COMPENSATORY GROWTH-RESPONSE OF THE LEGUME, MEDICAGO-SATIVA, TO DEFOLIATION AND DENODULATION

A laboratory study was conducted to determine the effects of defoliati on and denodulation on compensatory growth of Medicago sativa (L.). Pl ants grown hydroponically in clear plastic growth pouches were subject ed to 0 and 50% nodule pruning, and 0, 25, 50, and 75% defoliation by clipping trifoliate leaves. An additional experiment was conducted to determine if clipping leaves simulated herbivory by Hypera postica (Gy llenhal) larvae. Previously, we determined that nodule pruning accurat ely simulated herbivory by Sitona hispidulus (L.) larvae (Quinn & Hall , 1992). Results indicated that denodulation stimulated nodule growth and caused exact compensation in standing and total number of nodules per plant within 15 days and in standing nodule biomass within 22 days of treatment. Denodulation caused a significant reduction (13%) in fi nal shoot biomass, but did not affect significantly final root biomass . Percentage of change in number of trifoliate leaves per plant increa sed with the level of defoliation. Within 22 days of treatment, total number of trifoliate leaves per plant was similar to controls. However , final standing shoot biomasses were significantly less that controls , indicating undercompensatory growth. Shoot biomasses of the 25-, 50- , and 75%-defoliated plants were 18, 20, and 36% lower than controls, respectively. Nodule biomass per plant was reduced by 24 and 32% in 50 - and 75%-defoliated plants, respectively, but was not affected signif icantly by 25% defoliation. Root biomass was affected by all levels of defoliation. Clipping trifoliate leaves accurately simulated defoliat ion by H. postica larvae. Our results indicated that partial defoliati on affected shoot, root, and nodule biomass of M sativa, but that part ial denodulation only affected shoot biomass.