Reconstitution of photosynthesis upon rehydration in the desiccated leaves of the poikilochlorophyllous shrub Xerophyta scabrida at elevated CO2

We report the resynthesis of the photosynthetic apparatus and the restoration of its function in the monocotyledonous C-3 shrub Xerophyta scabrida (Pax) Th. Dur. et Schinz (Velloziaceae) following a period of 5 years in the air-dried state. Detached leaves were rehydrated at present (350 mu mol mol(-1)) and at elevated CO2 (700 mu mol mol(-1)). Elevated CO2 concentration had no effect on the rate of rehydration, nor on the de novo resynthesis pattern of the chlorophylls and carotenoids or the development of photochemical activity in the reviving desiccated leaves. The time required to fully reconstitute the photosynthetic apparatus and its function in the air-dried achlorophyllous leaves on rehydration did not differ at the two CO2 concentrations. However, respiratory activity during rehydration was more intensive and of longer duration at high CO2 and net CO2 assimilation first became apparent 12 h later than in the leaves rehydrated at present CO2. After reconstitution of the photosynthetic apparatus, the net CO2 assimilation rate was higher in the high CO2 leaves, however it rapidly declined to a value lower than that in the present CO2 plants due to acclimation. This acclimation to elevated CO2 occurred only after complete reconstitution of the photosynthetic apparatus. The downward acclimation of photosynthesis was accompanied by a decrease in content of photosynthetic pigments (chlorophyll a + b and carotenoids x + c) and stomatal conductance. The initial slope of the A/c(i) curve for the high CO2 leaves was much lower and net CO2 assimilation rates were lower at all c(i)'s than in the present CO2 plants. The rate of respiration also decreased and the C-balance of the high CO2 leaves therefore remained similar to that of leaves in present CO2.