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The process of photosynthesis in plants is of such import that were we to understand it completely and be able to duplicate it industrially, we could solve many of today's economic woes. For our purposes here, though, understanding the basics of the process is our objective . . .
Growing against the grain
Photosynthesis can be described as the conversion of light energy into chemical energy, and can be represented in a simple manner by stating that plants take six parts of carbon dioxide plus six parts of water plus light to yield one part of sugar and six parts of oxygen. Of course, the actual chemical reactions are numerous and more complex than this simple description. but the general idea is there. The process involves two sets of reactions, referred to as the light-dependent reactions and the light-independent reactions. The high-energy phosphorus-containing molecules are formed using light energy in the first set of reactions, while complex sugars are formed in the second set of reactions.
We grow our plants in sunlight most of the time, but the plants don't use all the light present in sunlight. The light that is used by the leaves is primarily red and blue, leaving the green light to be reflected. Plants with red leaves are still using red light, but since plants use much less than 100 percent of the light that shines on them, there is plenty left over to reflect and produce the variety of colors we observe. Seasonal differences in the frequency content of light can cause or inhibit flowering or dormancy in some plants.
A common idea is that photosynthesis is the opposite of respiration, which seems logical on the surface of it. However, even while the end result seems to confirm this comparison, the actual biochemical reactions are quite different. Besides, respiration happens in plants as well as animals, so photosynthesis does not take the place of respiration. Instead, it takes the place of having to acquire carbohydrates and proteins from the environment by eating, as we do. So a statement like, "plants breathe in carbon dioxide and breathe out oxygen, while we breathe in oxygen and breathe out carbon dioxide", is not accurate.
Real respiration
Photosynthesis must take place during the time when the sun is shining, so what do plants do in the darkness, at night? They experience respiration, which is not just breathing in and out as we do. Respiration in plants is the conversion of carbohydrates into energy to use in cellular activities, such as transfer of substances between cells and synthesis of amino acids. Carbon dioxide results from this respiration, just as happens in animals. However, in plant respiration, the amount of oxygen required is less than the amount of oxygen produced via photosynthesis, leaving a surplus for animals and us to use for our respiration!
Photosynthesis occurs in special cellular structures called chloroplasts; respiration also takes place in special structures, called mitochondria. These serve to parcel out the stored energy for metabolic functions in the cell and, thereby, to the whole plant. Since these functions must occur day and night in a living organism, respiration also takes place day and night in plants, whether photosynthesis is active or not.
News you can use
Knowing that phosphorus is involved in the production of carbohydrates in plants via photosynthesis, you can understand the important role that it plays in the development of storage tissue in plants, such as bulbs, corms and tubers. Phosphorus is the middle number in fertilizer analysis; for example, if you have a fertilizer that is a 14-26-6 mix, that middle number "26" is the amount of phosphorus in the formula. For some plants, phosphorus can be more important than nitrogen, particularly in regards to tuberous plants preparing for dormancy.
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About LariAnn Garner
LariAnn has been gardening and working with plants since her teenage years growing up in Maryland. Her intense interest in plants led her to college at the University of Florida, where she obtained her Bachelor's degree in Botany and Master of Agriculture in Plant Physiology. In the late 1970s she began hybridizing Alocasias, and that work has expanded to Philodendrons, Anthuriums, and Caladiums as well. She lives in south Florida with her partner and son and is research director at Aroidia Research, her privately funded organization devoted to the study and breeding of new, hardier, and more interesting aroid plants.
