51. Which one of the following statements about phytohormone ABA is correct?
    (1) High level of ABA predominantly promotes vivipary.

(2) ABA-Beeta-D-glucosyl ester is an active form of ABA.

(3) Inactivation of ABA involves its oxidation to phaseic acid.

(4)ABA biosynthesis occurs entirely in the plastids.

Correct answer: (3) Inactivation of ABA involves its oxidation to phaseic acid.

Explanation:

• (1) High levels of ABA do not promote vivipary; actually, low ABA levels are associated with vivipary (precocious germination) in seeds.

• (2) ABA-β-D-glucosyl ester is an inactive conjugated storage form of ABA, not an active form. It can be converted back to active ABA by enzymatic hydrolysis.

• (3) ABA is inactivated primarily through oxidation, which converts ABA to 8′-hydroxy ABA that spontaneously isomerizes to phaseic acid, a less active form. This is the main catabolic pathway for ABA in plants.

• (4) ABA biosynthesis occurs partly in plastids and partly in the cytosol; it is not entirely synthesized in plastids.

  ABA inactivation is crucial for regulating its levels and activity in plants, primarily occurring via oxidative conversion to phaseic acid, balancing its role in seed dormancy, stress responses, and development.