The investigation of some biochemical factors which may govern the rootability of Rhododendron stem cuttings

Abstract

The present study was conducted to compare the qualitative and quantitative differences in some of the biochemical parameters which may be involved in the variations in rootability of Rhododendrons. R. ponticum was taken as a representative of the easy-to-root category while R. Brittania and R. Jan Dekens were taken as representatives of the difficult-to-root variety.

The hormonal situation in R. ponticum was characterized by a high ABA, a low cytokinin and a low GA content. Such a situation has been reported to be beneficial to adventitious root formation. The difficult rootability of R. Brittania may be caused by a low ABA, a high cytokinin and a high GA content. The amounts of 'free' and acidic 'bound' IAA in the two plants were quite similar.

Rooting potential was positively correlated to the rooting cofactor activities. Paper chromatography separated extracts of R. ponticum and R. Brittania into four rooting cofactors but R. Brittania extracts showed reduced activities in such rooting cofactor regions. Further chromatographic purifications coupled with bioassay revealed a greater number of promoters in R. ponticum but a greater number of inhibitors in R. Brittania. By means of column and thin-layer chromatography, and ultra-violet and mass spectroscopies, simiarenol, ∝-amyrin, β -sitosterol and uvaol were identified as component promoters of cofactor 4 of both R. ponticum and R. Brittania. In addition two active unknowns, Yellow(0.71) and Violet(0.66) were detected in the cofactor 4 fractions. Vanillic, protocatechuic and tannic acids were identified as phenolic inhibitors of R. Brittania. These were present to a lesser extent or absent in R. ponticum.

Rootability cannot be accounted for in terms of nutrition. Most of the differences in the nutritional factors studied were marginal and the total carbohydrate content was found to be higher in the difficult-to-root R. Jan Dekens. The higher starch contents in R. Jan Dekens were not due to a lack of starch hydrolytic enzymes.

Striking qualitative and quantitative differences were observed in some oxidative enzyme fractions. The R. ponticum extracts showed higher in vitro PPO and PO activities and a greater number of PPO and PO isozymes than the R. Jan Dekens extracts. This may suggest a possible involvement of phenol oxidation. Very little IO activities were detected.

All these biochemical factors were subjected to seasonal fluctuations and it is likely that a proper balance of such factors is responsible for adventitious root formation.

Using the mung bean hypocotyl as a model system, the effects of hormones, catechol and selected promoters and inhibitors from R. ponticum and R. Brittania on the IO, PPO and PO enzyme systems were studied. The results did not support the involvement of a derivative of IAA oxidation in adventitious root formation. Only the PO system appeared to be under hormonal formation. The implications of these findings were discussed in the light of current theories of root induction.