Flowering and Endogenous Levels of Plant Hormones in Lemna Species

The occurrence and endogenous level of various plant hormones were measured for the short-day plants Lemna paucicostata 151 and 381 and the long-day plant Lemna gibba G3 to determine whether any of them are involved in the photoperiodic control of flowering. ABA, IAA, GA1, GA29, GA34, GA53, trans- and cis-zeatin, trans- and cis-ribosyl zeatin, N6-(Δ2-isopentenyl) adenine and N6-(Δ2-isopentenyl) adenosine were definitely detected in each species, while GA4 was only detected in L. gibba G3 and GA20 was only detected in L. paucicostata 151. The endogenous levels of ABA and IAA were in the range of 1–7 ng/g fr wt and were not significantly different in vegetative and flowering plants. The endogenous gibberellin levels were generally higher in Lemna grown under long-day rather than short-day conditions. The endogenous cytokinin levels were almost the same in both flowering and vegetative plants of L. paucicostata 151 and 381. In L. gibba G3, however, the level of cis-ribosyl zeatin, N6-(Δ2-isopentenyl) adenine and N6-(Δ2-sopentenyl) adenosine were higher in vegetative than in flowering plants. These results indicate that there is not necessarily a direct relation between endogenous plant hormone levels and flowering, and that the chemical basis for the photoperiodic control of flowering cannot be explained solely by changes in hormone levels. The possibility remains, however, that one or more of the plant hormones has some influence of secondary importance on the flowering process in Lemna.