Effect of Daylength on the Ability of Salicylic Acid to Induce Flowering in the Long-day Plant Lemna gibba G3 and the Short-day Plant Lemna paucicostata 6746.

When the long-day plant Lemna gibba L., strain G3 is grown on control medium the critical daylength is just under 10 hours. With 3.2 micromolar salicylic acid added to the medium substantial flower promotion is obtained on 9-, 10-, and 11-hour daylengths. On an 8-hour daylength salicylic acid treatment results in only a very small flowering response, and with daylengths less than 8 hours flowering is never obtained. Thus, salicylic acid treatment causes a shift in the critical daylength of about 2 hours, from just under 10 hours to just under 8 hours.Salicylic acid treatment also results in flower promotion in the short-day plant Lemna paucicostata Hegelm., strain 6746, and the optimal concentration of 1.8 to 3.2 micromolar agrees well with the results for L. gibba G3. In control medium the critical daylength for L. paucicostata 6746 is about 14 hours. When the plants are given 3.2 micromolar salicylic acid substantial flower promotion is obtained on daylengths of 13 and 14 hours, and the critical daylength is close to 15 hours. With daylengths longer than 15 hours, flowering is never obtained. These results are similar to those for L. gibba G3 since for both plants salicylic acid stimulates flowering by causing a shift in the critical daylength curve, but for L. paucicostata 6746 the shift is only about 1 hour and the critical daylength is extended rather than shortened.