, dark respiration and transpiration were influenced by growth regime temperature; the leaf temperature at which photosynthesis was maximal increased with growth regime air temperature up to at least 33°C. Plants transferred from one temperature regime to another exhibited the behaviour characteristic of the new regime within one day and two nights of the transfer. Estimates of gas diffusive resistances indicated that both the stornatal and CO₂ mesophyll (residual) resistances adapted to changed temperature conditions after about one thermoperiod. The temperatures at which these diffusive resistances were minimal varied in the same sense as the changes in growth regime temperature. The roles of these aspects of behaviour in the adaptation of A. lappacea to its environment are discussed.