EFFECT OF PLANT-AGE AND SOWING DENSITY ON NITROGEN-FIXATION OF MEDICAGO-TRUNCATULA AT 2 DIFFERENT DATES OF SOWING

An acetylene reduction (AR) assay was used to measure nitrogenase activity of soil cores as an estimate of the rate of nitrogen (N-2-) fixation in nodulated Medicago truncatula cv Jemalong in the field on several occasions during the growing season. The effects of date of sowing and of varying the density of sowing from 0 to 1 000 kg ha(-1) seed were investigated. Results showed an increase in AR activity with increasing medic sowing densities during the vegetative period of plant growth, whereas towards maturity AR activity was slightly higher at low than at medium or high densities. Soil core AR activity was better correlated with the weight of shoot dry matter (DM) present at the time of harvest during the vegetative period of plant growth than after flowering. At all sowing densities and irrespective of sowing date, AR activity increased from sowing to full flowering and thereafter decreased rapidly. The post-flowering decline in AR activity was sharper at the high than at the lower densities and coincided with a marked decrease in plant population in the medium and high-sowing density treatments. The relations between AR activity, shoot dry weight and plant number per unit area in the pre-flowering and post-flowering periods were examined via linear regression analysis.