Nitrate stimulation of mobilization of seed reserves in temperate cereals: Importance of water uptake

Relationships between nitrate (NO3-) supply, uptake and assimilation, water uptake and the rate of mobilization of seed reserves were examined for the five main temperate cereals prior to emergence from the substrate. For all species, 21 d after sowing (DAS), residual seed dry weight (d.wt) decreased while shoot plus root d.wt increased (15-30%) with increased applied NO3- concentration from 0 to 5-20 mM. Nitrogen (N) uptake and assimilation were as great with addition of 5 mM ammonium (NH4+) or 5 mM NO3- but NH4+ did not affect the rate of mobilization of seed reserves. Chloride (Cl-) was similar to NO3- in its effect on mobilization of seed reserves of barley (Hordeum vulgare L.). Increased rate of mobilization of seed reserves with additional NO3- or Cl- was associated with increases in shoot, root and residual seed anion content, total seedling water and residual seed water content (% water) 21 DAS. Addition of NH4+ did not affect total seedling water or residual seed water content. For barley supplied with different concentrations of NO3- or mannitol, the rate of mobilization of seed reserves was positively correlated (r > 0.95) with total seedling water and residual seed water content. The rate of mobilization of seed reserves of barley was greater for high N content seed than for low N content seed. Seed water content, was greater for high N seed than for low N seed, 2 DAS. Additional NO3- did not affect total seedling water or residual seed water content until 10-14 DAS. The effects of seed N and NO3- on mobilization of seed reserves were detected 10 and 14 DAS, respectively. It is proposed that the increased rate of mobilization of seed reserves of temperate cereals with additional NO3- is due to increased water uptake by the seedling while the seed N effect is due to increased water uptake by the seed directly. (C) 1996 Annals of Botany Company