The influence of elevated C02 on growth and age-related changes in leaf gas exchange

Rumex obtusifolius plants were grown for several months in daylitenvironment chambers (Solardomes) force-ventilated with aircontaining 350 or 600 µ;mol mol^–1 C0₂. ElevatedCO₂ was found to accelerate the natural ontogenic decline inphotosynthesis, but did not reduce leaf duration. In both CO₂treatments photosynthetic rates declined progressively withincreasing leaf age, the decline being greater for plants grownin elevated C0₂ such that rates became lower than in ambientCO₂. The degree of CO₂-induced photosynthetic down-regulationas determined by A/C₁ analysis was found to be dependent onleaf age. The major contribution to the decline in photosynthesiswas likely to be a reduction in Rubisco activity as changesin stomataland mesophyll limitations were small. Instantaneouswater use efficiency (WUE₁) was greater for plants in elevatedCO₂, but these values declined rapidly with leaf age, whereasin ambient CO₂ values were always lower, but were maintainedfor longer. Growth analysis indicated an increased root:-shootratio for plants grown in elevated CO₂, this occurring almostentirely as a result of increased root growth. Greater rootproliferation and increased WUE₁, are characteristics whichshould give this persistent and troublesome weed an increasedcompetitive advantage under projected conditions of climatechange Key words: tusifoliu, elevated CO₂, gas exchange, leaf age, senescence