Effect of abscisic acid on nitrate uptake, respiration and photosynthesis in green algae

In the unicellular green algae, Chlorella fusca and Ankistrodesmus (Monorhaphidium) braunii, nitrate uptake in the light as in the dark was stimulated by physiological concentrations (20 μM) of abscisic acid (ABA), up to 200% by 500 μM. Dark respiration was increased to a similar extent by low ABA concentrations, but up to 13-fold by 500 μM ABA. Addition of 50 mM glucose raised the respiration level to approx. 3-fold, but did not alter the ABA stimulation. Photosynthetic O₂ evolution showed only insignificant inhibition by ABA at high concentrations. The intracellular level of glucose, not that of sucrose, was elevated in the presence of ABA. This suggests that increased starch degradation may be involved in the stimulation of respiration and nitrate metabolism by ABA, rather than direct interference with the plasmalemma. The data show an action of ABA in algae although to our present knowledge, it is not produced in their cells.