Skinny Stems and Red Light

For high-latitude gardeners, the worst time of year is the span between the arrival of seed catalogs and the day the soil is warm enough for planting. Often enough, impatience leads to a clutter of potted seedlings on every windowsill, and elongated, scrawny plants at transplanting time.

This year I found a good scientific reason to give plants room. It explains why seedlings crowded on the sill may elongate to an unhealthy degree, even if they have plenty of food, water, and light.

Shaded plants usually become spindly and pale. Their stems are long and thin, with leaves far apart along puny stalks. They look as if they need light, and seem to be reaching for it. Well-lit seedlings don't become pale, but when they are crowded, may behave like shaded plants. They develop proportionately thin stems and big gaps between pairs of leaves as they shoot skyward.

That young plants growing in a crowd should race upward makes evolutionary sense. If a seedling's neighbors outgrow it, the infant plant may find itself dying in the shade cast by their leaves. Since the shaded-out plant wouldn't be successful in leaving offspring, passing generations would favor plants that grew more rapidly when they were in danger of losing light-gathering opportunities in a vegetal mob.

But what makes them behave as if they were already in the shade when they still have plenty of light?

A team of plant ecologists in Argentina think they've found out. As they reported in the journal Science, they first reviewed possibilities: What is affected by a high density of young plants per unit area? Air movement, humidity, and temperature alter substantially, but experiments in which these factors were manipulated to match less crowded conditions showed that none of them was critical.

The most promising possibility seemed to be light--and not just the quantity reaching the topmost leaves. Earlier work by others implicated the spectral distribution of light reaching the stems.

Those previous studies examined how different kinds of light affected plants growing under a well-established leafy canopy. When a big tree falls in a rain forest, for example, saplings that have been barely growing in the shadows suddenly increase their growth rates. The trigger speeding up the young trees wasn't simply more light; it was the spectrum of light reaching them. The crucial change in the quality of light was in the amount of red light compared to far-red light--the ratio of red to far-red.

Light filtering through a leafy canopy or bounced off adjacent greenery has a low ratio of red to far-red because the chlorophyll in leaves absorbs more light in the red band of the spectrum than in the far-red. Other studies indicate that the change in light ratio is more marked at stem level.

The double problem confronting the Argentinean researchers was how to establish that light quality is critical and that the sensory mechanism is in the stem. To solve it, they put tiny water-filled collars on seedling stems. Half of the collars contained a colored solution of copper sulfate, which absorbed much of the far-red light before it reached the stems--in effect, changing the light ratio from signaling "many leaves near here" (more far-red) to "vacant lot" (more red). The remaining collars contained only clear water, to be sure the mere presence of a collar wasn't affecting the plants' growth.

The researchers could keep constant the ratio of red to far-red light reaching a stem, no matter how close its neighbors were, by adjusting the copper sulfate concentration in the collars. That was enough to fool the plants. Even in dense groups, the plants with the copper-sulfate collars grew almost as if they were alone. Naked plants and those with clear collars reacted strongly to crowding--elongating from half again to twice as fast as the effectively blinded ones.

The team concluded that in even-aged stands, the ratio of red to far-red light striking the sides of stems controls their elongation well before leaves are affected by shading. Even though they worked with only two kinds of plants, a weed and a member of the mustard family, they believe the elongation effect will be found common in the plant kingdom.

So do I. This spring, my infant tomatoes live in solitary splendor, one to a windowsill.