LOW-TEMPERATURE LIMITATIONS OF PHOTOSYNTHESIS IN 3 TROPICAL VIGNA SPECIES - A CHLOROPHYLL FLUORESCENCE STUDY

The temperature-dependence of photosynthesis and chlorophyll (Chl) fluorescence quenching components was studied between 0 and 45-degrees-C in three tropical, chilling-sensitive Vigna species and in chilling-tolerant pea. Photosynthesis of the Vigna spp. was approx. 20% more reduced by temperatures between 7 and 30-degrees-C than in pea. The latter revealed significant changes in Chl fluorescence parameters at much lower temperature than the Vigna spp. Below 15-degrees-C, the reduction state of Q(A) increased quickly in pea, while in Vigna already below 30-degrees-C, an increase of reduced Q(A) was obtained. The analysis of different components of non-photochemical Chl fluorescence quenching (q(N)) revealed, that in pea photoinhibitory quenching (q(I)) occurred below 13-degrees-C. Below ca. 7-degrees-C, a sudden breakdown of both q(P) and the fast relaxing component of q(N) was observed in pea. In Vigna, susceptibility of LHC II phosphorylation or limitation of electron flow by damage to PS I, the PS II reaction centre or the water-splitting system were not responsible for the chilling-sensitivity of photosynthesis between 5 and 30-degrees-C. Instead, photosynthesis was gradually limited by an inefficient use of reduction equivalents. This, in turn may increase susceptibilty to photoinhibition, which occurred below 20-degrees-C in Vigna. The combined study of q(P) and of the different components of q(N) allowed the demonstration of the subsequent occurrence of different limiting processes with decreasing temperature in the chilling-sensitive Vigna species.