Why does giving barley more nitrogen stop increasing its growth beyond a certain point?

Details

Nitrogen availability and plant development both affect plants’ relative growth rate (RGR). This study of barley (Hordeum vulgare L.) examined which growth parameters respond to nitrate (N) availability and whether differences in plant size, reflecting differences in growth, confounded those responses.

Plants were grown hydroponically at six different nitrate concentrations for 28 days. Researchers performed nine harvests to assess how N supply affected growth parameters. Most growth parameters showed similar response patterns to N supply whether plants were compared at the same time points or at the same plant sizes.

N supply significantly affected biomass allocation: increasing N increased leaf mass ratio (LMR), the fraction of biomass in leaves, and decreased root mass ratio (RMR), the fraction in roots. Specific leaf area (SLA) was not significantly affected by N.

RGR increased as N supply increased up to 1 mM, associated with increases in both LMR and net assimilation rate (NAR). Increasing N supply above 1 mM did not increase RGR, because increases in LMR were offset by decreases in NAR.

The high RGR under suboptimal N supply suggests higher nitrogen use efficiency, expressed as biomass/N supply. The study discusses reasons for the maintenance of stable growth under suboptimal N levels.