BLOOM FLOWERS

  How do plants decide when it's time to make flowers?

 Finally scientist have some answers.

Maryland Mammoth was good for growers. Every ten-foot-high giant produced twice as many dryable, sellable, smokable leaves as the average plant could yield. It was to prove even better, though, for scientists who wanted to know how plants make flowers--more specifically, how they decide when to make them.

It's important for a plant, this flowering decision, since popping up one's delicate budlets in the midst of a bleak, dark winter could be seriously detrimental to reproductive success. It's important for people, too, since every grain, fruit, vegetable, and ornamental plant has its own prerequisites for flowering, and only by understanding these prerequisites can farmers and breeders work with them, maybe alter them, so that humankind can grow crops when and where it wants. As for those of us who garden at home--well, flowering science lies behind everything from knowing why lettuce bolts after a cold snap to why, if you planted the right flowers in an ideally situated garden, you could count off the weeks like a clock simply by noting what is blooming when. A cursory survey of the plant kingdom might discourage one from seeking clear, generalizable principles governing the whys and whens of flowering. All over the world, plants are doing different things for their own special reasons: flowering in springtime before their leaves unfurl so that their blossoms are showy for pollinating insects or are exposed to the wind that carries the pollen; flowering just in time to beat the frost; flowering when it's wet enough or dry enough or when other plants aren't hogging the birds or insects needed for pollination. Some of the strategies are extreme. The Brazilian jaboticaba tree waits for a drenching monsoon and then blooms like there's no tomorrow. The North American pink lady's slipper orchid flowers during the springtime flight of the queen bumblebee. Then there's bamboo, which grows without flowering for years (120 years in one remarkable species), and then suddenly whole acres decide it's time to flower. They then die en masse, leaving vast swaths of land bare and brown, and the giant panda wondering where its next meal is coming from. Clearly, if there is a grand unifying theory behind flowering, it can be summed up in one short phrase: find a strategy that works.

Despite all this variability, broad patterns behind flowering do exist--patterns that scientists can study in their own favorite, easy-to-breed-and-grow plant, with confidence that what they find out will be relevant to many other species. Maryland Mammoth, a mutant variety that appeared unexpectedly in a field of standard Narrowleaf tobacco back in 1906, was a perfect case in point.

The difference between Maryland Mammoth and other varieties of tobacco was that the former flowered so late in the year that frost often cut it down before seed had even set. This frustrated farmers, since it meant that the only way to get seed was to grow the plant in greenhouses. But it fascinated Harry Ardell Allard and Wightman Wells Garner, two scientists working in 1918 at a U.S. Department of Agriculture research farm in Arlington, Virginia (right where the Pentagon sits today). The duo set out to tackle the mystery.

They learned that while most commercial varieties of tobacco grow until they reach a certain size and age, Maryland Mammoth stops growing only if the days shorten to a certain critical length. Picture Garner and Allard hefting pots of Maryland Mammoth into light-tight little houses every afternoon at four, then hefting them back outdoors again at nine the following morning, to artificially give the plants long nights of seventeen hours and short days of just seven hours. The result was flowers. And seeds. In July. Meanwhile, out in the field, Maryland Mammoth was basking in fourteen-hour summer days and was a long way from flowering. Garner and Allard concluded that the plants were measuring day length and that sensing either light or dark was the way they were doing it.

With this knowledge, it was now easy enough to get Maryland Mammoth seed by growing the plant in winter in southern Florida, where the weather was mild and the days short. More important, it soon became clear that many, many plants, both wild and commercially bred, exhibit a similar photoperiod response--that is, they have daylength requirements for flowering, although the length varies with latitude and lifestyle. Take plants like chrysanthemums or asters or summer wheat. All flower when the days start getting shorter, giving them time to enjoy the long sunny summer but still get their blooms out before the frost. Or think of the lilies that bloom in time for Easter, when the days are getting longer and winter has finally passed.

Decades of research are beginning to reveal how plants perform this trick. To sense the light, plants use molecules called photoreceptors. Some, known as phytochromes, were discovered four decades ago; they are most sensitive to wavelengths in the red portion of the electromagnetic spectrum. Other, very recently discovered photoreceptors called cryptochromes sense blue light. Yet other molecules, which sense ultraviolet light, are still lurking out there, waiting to be found.

Having sensed the light, plants need to be able to "count" and to leap into flower-making action when day (or night) length is just right. Here's where the science of circadian rhythms enters the picture. Circadian rhythms are regular rhythms of growth and activity, occurring roughly every twenty-four hours. They control everything from when animals sleep, wake, and are most alert, to when an insect emerges from its pupal case or a flower produces nectar.

