Endomycorrhizal Synthesis by Gigaspora Margarita in Poinsettia

SUMMARY

The roots of poinsettia (Euphorbia pulcherrima) cuttings became mycorrhizal when inoculated with Gigaspora margarita. Maximum endomycorrhizal formation occurred following application of azygospores to both the rooting medium in the mist bed and, subsequently, to rooted cuttings at transplant. Cuttings inoculated in the mist bed or at transplant only, however, developed fewer mycorrhizae. Application of 125 ppm N-P-K per potted plant weekly favored mycorrhizal formation but there was no associated plant growth stimulation. Increasing N-P-K fertility to 250 ppm resulted in a mycorrhiza-induced growth response, but the plants eventually became chlorotic. Application of 125 ppm or 250 ppm N-P-K twice weekly significantly improved plant growth and appearance. At the lower rate, mycorrhizal synthesis was repressed, occurring only as scattered hyphae in the roots, and at the higher rate mycorrhizal synthesis was completely inhibited. Gigaspora margarita, in conjunction with Rootone, stimulated rooting of cuttings in the mist bed by markedly increasing the number and weight of roots over cuttings treated with Rootone alone. The mycorrhizal cuttings withstood transplant shock under high temperatures and low moisture conditions better than nonmycorrhizal cuttings. Thus, endomycorrhizal plants grew better than nonsymbiotic plants under conditions of low fertility and moisture stress, but the relationship appeared to be of minimal importance under cultural conditions favorable for the plant