Responses of wild plants to nitrate availability : Relationships between growth rate and nitrate uptake parameters, a case study with two Bromus species, and a survey.

Two annual species of Bromus, an invader (B. hordeaceus, ex B. mollis) and a non-invader (B. intermedius), were grown for 28 days in growth chambers, at 5 and 100 μM NO 3- in flowing nutrient solution. No differences between the two species were observed at either NO 3- level, in terms of relative growth rate (RGR) or its components, dry matter partitioning, specific NO 3- absorption rate, nitrogen concentration, and other characteristics of NO 3- uptake and photosynthesis. The effects of decreasing NO 3- concentration in the solution were mainly to decrease the NO 3- concentration in the plants through decreased absorption rate, and to decrease the leaf area ratio through increased specific leaf mass and decreased leaf mass ratio. Organic nitrogen concentration varied little between the two treatments, which may be the reason why photosynthetic rates were not altered. Consequently, RGR was only slightly decreased in the 5-μM treatment compared to the 100-μM treatment. This is in contrast with other species, where growth is reduced at much higher NO 3- concentrations. These discrepancies may be related to differences in RGR, since a log-linear relationship was found between RGR and the NO 3- concentration at which growth is first reduced. In addition, a strong linear relationship was found between the RGR of these species and their maximum absorption rate for nitrate, suggesting that the growth of species with low maximum RGR may be partly regulated by nutrient uptake.