A Garden of Marvels Page 22
Small bees do occasionally visit both species’ flowers during the day, so one would expect that at least a few hybrid seeds would set. But without a breeder’s intervention, they rarely do. Why not? The two species are compatible enough that pollen grains of both species send out pollen tubes no matter which of the two species’ stigmas they land on. But when both integrifolia pollen and axillaris pollen fall on axillaris stigmas, the pollen tubes of axillaris always grow faster and win the race down the style to the ovary. Likewise, on integrifolia stigmas, integrifolia tubes always beat axillaris tubes. This home-field advantage is part of what makes a species a species. There are many other possible barriers to hybridization—incompatible chromosome numbers is the biggest one—but between these two petunia species, the homospecificity of pollen is the stopper. Only when a breeder or a lab ensures there is no competition do the two species hybridize.
By 1837, Joseph Harrison was writing in the English Floricultural Cabinet magazine that breeders had produced many charming color variations from crosses of the two species made in the greenhouse, including “pale pink with a dark center, sulphur [a deep pink] with a dark center, white with a dark center, and others streaked or veined.” Since that time, breeders around the world have tinkered with Atkins’s hybrid and made it one of the world’s most popular garden plants. By repeated crossings, careful selections, and inbreeding, plus the occasional fortuitous mutation, they have changed the simple purple and white petunias radically. Now some varieties have double the number of petals or edges so deeply frilled that they are hard to recognize as petunia blossoms. Breeders have coaxed yellows out of the gene pool, and fashioned a host of patterns, including stripes, stars, bicolors, streaks, freckles, contrasting edges (“picotees”), and “morns” that slide from hue to hue like a Turner sunset. In 1837 Harrison was impressed by petunia corollas (the petals, collectively) that had grown to three inches in diameter; twentieth-century breeders have bred blossoms as broad as seven inches. Petunias now flower more prolifically and branch more abundantly, tolerate drought better, and are more resistant to fungi. Their soft petals, once easily damaged in a hard rain, are sturdier.
Until twenty years ago, however, one trait remained the same: All petunias on the market had an upright growth habit. Then, in 1995, Ball introduced a new type of petunia, a spreading petunia called Wave. Developed by a breeder at the Kirin Brewing Company in Japan, Wave opened an entirely new market for the petunia as a full-season, flowering ground cover. Put one Wave petunia in a garden and it will cover a four-foot-diameter area in blossoms. In 2001 when Ping arrived at Ball, Wave was where the action was in petunias.
“To be honest,” Ping tells me, “when I was assigned to regular petunias, I was a little bit discouraged. Everyone already knows what a regular petunia is. But since it was my assignment, there’s got to be something I can do, so I worked really hard.”
A new flower color or a unique pattern is just one objective for commercial breeders like Ball. Ball sells seeds to wholesale growers who are keen to buy varieties whose buds open simultaneously. Growers sell to retail garden centers and “big-box” stores that like to have all the colors of a series (say, the heat-tolerant “Madness” petunias) in a blooming display all at once. A petunia that blooms especially early in spring also appeals to growers because they might squeeze in an extra crop that season. They also prize varieties that bloom profusely but remain compact, so they can maximize the number of individuals in expensive greenhouse space. Such factors are equally, if not more, important than new colors to the wholesale growers who buy Ball seed, and Ping found room for improvement.
Ping takes me into one of her light-filled petunia greenhouses where she evaluates new parent lines and promising new crosses. In a commercial greenhouse, the wide waist-high tables (called “benches”) are jammed pot to pot, making it difficult to distinguish one individual from another. Here, larger, vibrantly healthy, blooming specimens are generously spaced on long, narrow benches that run the length of the greenhouse. Each specimen has room to demonstrate its tendency to branch and its willingness to flower. The benches are arranged according to color groups and to hues within each group. If you could hover at the ridgeline of the greenhouse and look down, the benches would look like those paint store sampler strips, laid end to end.
