Plant Physiology 102

Like animals, the life span of plants varies differently among species and is also affected by the environment they live in. Basically, we can divide plants as annuals, biennials and perennials referring precisely to whether they live just for one growing season (about a year), two growing seasons (two years) or many years; in some cases thousands (some conifers).

Most small plants fall into annuals or biennials, though there are many perennial herbaceous species. However, the ability of plants to live longer has been explored and used by man. A typical and very well studied example is maize, or corn.

The wild species closest to our common corn plant are in fact perennial plants, but due to man-made selection and manipulation, domesticated maize is at best a biennial plant in some varieties. Most maize varieties, or cultivars, are in fact annual plants. Just out of curiosity, shortening its life span was not the only modification that man made to maize.

The domestication of maize took a toll on its ability to fight natural enemies, e.g. nematodes.
Living and surviving throughout different growing seasons is in fact quite a challenge for most plants, as they have to face very different climate conditions (e.g. temperature, water availability, light) through time. In some cases these climatic changes can happen quite fast, thus plants have to adapt and cope with that. In tropical regions, this is not a major problem as we all know that the hot and humid weather is almost constant; changing very little between months.

This constancy is even more evident closer to the Equator. In these regions all tropical plants grow continuously at very similar rates throughout the year and this is why tropical trees don’t tend to have distinct growth rings, as happens with trees of temperate regions. However, some tropical regions face some extreme changes between dry and wet seasons (for example the regions affected by the Monsoon).There the trees do show different growth rates between the two seasons thus showing apparent grow rings, e.g. teak (Tectona sp). In temperate regions the scenario is quite different, and rather diverse.

To best face winter of temperate regions, where the temperature is far from optimum, the sun does not shine, and it rains or snows or gets below zero most of the time, plants lose their leaves and stop growth all together. Oak, lime, ginkgo, ash, and tulip tree are very well known examples of deciduous trees. Why waste energy on keeping their leaves to photosynthesize at minimum levels? Better give it a rest and wait for better times.

The process by which these plants, from annuals to perennials, lose their leaves, thus being classified as deciduous, is called abscission. Contrary to what you may think, what gives them the signal to initiate such a physiological process is not the air temperature becoming lower as winter approaches but the day getting shorter. Whether the sun shines or not, day length is a more regular and predictable phenomenon than the drop in temperature that plants use to “know” that not so favorable times come ahead and so it is time to start preparing things. Clever isn’t it?

However, not all plants from temperate regions lose their leaves. That is, not all of them are deciduous. As we know, pines do not lose their leaves and most conifers like cypresses, sequoias, spruces, firs, cedars, and yews , well as eucalyptus and most tropical species keep their leaves.

In temperate regions, evergreen plants have adopted the strategy to survive by growing very very slowly and thus consuming, or needing of, very little. Losing leaves is not an exclusively winter phenomenon. In fact it may happen in summer or in dryer seasons, e.g. our native baobab tree (Adansonia gregorii). Some species live under very arid conditions in summer, or winter depending on the location, and water is scarce.

The best way to face this cruel summer is to stay dormant or stop growing and save themselves for winter when they really grow. This is quite common in many Mediterranean species where the growing season is the mild and cool winter, e.g. olive trees. Although many plants can adapt and live far from what their normal habitat conditions are, dormancy in plants is in fact a necessary strategy for their healthy growth. Growing deciduous species under “eternal sun” conditions although it may work for some species for some time (usually up two years), it is in fact quite stressful and usually results in death and very short life spans.

Special thank you to Dr Pablo Cabrita (Phd. Plant Physiology)

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