Combined Nitrogen and Vegetative Regrowth of Symbiotically‐grown Alfalfa¹

Abstract

Few studies clearly compare N source effects on herbage yield of alfalfa (Medicago sativa L.) grown with Rhizobium over several harvest cycles. Therefore, the effects of combined N on nodulation, growth, N accumulation, and development of alfalfa were studied over four harvest/regrowth cycles with a genotype cloned from the nondormant cv. ‘Dohfari.’ Ramets inoculated with Rhizobium meliloti Dang. strain 102F28 and irrigated with 0,2,8, or 16 mM N provided as NH4NO3, NO3‐, or NH4+ were grown in vermiculite in a naturally illuminated greenhouse in two replicate experiments from March through June and July through October. All combined N treatments enhanced herbage dry weight and total crude protein above the nil N control during the first growth period. The promotive effect of combined N decreased with each harvest until during the fourth regrowth, plants totally dependent on Rhizobium for reduced N produced herbage and crude protein yields equivalent to plants supplied 8 or 16 mM N as NH4NO3. Plants grown with 2mM N, a more reasonable N concentration for the soil solution, were more productive than nil N plants during the first two growth cycles but had no advantage during the third and fourth cycles regardless of the form of combined N used. Nitrate suppressed nodule development more than either NH4NO3 or NH4+. Greater availability of combined N increased the soluble sugar:starch ratio in the root and crown tissues. No significant differences were observed between the two experiments except that plants grown later in the year (July to October) took longer to flower during each regrowth cycle. The data indicate that the Rhizobium symbiosis cannot produce sufficient reduced N for optimum alfalfa growth during the first few harvest/regrowth cycles. During subsequent regrowth cycles, however, plants totally dependent on Rhizobium for N compounds had overcome the period of N‐limited growth and produced as much herbage and crude protein as plants grown with levels of combined N that might be expected under normal field conditions.

A Chalcone and Two Related Flavonoids Released from Alfalfa Roots Induce nod Genes of Rhizobium meliloti

“…Kapulnik et al (14) observed significant increases in nodulation, N2 fixation, and seedling growth of 'Hairy Peruvian' alfalfa when the rhizosphere was supplemented with luteolin. This is consistent with evidence that some N2-dependent alfalfa seedlings are initially N-limited due to insufficient root nodule formation (10) and suggests that more nodules might be formed if more rhizobial cells are induced to initiate the infection process.…”

Nitrogen Reserve Mobilization during Regrowth of Medicago sativa L. (Relationships between Availability and Regrowth Yield)

“…3 and 4). This effect of low N supply on starch accumulation was previously noted by Fishbeck and Phillips (1981). They showed that in nodulated alfalfa, greater availability of combined N decreased the starch content in roots and crown tissues but increased shoot yield.…”

Flavone Limitations to Root Nodulation and Symbiotic Nitrogen Fixation in Alfalfa

“…flowering and the plants are allowed to regrow, the differences in N assimilation between N2-dependent plants and those supplied with 8 mm nitrate disappear (8). Thus, one might suggest that earlier or more prolific formation of effective root nodules could promote N2 fixation and plant growth.…”

“…There is clear evidence in alfalfa (8) Analyses. Dry weight, Kjeldahl N analyses, and '5N determinations by mass spectrometry were done as reported previously (17,18).…”