We start along a bench devoted to variations on the theme of burgundy, then walk along a bench filled with various shades of magentas. I trail Ping as she walks up one aisle and down the next, along benches of lavenders, royal purples, purplish blues, sky blues, peaches, pale pinks, and hot pinks. She is explaining some of the intricacies of the genetics behind plant breeding—in each pot is a stick with a white card that displays strings of numbers and letters that indicate the plant’s heritage—but I find to my dismay that I’m having a hard time listening. The visual stimuli are overwhelming. Actually, it is all I can do to resist stroking the gorgeous petals or, even more tempting, brushing my cheek against them. The amber flowers of one plant are exactly the color of toffee, and I stifle an impulse to nibble one. One piece of information that does penetrate my stupefied brain is that out of every thousand plants Ping produces, 999 will end up in the compost heap. Of these beauties before me, none will be seen by anyone but Ping, her assistants, and now me.
After we pass along the creams and whites—a welcome interlude, like a palate-cleansing sorbet between courses—we stop at a cluster of petunias whose flowers are green. I realize I’ve never seen a green petunia before. That is not surprising, Ping tells me, because until a few years ago, no one else had, either.
“We must stop here,” she says, “because talking about black has to be talking about green” when it comes to petunias. And talking about green petunias takes her back to the spring of 2003, when one of Ball’s customers, a wholesale grower in Minnesota, called her up. He had purchased seeds of Ball’s white Supercascade, an upright variety with blossoms up to five inches across, but something odd had happened: One of the plants was bearing green flowers. Was anyone at Ball interested in this green petunia?
Ping was. She asked the grower to send her the plant, and found its flowers were a pale lime, almost a yellow green. The color wasn’t particularly attractive and the plant’s architecture wasn’t very good. “It was very loose, didn’t branch well,” Ping said. Nonetheless, she wanted to acquire this individual’s genes, and Ball licensed the plant from the grower.
A green pigment in a plant is obviously not unusual; leaves and stems are green with chlorophyll. The purpose of flowers, however, is not to produce energy but to attract pollinators, and the corolla needs to stand out from the foliage background like a glowing neon EAT HERE sign. Flowers dressed in green petals are generally not dressed for evolutionary success. They are less likely to catch the eye of a pollinator, and therefore less likely to produce offspring. (Wind-pollinated plants, like grasses and many trees, needn’t invest in gay apparel.) There is, though, a period when a plant with chlorophyll in its petals might have a slight edge in the competition for survival. Look at a white petunia, Ping said, just as the bud begins to open, and you will see that its immature petals have a green tinge. The immature petals have chloroplasts that are contributing their small bit to the plant’s overall photosynthetic capacity. As the bud matures and fully opens, the chlorophyll degrades, and the corolla appears white. In the Minnesotan green petunia, Ping explains, a genetic mutation stopped the chlorophyll from degrading.
Ping started a careful program of crossing the green into various parent lines based on her knowledge of their genetics. At first, nothing interesting emerged, but eventually “all kinds of new colors come out, an incredible color range. Most colors look dirty and are not too pretty—one looked like old blue jeans. But the thing is, many of these colors have never been seen before in petunia.”
Petunias inherit two copies of each gene, one from its mother and one from its father. So, which form of the gene (that is, which allele) will be expressed in the individual, the m
other’s or the father’s? When Gregor Mendel crossbred pea plants in 1865 and 1866, he focused on two traits: plant height and pea texture. Fortunately for him, the pea plant’s height (either tall or short) is attributable to one gene while pea texture (either wrinkled or smooth) is attributable to another. Even better, in both cases one of the two alleles—tall in the case of height; smooth in the case of texture—is dominant while the other is recessive. Whenever a pea plant inherits both a “tall” allele and a “short” allele, the plant’s phenotype (meaning its appearance) will be tall. A “smooth” allele trumps a “wrinkled” allele. Only when a pea plant inherits two recessive alleles will its phenotype reflect the recessive trait.
