Dark treatment effects on symbiotic nitrogen fixation and related processes in Cicer arietinum L. (Chickpea)

Abstract

Seven-week-old chickpea plants grown in earthenware pots were given dark treatment by shifting them to a dark room after they had received a 9 hr exposure to natural day light. The plants were retained in the dark till the end of the experimental period, while a set of plants growing under natural daylength (average 10–11 hr) served as control. A dark period of 18 hr was sufficient to result in a 69% decrease in the rate of nitrogen fixation as determined by an acetylene reduction bioassay. With prolongation of the dark period to 48 hr, the nitrogenase activity was decreased to 10% of the control value. However, the decrease in nodule respiration was not correspondingly sharp. Dark treatment of 18–66 hr resulted in 34–68% decrease in nodule respiration, respectively. During the same period nodule carbohydrates decreased only by 25–40%. Dark treatments led to an accumulation of free amino acids in the nodules. This accumulation appeared to be mainly due to protein hydrolysis. A dark period of 18 hr resulted in a small decrease in the nodule leghemoglobin content, which, however, remained unaffected with further prolongation of the dark period. It appears that the metabolic factors investigated are unable to explain such a sharp decline in the rate of N-fixation. It appears that some other critical and faster acting factor(s) is (are) involved.