BIOMASS ALLOCATION AND NITROGEN DISTRIBUTION IN RYEGRASS UNDER WATER AND NITROGEN SUPPLIES

Ryegrass (Lolium perenne L.) in grassland is known to sustain with water and nitrogen (N). This study investigates biomass and N partitioning in plant organs (roots, main and the youngest tillers) under water-nitrogen interactions. Nitrogen was applied at the rates of 50 and 100 mg N kg-1 as N1 (low N) and N2 (high N) treatments, respectively, with uniform irrigation until 440 growing degree-days (GDD). Thereafter, the water supply was restricted to 50 mL on a weekly basis (W1) against 50 mL on a daily basis (W2) and concurrently, N enriched with 1 atom% 15N isotopes. Cumulative tillers' biomass increased linearly from 1st to 8th order, but thereafter reached a plateau with further increases in number of negligible weights. Initially tiller mass and number per plan did not differ (P 0.05) with water and/or N applications but changed at 788 GDD with clear differences at 911 GDD with the highest under N2W2 and lowest under N1W1. Nitrogen concentration sharply decreased from 530 to 700 GDD and then levelled off with age. The decline was more pronounced in tillers than roots. The high N treatment showed elevated N-concentration under both water treatments. Watering on a daily basis promoted vegetative growth. High water and N levels significantly (P 0.05) influenced concentration of N absorbed during 15N labeling (NL) in all organs with relatively pronounced NL under N2. The additive positive effect of W2 and N2 was obvious on NL as compared to NT, which showed that plants discriminate N-uptake on mass basis. Nitrogen (mobile) was higher in young and 15N (heavier) was low in young tillers and vice versa. Accumulation of N absorbed during 15N labeling (15NA) was significant knowing that water is a strong determining factor of N concentration in ryegrass organs.