The Effect of Nodulation Inducers on Alfalfa (Medicago sativa L.) Yield and Nodulation under Optimal and Salinity Conditions

Luteolin is one of the most important flavonoids, which release from seeds during the first four hours of imbibition. On the other hand, at present, salinity is one of the most important factors in reducing crop production. The results of some studies show that the use of external flavonoids increases expression of nod genes, yield and nodulation in some legumes species under stress conditions. Therefore, this experiment was conducted with the aim of investigating the effect of luteolin and alfalfa seed exudates external inducers on the expression of Rhizobium nod gene and the yield and nodulation of alfalfa in normal and salinity condition.

We studied the effect of luteolin on the induction of nod genes in R. Meliloti carrying a plasmid with a translational fusion between R. Meliloti nodA and lacZ of Escherichia coli, and the expression activity was measured by β-galactosidase activity. Luteolin strongly induced the expression of nod genes inhibitory effects. We further studied the effect of luteolin and Seed exudate on alfalfa (Medicago sativa L.) yield and nodulation under optimal and salinity condition. One greenhouse was conducted to determine whether the pre-incubation of Rhizobium meliloti with luteolin and application of luteolin and seed exudate directly on to the seed surface can increase alfalfa nodulation and yield. The factorial experiment was arranged based on randomized complete block design, with three replications. Treatments were two cultivars, two bacterial strains (Sensitive and resistant strains), two levels of salt (0 and 15 dS.m-1 of NaCl) and two levels of application of inducers along with control.

The results from this experiment clearly indicated that inoculation of alfalfa seeds with luteolin and seed exudate increase alfalfa nodulation (47%) and yield (30%) significantly under salinity condition that these treats affect on traits more under normal condition. But significant effect was not observed on chlorophyll content. In this experiment a significant difference was not observed between the control and pre-incubation of R. meliloti. Also, salinity increased proline four times compared to normal condition. Luteolin and seed exudates increased proline 1.66 and 1.35 times respectively in salinity condition, but under normal condition they did not have significant effect. Salinity increased the content of sodium in the leaves (7 times) and roots (8.5 times) and decreased the content of potassium 29% and 24% in these organs of the plant, respectively, that, luteolin and seed exudate partially moderated these changes.

The results of this experiment indicated that luteolin and seed exudate can be used as exogenous inducers to improve the growth and nodulation of alfalfa under salinity and normal condition. Flavonoid inducers act in low concentration, and their negative effects relate to reduced germination and growth. In general, it seems that the application of flavonoid inducers is more suitable to improve nodulation, but the direct application of them in agriculture should be used only in specified cases and when flavonoids are as a limiting factor.