My Tomatoes Not Developing Good Red Color

Interveinal (between the veins) chlorosis (yellowing)

In desert regions, we sometimes notice that plants growing under higher temperatures develop yellowing of the leaves when exposed to intense sunlight for long periods of time. This yellowing does not appear to be associated with a lack of any particular type of nutrient such as iron or manganese, a common problem in desert soils. We don't see the typical micronutrient deficiency symptoms such as interveinal chlorosis; yellowing between the veins while the veins remain green. I have talked about this in previous postings, probably relating to intense sunlight. Usually, growing tomatoes under light shading (no more than 30%) during times of intense sunlight helps to alleviate this problem.

Retired vegetable specialist from the University of California, Hunter Johnson, explains it much better. He is talking to green house growers but the message is the same. Reduce intense sunlight on tomatoes for better color development. I borrowed this posting from a University of California publication several years ago. http://vric.ucdavis.edu/pdf/TOMATO/tomatoes_HomeGarden.pdf

Solar Yellowing in Tomatoes

Yellow discoloration invariably occurs beginning in the late spring months under greenhouse conditions, or from that period on through the summer in the open field in inland areas where daytime temperatures regularly exceed 85^OF. An accurate term for this condition is “solar yellowing" because the source of the problem is the sun. It isn't only the heat of the sun or the temperature increase in the

Tomatoes not developing good red color in heat

fruit that creates the problem, but also high light intensity. This was shown by Dr. Werner Upton, who coined the term "solar yellowing" in research he conducted on the subject in 1970. His treatments involved shading or painting the fruit either black or white. Black-painted fruit were higher in temperature than exposed fruit, but discoloration was highest in the exposed fruit. His conclusion was that short-wave radiation was largely responsible for defective coloration.

The reason for the yellow or yellow-orange color, rather than the normal red, is that the red pigment (lycopene) fails to form above 30OC (86^OF). This phenomenon was first described by researchers in 1952 and was later confirmed by others. When lycopene fails to form, only carotenes remain for fruit color. In the field, some red color forms when day temperatures rise above 85^OF because of fluctuation in noninhibiting temperatures during other parts of the day or night. An orangey-red color results. 

In production areas where temperatures do not exceed 85^OF, much higher red color develops.

For good uniform red color to develop, high temperatures should be avoided and fruit should be protected from short-wave radiation in high light intensity areas. 

Dr. Upton showed that sprays of non-phytotoxic whitewash will help. In greenhouses, growers who intend to mature fruit in May and June should begin to alter their pruning practices in March by allowing two leaves to develop on axillary branches instead of the standard practice of removing these branches.

The author is Hunter Johnson, Jr., retired University of California Cooperative Extension Vegetable Specialist, Riverside Campus.