USE AND DEGRADATION OF LIGHT ENERGY IN WATER-STRESSED DIGITALIS-LANATA

Using (CO2)-C-12, (CO2)-C-14 and H2O gas exchange as well as metabolit e analysis, net CO2 uptake (P(N)) and transpiration rate (E) were meas ured in the water-stressed plants of Digitalis lanata EHRH. The leaf c onductance (g(CO2)), the gross CO2 uptake (P(G)), the photorespiration (R(P)) and reassimilation (RA) rates were calculated from measured pa rameters. The pulse modulated fluorescence was measured during the ste ady state photosynthesis. After withholding irrigation, the leaf water potential decreased to -2.5 MPa, but leaves remained turgid and fully exposed to irradiance even at a severe water stress. Due to the stres s-induced reduction of g(CO2), P(N) and E were drastically reduced, wh ereas P(G) and R(P) were less affected. Water use efficiency (WUE), wh ich was higher in 1 000 than 350 cm3(CO2) m-3, increased as the water stress developed. The stomatal closure induced an increase in the reas similation (RA) of internally liberated CO2 (R(P)). The increased CO2 recycling in relation to the water stress was high in 350 cm3(CO2) m-3 and still substantial in 1 000 cm3(CO2) m-3, and consumed a substanti al amount of radiant energy in the form of ATP and reduction equivalen ts. Consequently, the metabolic demand for radiant energy was reduced by less than 40 %, whereas P(N) was diminished by more than 70 % in se verely stressed plants at 350 cm3(CO2) m-3. Additionally, the quantum efficiency of photosystem 2 as a measure for the flux of photosyntheti cally generated electrons was reduced upon the stress. This (and possi bly other mechanisms) enabled the stressed plants to avoid overreducti on of the photosynthetic electron transport chain.