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 rest oration of its function in the monocotyledonous C-3 shrub Xerophyta sc abrida (Pax) Th. Dur. et Schinz (Velloziaceae) following a period of 5 years in the air-dried state. Detached leaves were rehydrated at pres ent (350 mu mol mol(-1)) and at elevated CO2 (700 mu mol mol(-1)). Ele vated 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 desiccat ed 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, res piratory 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 reconstit ution of the photosynthetic apparatus, the net CO2 assimilation rate w as higher in the high CO2 leaves, however it rapidly declined to a val ue lower than that in the present CO2 plants due to acclimation. This acclimation to elevated CO2 occurred only after complete reconstitutio n of the photosynthetic apparatus. The downward acclimation of photosy nthesis was accompanied by a decrease in content of photosynthetic pig ments (chlorophyll a + b and carotenoids x + c) and stomatal conductan ce. 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 tha n in the present CO2 plants. The rate of respiration also decreased an d the C-balance of the high CO2 leaves therefore remained similar to t hat of leaves in present CO2.