Plants and light
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Issue: April 19, 1999

Plants and light

Question:

At the Master Gardener class in Los Alamos you said that some plants don't need full sunlight, while others use full sunlight and can be used to shade the others in the garden. Will you explain that in print?

Answer:

Gardeners have long known that some plants are shade plants which do better in shady locations, while others do better in full sunlight. Some are adaptable and may adapt to either condition if they are not suddenly moved from shade to full sunlight.

There are several mechanisms which allow for this. One is that the plants develop their own sun screen by producing additional silvery, reflective hairs to reflect excess light. This is common in many arid land plants. Many of these are silvery or gray in color. These hairs also increase the "boundary layer" or area of still air next to the leaf, reducing water loss from the leaf.

Other plants increase the concentration of auxiliary pigments in the leaf, especially yellow colors, to absorb excess sunlight preventing it from reaching the chlorophyll which would be damaged by too much light. Chlorophyll is the pigment which gives the plant its green color and is essential for photosynthesis, the process by which the plant produces food from sunlight, water, carbon dioxide, and nutrients from the soil.

Finally, plant physiologists have discovered several forms of photosynthesis in plants. One is based on organic acids with three carbon atoms (C-3), which receive the carbon from carbon dioxide to form sugars, another is based on 4-carbon organic acids (C-4), and another is used by plants in arid lands so that carbon dioxide may be absorbed at night when it is cooler and less water is lost as the plant absorbs carbon dioxide from the air.

Your question relates to the C-3 and C-4 photosynthetic pathway. Plants with the C-3 photosynthesis do not need full sunlight. In high levels of light they waste energy and carbon through a process called "photorespiration," while the C-4 plants do not lose energy through photorespiration. At lower light levels, C-3 photosynthesis plants are more efficient while C-4 plants are most efficient at high light levels. Tall plants with C-4 photosynthesis are useful in New Mexico's high light levels as they can use the sunlight and cast a shade under them to protect the C-3 plants. This is the reason that Native American gardens mixed tall corn plants with the shorter squash, beans, and chile plants. The C-4 corn plants shaded the C-3 squash plants maximizing utilization of light and protecting the C-3 plants. Their planting style was not the same as our contemporary row agriculture; they often planted the plants in "hills" or clusters of several corn plants in a hill, then several feet away several squash, bean, or chile plants in another hill. The spacing was for water efficiency as well as to assure that the tall corn did not cast too much shade for the C-3 plants.

Intercroping like this can be used in our gardens today. Here in New Mexico, the shade from tall plants can be very useful in our gardens if the plants are not too densely planted. Remember also that if you plant too densely, competition for water and nutrients can limit production.