Cytokinins in Lupinis albus L.
A review of the literature pertaining to the sites of synthesis, distribution and metabolism of cytokinins, established that there was a need to investigate the quantitative and qualitative changes in cytokinin levels during the growth cycle of a whole plant. An annual legume, Lupinus albus L. (the white lupin), was selected as the experimental plant for such a study. The levels of cytokinin activity in the root exudate, leaves and terminal (primary) apices of mature plants were examined between 5 and 17 weeks after planting. The results indicated that the total level of activity in the root exudate of a single plant is much greater than that which accumulates in the leaves and apices up to the time of flowering. This was considered to be indicative of rapid utilization of cytokinins in the actively growing shoot. It was also established that, after flowering, the level of activity in the terminal apices increases markedly, while that in the root exudate decreases. The latter finding was examined more closely in the next experiment, during the course of which cytokinin levels in the root exudate, leaves, fruits and fruit sap were determined at 2 weekly intervals after flowering. As in the previous experiment, the results showed that, although there may be a slight increase in the amount of activity in the root exudate, this activity was insufficiently high to account for the levels of cytokinin which accumulate in the fruiting apices. Cytokinin activity was however detected in the sap passing into the fruits (largely phloem sap), thereby indicating that at least some of the cytokinins detected in both pod wall and seed extracts could have originated in other parts of the plant. Embryonic development was followed at the gross morphological, anatomical and ultrastructural levels, and estimates were made of the cytokinin levels in the embryos and adjacent tissues. The results indicated that the cytokinin levels in the flowers, and young fruits, were low, but gradually increased with time. At 2 weeks after anthesis the level of activity in the pod wall was higher than that in the individual seeds. By 6 weeks, however, the activity in each seed was in excess of that recorded in the pod wall. Decreasing levels of activity in the seeds, and increasing levels in the pod wall, reversed this trend at later developmental stages. The level of activity recorded in the maturing embryo was low, while in the adjacent tissues it was high. This was especially noticeable at 6 weeks after anthesis when the cytokinin activity in the endosperm reached a high level. It would appear that these cytokinins facilitate nutrient mobilization to the tissues surrounding the embryo, and that they are possibly utilized in order to promote certain growth processes taking place in the embryonic cells during development. The cytokinin activity in the mature embryo was low, but increased slightly in the cotyledons, radicles and young shoots during germination and seedling establishment. The cytokinins present in all white lupin extracts had chromatographic properties, on paper and/or Sephadex LH-20, similar to the naturally occurring cytokinins zeatin and zeatin riboside and their glucosylated or phosphorylated derivatives. Mass spectrometric-analysis of the compound co-eluting with zeatin showed that this cytokinin was present in white lupin extracts, while analysis of the peak co-eluting with zeatin riboside indicated that the cytokinin at this elution volume was an adenine derivative. The possible presence of dihydroderivatives of these cytokinins was also indicated. The results did not exclude the possible presence of other cytokinin-like compounds. The significance of qualitative as well as quantitative changes in cytokinin activity were assessed. It is concluded thatcytokinins do not accumulate at excessively high levels in actively growing tissues, unless these tissues function as a transitory store for nutrients, as for example the endosperm, testa and pod wall. Cytokinins are probably involved in the control of a number of growth processes of which nutrient mobilization may be of considerable importance to the plant.