Growth and Development of Horticultural Crops (1+1)

Occurrence and distribution of ABA in plants

• Within the plant, ABA has been detected in all major organs or living tissues from root caps to apical buds such as roots, stems, buds, leaves, fruits and seeds and also in phloem and xylem sap and in nectar. ABA is synthesized in all types of cells that contain chloroplasts or other plastids.
  
• It occurs predominantly in mature green leaves. Most plant tissues contain ABA in concentration of 20-100 ng per g fresh weight, but higher conc. of 10 μg and 20 μg per g fresh weight have been reported in avocado fruit pulp and dormant buds of cocklebur (Xanthium spp.) respectively.
  
• The concentration of ABA in specific plant tissues varies greatly at different developmental stages or in response to environmental conditions especially water stress. For instance, in developing seeds ABA conc. may increase 100 fold within a few days and decline as the seed matures. Similarly, under water stressed conditions, ABA level may increase 50 fold in the leaves within a few hours and declines to normal when plant water potential is restored.
  
• The concentration of ABA in plant tissue is regulated by
  
(i) its synthesis,
(ii) degradation,
(iii) compartmentation and
(iv) transport.
• In plant, ABA predominantly occurs in its free form but it may also occur in conjugated form as glycoside with some simple sugar molecule such as glucose forming ABA-β-D-glucosyl ester. ABA is biologically inactive in its conjugated or bound form.