All these organisms have biochemical clocks that enable them to perform a host of activities and cellular processes in a neat, timely fashion. With its clock, a plant can crank up production of its light-harvesting enzymes before dawn and make the most of every iota of daylight. It can time the opening and shutting of the pores in its leaves so that gases enter when they're needed and precious water is retained when they're not. Some plants shut their flowers at night to protect them from the cold and the pollen-damaging dew and open them up in the day so that insects and birds can visit and pollinate them. Others open theirs at night to greet bats and moths. So precise is this flower-opening stunt that a gardener could conceivably tell not only what week it is by when different species bloom but also what hour it is by noting when certain blooms open up. Researchers have pinpointed many of the genes involved in circadian rhythms and have come up with an elegant model for just how proteins tick and tock and thus orchestrate an organism's daily functions. Circadian clocks, which have proved remarkably similar in nearly all the organisms studied (the one exception so far appears to be blue-green algae), are complicated and not yet fully understood. In fruit flies, the focus of most of the research, the story goes something like this: Two proteins, called period and timeless, build up slowly in a cell. When these become plentiful enough, they prevent the production of two other proteins--clock and cycle--without which, however, they themselves can no longer be made. So now period and timeless disappear from the cell. This allows clock and cycle to be produced again, which eventually brings period and timeless back into the picture. And on and on, in an approximately twenty-four-hour rhythm.
A circadian clock is essential for responding to daylight: destroy it and you're left with a befuddled plant that doesn't know when to flower. But even with a clock in good working order, a "Time to Flower!" signal must travel up from the leaves to the apex of the plant, which will continue churning out leaf after leaf until it gets instructions to do otherwise. Scientists know the signal must come from the leaves, because if you provide plant A with the appropriate flower-triggering day length, then cut off a leaf and graft it onto plant B--which has never experienced that day length--plant B will dutifully flower. Something in the foreign, transplanted leaf travels up the plant and trips some developmental switch.

The identity of that something is still a mystery, sixty years after it was given the name florigen. Which isn't to say that scientists haven't sleuthed away like crazy trying to find out. Yeast extract, acids, vitamins--you name it--they've all been spritzed onto plants at one time or another in the hope that a blossom would ensue. Frustratingly, no clear and simple picture has emerged from all these efforts. But some scientists today are leaning toward an idea known as the theory of multifactorial control. This means that a medley of substances--perhaps sundry plant hormones such as gibberellins and cytokinins, along with a burst of sucrose and calcium ions--travel up to the bud, triggering flowering only when all the substances are present and accounted for in just the right ratios.

The complexity of flower induction is also reflected in the number of genes involved in the process. In Arabidopsis thaliana (most of the genetic work on flowering has been done on this little mustard weed), there are now in excess of forty known flowering-time genes. When inactivated, some of these genes, such as those named hasty and speedy, result in mutants that flower too soon; others, such as gigantea, sca, and constans, produce mutants that flower too late. Many of these genes are involved in controlling the plant's response to day length. But many others aren't. When it comes to flowering, photoperiodism is by no means the only game in town.

Most plants, for instance, go through a juvenile phase, just as humans do. Reproduction during that phase is strictly verboten, and genes like hasty appear to be part of the brake. And many plants, such as sunflowers and most tobacco varieties, pay no attention to the length of day or night; they grow until they reach a certain size, and then kaboom! they bloom. The timing of that decision depends on the balance of a whole slew of genes, some of them pushing the plant toward reproduction, others cold-showering the inclination. Different balances of these pluses and minuses translate into different flowering times.

Then there's temperature. With some plants, such as lettuce, a quick cold snap tells them winter is coming, and they'll flower posthaste. But many others need a more protracted period of cold before they deign to put out blooms, which is why inhabitants of warm climes have to shove their tulip and daffodil bulbs into the fridge if they're ever to get more than one springtime's worth of color from them. It's also why farmers who plant wheat and barley in the fall choose varieties that require this same chilly treatment, known as vernalization. This way they'll be sure their crops will grow vegetatively through the winter and yield an abundant harvest in the spring.

The more scientists know about flowering, the more they can play with the process in the plant of their choice and this makes for great agricultural potential. Treating lily bulbs with cytokinin hormones can mean early flowers and more of them. As for vernalization, growers would like to save money by chilling bulbs for the shortest time possible, yet they have to be sure they've successfully flipped the molecular flowering switch, or their bulbs will be no good. Diagnostic tests for successful vernalization, based on genes that turn on when this switch gets flipped, are under development.

Genetic engineering, too, can do remarkable things to flowering. Not everyone agrees it's desirable to fool Mother Nature this way, but there's no denying the potential significance of the results. Getting a plant to flower even slightly earlier or later than normal can extend the geographical range of a particularly favored variety, and fast flowering also means faster breeding. Experimenting with Arabidopsis, for example, molecular biologists have created early bloomers by inserting extra copies of various flowering genes into the plant. They've even engineered plants that will flower on command, in response to a gene that turns on when exposed to the right chemical trigger. Efforts are afoot to do similar things with economically valuable plants such as sunflowers, lettuce, and cereals.

One flowering gene, called leafy, has already been inserted into aspen trees, with spectacular results. Normally aspens wait ten to twenty years before they flower, which makes them less than popular for breeding. In the leafy aspen, flowers spring forth from inch-high seedlings after a mere three months. Tree breeding at last becomes a viable, do-it-in-your-own-lifetime proposition.

With all this going on, who knows what the plants of the future will be like?
 

Her last Natural History article was "The Genetic Battle of the Sexes" (2/98). Rodica Prato, born in Bucharest, Romania, studied architecture before becoming a freelance illustrator. Her work has appeared in dozens of books and magazines. She illustrated "Searching for the Wild Bactrian Camel" in last month's Natural History.

COPYRIGHT American Museum of Natural History & Gale Group