Response to low temperature in dinitrogen fixing soybeans

Dinitrogen fixation is an energy intensive process and photosynthesis in soybean (Glycine max L.) adapts to the carbon requirements of the nodules. Chlorophyll fluorescence measurements were used to assess the photochemical response of soybean (cv. Maple Arrow) fed exclusively on either mineral or symbiotic nitrogen, to a 12-day-long low temperature stress. In the nodule bearing plants, N-2 fixation resulted in an earlier decrease in the photochemical efficiency of open reaction centers, F-v/F-m (dark-adapted leaves), and in the photochemical efficiency of PSII at actual closure of reaction centers (Phi PSII), which reflected the occurrence of earlier leaf senescence. As a result of the specific effect of the low temperature stress, the nitrogen fixing plants exhibited a stronger decrease in F-v/F-m, ratio, with a higher increase in F-0 (minimum chlorophyll fluorescence yield) than the plants supplied with nitrate. The lack of enhancement of the non-photochemical quenching NPQ with the low-temperature treatment and the persistance of high F-0 values after return to optimum growth temperature suggested the occurrence of a low photoprotective ability accompanied by photodamage in the N-2 fixing plants. The application of oversaturating irradiance during the low temperature treatment corroborated the greater susceptibility of the N-2 fixing plants to photoinhibition. If the energy cost of symbiosis can be met by improved photosynthesis, accelerated leaf ageing and higher susceptibility to photoinhibition under stress reduce N-2 fixation-related plant performances. (C) 1997 Elsevier Science Ireland Ltd.