Dr. Lew Feldman, Garden Director
One of the more common and challenging activities for both indoor and outdoor gardeners is changing the location of a plant, which is sometimes necessary when a plant has become too large for a particular location, or, for indoor horticulturists, perhaps associated with rearranging the layout of a room. And while moving a plant might seem like a fairly straightforward task, such activity is often stressful for the plant, and sometimes, even fatal.
Often these relocations are accompanied by changes in the lighting, resulting in the plants being exposed to different (lower or higher) light intensities. In order to understand and appreciate why adapting to a different level of illumination can be formidable for a plant, we need to remind ourselves about the process of photosynthesis, through which light energy is captured and used to convert carbon dioxide (CO2) from the atmosphere into energy-rich, organic compounds which provide the building blocks for the plant. The process of making and then consuming organic compounds allows the plant to grow and is accompanied by the return of CO2 to the atmosphere via the process of respiration. However, a plant grows only if the amount of CO2 incorporated into organic compounds via photosynthesis exceeds the amount of CO2 returned to the atmosphere.
The point at which the incorporation and loss of CO2 balance each other is called the CO2 compensation point. In most plants, in order for there to be available sufficient organic building blocks for growth, the amount of CO2 incorporated into organic compounds must exceed the amount lost to the atmosphere. Each plant, therefore, depending upon its environment, establishes its own CO2 compensation point. In bright light, where photosynthesis is highly favored, the amount of respiration can also be high, allowing the plant to take advantage of the abundance of light and to grow extensively.
So what does the CO2 compensation point have to do with success (or failure) in moving a plant? Let’s begin with the situation in which a plant is growing in bright light and then is moved to a shady environment. Leaves originally produced in the bright light have established a particular CO2 compensation point that is related to the amount of photosynthesis. Suddenly moving a plant to shade now results in an immediate reduction in the level of photosynthesis, but without a corresponding, immediate reduction in respiration. In this situation the plant is using up its energy rich compounds faster than it is making them and releasing more CO2 to the atmosphere than it is taking in. The result is that leaves are eventually starved of energy and they often fall off and the whole plant frequently dies. Now let’s look at the reverse situation; moving a plant from a dimly lighted room to one with bright light. In locations with reduced illumination the plant has had to adapt to gathering what limited light is available. It does this by making large, often thin leaves with increased surface area for capturing the light.
Also, importantly, the leaves in the shade do not have any “filters” (usually other pigments) which function as sunscreens for reducing the intensity of the light; in shade, these leaves want all the light they can get, so they do not develop sunscreens.
Under these circumstances, moving a plant from the shade into the sunlight essentially overwhelms the photosynthesis light capture system, for now, high intensity light falls on a large area of light capturing pigments, unprotected by sunscreening pigments, usually resulting in damage to the photosynthesis machinery. As a consequence, such leaves typically die.
So, is it possible to successfully move a plant into an environment with a light intensity differing from where the plant was originally located? Moving plants from bright light to shade can be successful if this is done gradually, thereby allowing the already-existing leaves to slowly shift their CO2 compensation points. However, moving a plant from shade to bright light is difficult, since existing leaves cannot change their size nor thickness and usually lack the necessary sunscreen filters for moderating light intensity. In general, you have more success in moving a plant if it is healthy before the move, and if you don’t mind the loss of the leaves that were present at the time of the move. The production of new leaves, with new CO2 compensation points, plus other adjustments to the changed light levels, are generally what favors success in relocating a plant.
So on your next visit to the UC Botanical Garden, look for plants growing in both shade and sun and see if you can notice differences in the features of the leaves in these two dissimilarly illuminated environments.