lemon and orange. . Physiological Effects • Ethylene stimulates respiration and ripening in fruits. • It breaks the dormancy of buds, seeds and storage organs.
• It stimulates formation of abscission zone in leaves, flowers and fruits. This makes the leaves to shed prematurely. • Inhibition of stem elongation (shortening the internode). • Growth of lateral roots and root hairs.
This increases the absorption surface of the plant roots. • Ethylene normally reduces flowering in plants except in Pine apple and Mango. . .
Abscisic Acid (ABA) (Stress Phyto hormone) . Discovery In , the hormone was first isolated by Addicott et al., from young cotton bolls and named as Abscission II . Eagles and Wareing during – isolated a dormancy inducing substance from leaves of Betula and called it as dormin. In , it was found by Cornsforth et al ., that both dormin and abscission are chemically same compounds and called Abscisi c Acid (ABA) .
. Site of Photoinductive perception Photoperiodic stimulus is perceived by the leaves. Floral hormone is synthesised in leaves and translocated to the apical tip to promote flowering. This can be explained by a simple experiment on Cocklebur ( Xanthium pensylvanicum ), a short day plant.
Usually Xanthium will flower under short day conditions. If the plant is defoliated and kept under short day conditions it will not flower. Flowering will occur even when all the leaves are removed except one leaf. If a cocklebur plant is defoliated and kept under long day conditions, it will not flower.
If one of its leaves is exposed to short day condition and rest are in long day condition, flowering will occur (Figure . ). A B C D E F Short Day Long Day Short Day Figure . : Experiment on Cocklebur plant showing photoperiodic stimulus .
Importance of photoperiodism . The knowledge of photoperiodism plays an important role in hybridisation experiments. . Photoperiodism is an excellent example of physiological pre-conditioning that is using an external factor to induce physiological changes in the plant.
. Phytochrome P 660nm 730nm Physiological response X