Studies on the Carotene-Destroying Processes in Drying Bean Leaves

Abstract

By pretreating (vacuum-drying, freezing, or autoclaving) matched half-leaves of Red Kidney bean plants (Phaseolus vulgaris) and incubating them in the dark and in the light, the effects of varying external and internal conditions on the photodestruction (P.D.) and enzymic destruction (E.D.) of carotene were investigated. Maximum E.D. and P.D. were found in leaves which had lost approx. 85% and 75% of their water, respectively. E.D. at 37[degree]C was rapid during the first hr., declined and virtually ceased after 40-50% of the carotene had been lost. Although P.D. in predried leaves also decreased with time, it continued at a slow rate after E.D. had ceased. In autoclaved leaves, P.D. continued at a high rate until 80-90% of the carotene had been destroyed. Q10 values for E.D. and P.D. of carotenoids are 1.6-1.7 (4-25[degree]C) and 1.2-1.3 (24-64[degree]C), respectively. Atmospheric O2 was found to be essential for E.D. of carotene, since no loss occurred in atmospheres containing 0.02% O2 or less. P.D. of carotenoids was shown to be proportional to logarithm of O2 tension from approx. 0.5 % to 20% O2. Below 0.5% O2, P.D. did not appear to be significantly influence 1 by O2 tension although P.D. was still approx. 20% of the controls in air. Amt. of P.D. was shown to be approx. proportional to light intensity up to 300 ft.-candles. P.D. occurred in orange and yellow-green light as well as in blue, proving that light energy was not absorbed directly and exclusively by carotenoids. Etiolated leaves as well as normal green leaves lost carotenoids in light of these 3 spectral regions. Naturally-occurring H2O-soluble substance(s) in bean leaves were shown to act as antioxidant(s) in retarding P.D.