Exogenous Applications of Abscisic Acid Increases ‘Micro' Tomato Leaf Chlorophylls and Carotenoids

Carotenoids are secondary plant compounds divided into two groups; the oxygenated xanthophylls such as lutein, zeaxanthin, and violaxanthin, and the hydrocarbon carotenes such as b-carotene, a-carotene, and lycopene.  Within the thylakoid membranes of chlorophyll organelles, carotenoids are bound to specific protein complexes of photosystems I and II.  Carotenoids function in harvesting light energy during photosynthesis and dissipation of excess solar energy.  When the absorption of light reaches saturation, excess excitation energy can result in the formation triplet excited chlorophyll (³Chl) and reactive singlet oxygen (¹O₂).  Carotenoid pigments protect photosynthetic structures by quenching ³Chl and by binding ¹O₂ to inhibit oxidative damage.  Abscisic acid (ABA) plays a regulatory role in plant growth and development, seed dormancy, and stress responses.  Abscisic acid is an apo-carotenoid derived from the same isoprenoid pathway as chlorophyll and carotenoid pigments.  Our objective in this study was to determine the effects of exogenous ABA on chlorophyll and carotenoid concentrations in ‘Micro-Gold’ tomato leaves.  ‘Micro-Gold’ tomatoes were grown in 10-L reservoirs filled with nutrient solutions.  Abscisic Acid was applied to the solutions at flower initiation at concentrations of 0.0 (Control), 0.5, 5.0, and 10.0 mg ABA·L^-1.  Leaves were harvested at fruit ripening and freeze dried prior to HPLC pigment analysis.  Total chlorophylls and carotenoids were significantly increased (66% and 73%, respectively) with ABA.  Chlorophyll a ranged from 7.7 to 12.4 µg·g^-1 DW and chlorophyll b ranged from 4.3 to 5.7 µg·g^-1 DW.  Chlorophylls a and b, Lutein (1.14 to 1.49 µg·g^-1 DW), antheraxanthin (0.2 to 0.3 µg·g^-1 DW), neoxanthin (0.4 to 0.6 µg·g^-1 DW), and oxygenated carotenoids concentrations were all positively correlated with ABA treatments.  This data suggests that the application of the plant growth regulator ABA may act to induce a stress response in tomato, as evident by increasing concentrations of photosynthetic and protective leaf pigments.