Effect of ammonium ions and cytokinins on hyperhydricity and multiplication rate of in vitro regenerated shoots of Aloe polyphylla

In search for the optimal culture conditions resulting in a high production of healthy plants and low occurrence of hyperhydricity in tissue cultured regenerants of Aloe polyphylla, we investigated the relationship between ammonium ions in the medium, applied cytokinins (CKs) and CK concentrations in the induction of hyperhydricity. Shoots were grown on media with different NH4+ concentrations (10.3, 20.6 and 61.8 mM) and supplemented with N-6-benzyladenine (BA), zeatin or thidiazuron (TDZ) at 0, 5 or 15 mu M. Elevating the levels of NH4+, in the absence of CKs, could not induce hyperhydricity. Similarly, very low hyperhydricity was observed when CKs were added to media containing low NH4+ (10.3 mM). However, in the presence of higher NH4+ concentrations, CKs increased hyperhydricity in a concentration-dependant manner, suggesting that they were capable of inducing this syndrome only when other factors in the culture system were not optimised. High numbers of healthy looking shoots were produced on media with low NH4+ and low BA or zeatin (5 mu M). The use of TDZ resulted in the formation of buds, which did not develop into shoots. Identifying the factors responsible for hyperhydricity is an important step in the successful use of the micropropagation technique for the conservation of this species.