Growth and Nitrogen Fixation of Phaseolus vulgaris L. at Two Irradiances II. Nitrogen Fixation

Abstract

Phaseolus vulgaris cv. Glamis plants grown at 7 W m2 and 28 W m-2 in controlled environment cabinets showed copious nodulation and high levels of acetylene reducing activity. Early differences in nodulation were apparent before differences in photosynthesis and were attributed to an effect of far-red light on nodule development. Total plant nitrogen content was greater at 28 W m-2 than at 7 W m-2 but nitrogen content as a percentage of d. wt was greater at the lower irradiance level. Total acetylene reducing activity (nmol. min-1 root-1) was greater at 28 W m-2 than at 7 W m-2, but there was no irradiance effect on specific activity (nmol. min1 g d. wt of pink nodules-1 or nmol. min-1 pink nodule-1). Transfer of 40-day-old plants from 7 W m-2 to 28 W m-2 resulted in increased nodule size (due to increased size of infected cells), accompanied by increased total, but not specific, acetylene reducing activity. Transfer of plants from 28 W m-2 to 7 W m-2 resulted in a fall of total acetylene reducing activity within 24 h, and senescence of large nodules. Specific acetylene reducing activity was unaffected. The results are interpreted as an effect of light on the production of nitrogen fixing tissue, rather than on nitrogenase activity.