Chlorophyll synthesis requires light. Chlorophyll and most pigments are in a balance between synthesis and destruction. Placing fruit in darkness will result in bleaching of the green color. The result might be white or yellow fruit depending on which pigments are present. Light exclusion is commercially practiced by placing bags over developing fruit. One desirable result is the loss of green color, making the fruit look brighter. The bags must be changed or removed to allow light for anthocyanin synthesis. Controlled atmosphere stor age, even for short periods of time, will reduce chlorophyll loss, which is advantageous for green apple cultivars.
Anthocyanin synthesis is light dependent in many but not all fruit. The radiation intensity required for color development is species specific, and some fruit are not considered to require light. The light requirement is in addition to photosynthesis, and although similar in spectral response to the light reactions of phytochrome, it is distinct and high energy. The light response is not translocated from cell to cell, so maximum exposure of the fruit is the best solution. Tree training systems have been developed to maximize the quantity of light energy reaching the fruit. Reflective materials spread under the tree increase the amount of light reaching the fruit and therefore increase color. For many years it has been known that apples will color after harvest, but as initially practiced by spreading the apples under the tree for days, there was significant loss of storage quality.
The influence of temperature on fruit color has been widely studied. Poor color of apples was the traditional problem. 'McIntosh' apples, in addition to a light requirement, color better during cool temperature conditions than during warm periods. The mechanism appears similar to "autumn coloration" of foliage, which also involves senescence and is enhanced by cool temperatures. A proposed biochemical mechanism is based on differentially temperature-dependent enzyme turnover. Tree cooling with overhead sprinklers will increase anthocyanin content in apples.
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