Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
I have had interesting conversations recently regarding the fruit color in pickling cucumbers. Processors want a dark green color and eastern grown pickles often are much lighter in color than ideal for processor needs. There has been considerable research on the development of color in different types of fruits and the physiology is often complex. The following are some of the major factors affecting color in fruiting vegetables.
Genetics
The primary factor affecting fruit color is genetics. Vegetable cultivars are bred and selected for specific color attributes in fruit – dark or light, immature color, ripe color, blush over background. The expression of this genetically determined fruit color can be modified somewhat by environmental and cultural conditions.
Chlorophyll and Other Pigment Production
The amount pigments produced will determine ultimate color. Chlorophylls, carotenoids, and flavonoids (such as anthocyanins) are the pigments produced by plants that give specific colors. Anything that reduces the production of pigments will result in reduced color. Chlorophyll gives the green color for cucumbers, zucchini, green peppers and other green fruit. Chlorophyll production can be impacted by many factors. Under high populations or heavy leaf cover, less chlorophyll is produced in shaded fruits and therefore they are lighter in color. On the other hand, over exposure of fruits subjected to high temperatures will result in degradation of chlorophyll and sun scalding. In fruits that develop red, orange, yellow, or purple colors, light is also critical for good color development. It is critical to maintain a balance between enough leaf cover to prevent over-heating, pigment destruction, and sun scalding with enough light penetration to develop color.
The chlorophyll molecule; apart from carbon, hydrogen, and oxygen; also contains nitrogen and magnesium. Production of all pigments requires enzymes that contain nitrogen and sulfur. Deficiencies of these nutrients can lead to reduction in color.
Pigment production requires significant plant resources (carbohydrates as building blocks and energy sources) so any factors that reduces the production of these resources may have an impact on initial or ripening color. For example, with the hot year we are having, many vegetables will have higher respiration and reduced photosynthesis leading to less food resources to go into fruit pigments, again leading to reduced or less intense color. Other stressors such as drought, insect damage, or loss of leaf area to disease can also lead to poor color because of the reduction in carbohydrate resources to produce pigments.
Ground Contact
Ground contact will result in loss of light sensitive pigments, particularly chlorophyll. Therefore you will often see white or yellow colors on fruits where they touch the ground. For instance, dark rind watermelons will not have much chlorophyll on the ground spot but will still have carotenoids therefore showing the yellow color when ripe. Crops that are trellised or that vine up windbreaks or nearby plants will have more uniform color.
Destruction of Pigments by Pests
Disease organisms, insects and mites can destroy pigments when infecting or feeding on fruits. For example, many plant viruses will cause loss of color, mottled color, or irregular color in fruits.
Plant Hormone Activity
Plant hormones can help maintain or cause deterioration in color depending upon the hormone involved. For example, cytokinins help to maintain green color while elevated ethylene will cause premature yellowing. Plant hormone activity is tied closely to stress and environmental responses.