PHOTOSYNTHESIS AND NITROGEN-FIXATION IN RED-CLOVER GROWN UNDER DIVERSE CONDITIONS OF IRRADIANCE AND NITROGEN NUTRITION

Red clover (Trifolium pratense L.) plants were grown in water culture either containing or lacking nitrate. Plant growth, chlorophyll, starc h, sugars and protein content in leaves, as well as the rates of symbi otic nitrogen fixation and root respiration, were examined in relation to diverse illumination treatments. In the the presence of exogenous nitrate, plant growth was enhanced by increasing irradiance from 30 to 100 W/m2. In the plants deprived of nitrate, as compared to the plant s that received it, growth was inhibited at all light intensities stud ied, especially at 100 W/m2; the protein and chlorophyll content, on a leaf dry weight basis, was lower, whereas the starch level was higher , the rate of symbiotic nitrogen fixation was considerably greater (so metimes by two orders of magnitude) and was independent of the irradia nce changes studied. The low values for soluble sugars to starch ratio s and the higher ratios for non-structural carbohydrates to protein an d chlorophyll in plants deprived of exogenous nitrate are considered t o be related to nitrogen deficiency, despite the high rate of nitrogen fixation. The reduction in clover growth in the absence of combined n itrogen is suggested to result from the release of nitrogen fixation p roducts from nodules to plant shoots rather than from the disturbance in photosynthesis and photosynthate translocation to roots.