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Studies on the Carotene-Destroying Processes in Drying Bean Leaves

1947; University of Chicago Press; Volume: 109; Issue: 2 Linguagem: Inglês

10.1086/335469

1940-1205

Leon Bernstein, John F. Thompson,

Antioxidant Activity and Oxidative Stress

1. The enzymic and photodestructive processes affecting the carotene content of bean leaves have been studied. By pretreating leaves (drying or freezing) and controlling the conditions of experimental exposure, we have been able to measure quantitatively the separate processes of carotene destruction in leaves in which both enzymic destruction and photodestruction have occurred simultaneously. 2. In partially dried leaves, carotene is destroyed rapidly both by enzyme- and by light-mediated processes until about 50% of the carotene is gone. The rates of destruction, especially the enzymic rate, then decrease, although in the light a further gradual decrease in carotene content has been observed. In autoclaved leaves, however, photodestruction continues at a high rate until 80-90% of the carotene is destroyed. In prefrozen leaves, incubated at 37⚬ C⚬. in the dark, 45% of the carotene is destroyed by enzymic action in 2 hours, but the remaining carotene is stable. Our results indicate that both the photodestructive and the enzymic processes are important in accounting for the losses of carotene which occur in field-curing of hay. 3. As a leaf dries out, the cells in the margin of the leaf suffer injury or death first, and carotene destruction, is initiated in progressively increasing numbers of cells. In tests of the enzymic destruction of carotenoids in leaves held at given moisture contents, maximum destruction of the pigments was found in leaves which had lost 80-95% of their water by predrying. In completely dehydrated leaves, no enzymic destruction of carotenoid pigments occurred. Photodestruction also becomes progressively greater in leaves which have decreasing moisture contents and reaches a maximum when about 68-80% of the water had been lost by predrying. Completely dehydrated leaves show a low but distinct rate of photodestruction. 4. Enzymic destruction, in the time interval used, is at a maximum at approximately 37⚬ C. and decreases at higher temperatures. The temperature coefficient (Q10) for enzymic destruction is 1.6-1.7 between 4⚬ and 25⚬ C., and is 1.2-1.3 for the hotodestructive process between 24⚬ and 64⚬ C. 5. Atmospheric oxygen is essential for enzymic destruction of carotenoids. In an atmosphere containing 0.02% oxygen or less, enzymic destruction does not occur, Photodestruction is also influenced by atmospheric oxygen, and the rate of photodestruction is proportional to the logarithm of oxygen concentration from about 0.5 to 20% oxygen. Below 0.5% oxygen, further decreases in oxygen tension do not appear to influence photo-destruction appreciably. It is suggested that some cellular constituents can replace atmospheric oxygen in the photodestruction of the carotenoid pigments or that the carotenoids are destroyed by photoreduction in the absence of oxygen. 6. Photodestruction of carotenoids is roughly proportional to light intensity up to approximately 300 foot-candles. Although photodestruction continues to increase at higher light intensities, the relationship is not the same as at low light intensities. Photodestruction has been shown to occur equally well in orange, yellow-green, and blue light, proving that the light energy is not absorbed directly and exclusively by the carotene. The influence of chlorophyll content has been investigated. The photodestruction of carotenols was found to be as great in etiolated leaves (having only 1-2% of the chlorophyll content of normal green leaves) as in green leaves. Furthermore, photodestruction occurred in orange light as well as in blue light in the etiolated leaves. 7. Naturally occurring antioxidant(s) have been demonstrated in bean leaves. An induction period is noted in the photodestruction of carotenols.