Abscisic acid

As mentioned earlier, abscisic acid (ABA) was discovered for its role in regulating abscission and dormancy. But, like other PGRs, it also has other wide-ranging effects on plant growth and development.

The main roles of ABA are:

• acting as a general plant growth inhibitor and an inhibitor of plant metabolism
• inhibiting seed germination
• stimulating the closure of stomata and increasing the tolerance of plants to various kinds of stresses — for which reason ABA is also called the stress hormone
• playing an important role in seed development, maturation and dormancy

By inducing dormancy, ABA helps seeds to withstand desiccation and other factors unfavourable for growth. In most situations ABA acts as an antagonist to the gibberellins (GAs): where one tends to promote a process, the other tends to inhibit it. Overall, for any and every phase of growth, differentiation and development, one or another PGR — sometimes several acting together, complementarily or antagonistically, individually or synergistically — has a role to play. …