COMPONENTS OF RELATIVE GROWTH-RATE AND NITROGEN PRODUCTIVITY OF BRUSSELS-SPROUTS AND LEEKS GROWN AT 2 WIDELY DIFFERING LIGHT INTENSITIES

Early in the season, Brussels sprouts have a higher rate of biomass pr oduction than leeks at recommended fertilizer (nitrogen) application r ates. Furthermore, they have a considerably higher annual biomass prod uction and a higher annual biomass production per unit of nitrogen tak en up. In this paper we explore and discuss the possible explanations for these field observations, using information obtained from growth c hamber studies and the literature. Young vegetative plants of Brussels sprouts and leeks were grown in a growth chamber at a light intensity of 550 mu mol m(-2) s(-1) and at a light intensity which was 20% of t hat of the high-light treatment. At both light treatments, Brussels sp routs had an approximately 90% higher relative growth rate than leeks. At both light treatments the higher relative growth rate of Brussels sprouts was mainly explained by its higher leaf area ratio. Only minor differences in the physiological component, the net assimilation rate , between the two species within a light treatment were observed. The higher leaf area ratio of Brussels sprouts was mainly explained by its higher specific leaf area. Brussels sprouts had a higher rate of biom ass production pet. unit of internal nitrogen (nitrogen productivity) than leeks. This was mainly explained by a higher allocation of nitrog en to leaves and a higher rate of biomass production per unit leaf nit rogen. We suggest that the higher biomass production per unit of nitro gen taken up of Brussels sprouts compared to that of leeks as observed in the field is explained by its higher nitrogen productivity. The re sults obtained from the growth chamber studies are discussed in relati on with field experiments and data from the literature.