Nitrogen and Growth Regulator Effects on Shoot and Root Growth of Soybean in vitro
Effects of different nitrogen sources and growth regulator concentrations on expiants from soybean seed embryos [Glycine max (L.) Merr. cv. Braxton] were studied to identify reasons for the general lack of regeneration of soybean plants from callus. The nitrogen source supplied in the agar medium was critical to shoot growth. Ammonium stunted shoot growth unless high relative concentrations of nitrate were also supplied. However, stunting of shoots was reversible upon transfer to a medium more promotive to growth, i. e., low ammonium, high nitrate. Actively growing shoots were also adversely affected by a high ammonium concentration as indicated by cessation of apex growth and severe leaf senescence. A test of 4 diverse genera showed that ammonium/nitrate inhibition of shoot growth was neither limited to soybean, nor common to all legumes. Depressed pH of the medium due to high ammonium/nitrate ratios could not account for decreased shoot growth. While glutamine, substituted for ammonium, did not depress shoot growth, nitrate was still required for growth to proceed. Root initiation was prolific on all nitrogen combinations tested, but root growth was reduced by ammonium and promoted by nitrate. Glutamine had little effect on root initiation or growth.