Mendel was lucky in his choice of pea plants as experimental subjects. Height and pea texture follow the simplest, “Mendelian” inheritance pattern. It’s a good thing he didn’t run his experiments by crossing Petunia axillaris and Petunia integrifolia. Instead of a mix of white- and purple-flowered offspring in a 3:1 ratio, he would have gotten all pink-flowered plants, and might never have formulated his laws governing inheritance. Those nineteenth-century pink petunias arose thanks to “partial dominance,” where neither color is fully dominant over the other, so a mixing of characteristics results. On the other hand, if Mendel had crossed Ball’s green petunias and certain pink petunias, he might have produced something like the Sophistica Lime Bicolor, a Ball petunia whose petals are piebald, with patches of green and pink. He would have discovered “co-dominance” where both alleles are dominant and both traits are expressed. Stripe, star, and picotee patterns also involve co-dominance.
Breeding for flower color in petunias requires an instinct, an education in floral genetics, and long experience. Flower pigments fall into three major groups: chlorophyll (which produces green), carotenoids (yellows and oranges), and flavonoids (primarily reds, purples, and blues). Among the many flavonoids are also “co-pigments,” which appear colorless to us but shift the appearance of other colors. A petunia has about two dozen “structural” genes that direct the multiple steps necessary to produce pigments or co-pigments. It also has another two dozen “regulatory” genes that determine when and how those structural genes are turned on or “expressed.” Given all these genes, and the two different alleles present for each of those genes, Mendelian inheritance alone would produce a huge number of corolla hues and shades. Add the impact of partial dominance and co-dominance, and the color possibilities become astronomical. Still more factors influence color, including the shape of the epidermal cells of the petal and the pH of its pigmented cells. Change the pH of a petunia pigment cell by 10 percent and you can change its color from red to blue.
That’s just pigment inheritance. We haven’t gotten to pattern yet. Consider the effect of a regulatory gene called MIC, which produces co-dominance. MIC turns on the blue-red pigments called anthocyanins. If MIC never gets turned on, no matter what other colors lie in the plant’s DNA, its flowers will be white. If, as the petals begin to develop in the bud, MIC activates early in development, you get thin white pinstripes on a dark background. If MIC turns on late, you get a broad, white star pattern. Because MIC is influenced by environmental conditions—heat, light, humidity, and temperature—and because buds on any one plant develop at different times, you may find solid, pinstriped, and star-patterned petunias on one plant.
So, while I may see a simple pink petunia on one of Ping’s benches, a complex genetic heritage has influenced its appearance. Make a cross, and a color hidden in a recessive allele and unexpressed in either parent may suddenly appear. Which is just what happened one day in 2005. Ping had placed pollen from a purple-with-bright-yellow-star petunia on the stigmas of burgundy-with-white-star petunia. Of the two hundred seeds that she sowed, one seedling grew up and opened to reveal truly black petals. “When we see it,” Ping said, “it is so surprising. It is black, black, black. Nobody can believe it.”
When she looked at the parents’ pedigrees, Ping realized what had happened. In addition to the genes for burgundy, purple, white, and yellow, the parents’ lines contained genes—including the mutation for green—for the entire color spectrum. Through genetic reassortment, this one individual expressed all the colors of the rainbow. With every visible wavelength absorbed, no light is reflected, and human eyes see no color at all.
It took five years to bring Black Velvet to market. Neither crossing the parents of Black Velvet again nor crossing two Black Velvet offspring results in seeds that “breed true,” that is, consistently produce seedlings with perfectly black flowers. This means that, so far, the variety must be produced from cuttings rather than seeds, a labor-intensive and more costly method of producing a crop.
At the end of my visit, I tell Ping I am going to buy some Black Velvet when I get home, but she tells me it too late in the season to buy petunias at all. Instead, she gives me a small plant that was headed for composting, and I take it with me in a cardboard box on the plane. During the flight, I see that one of the blooms, soft and deeply black, has fallen off, and I am inspired to dissect it. (To think how far I’ve traveled that a midnight blossom now stirs in me not poetry but a deconstructive curiosity.) I put on my reading glasses and pull the flower apart. And here is a discovery, at least for me. That brilliant dot of yellow, those photons trapped and blazing in the depths of the flower’s black hole? In fact, it is a tight cluster of five, pinhead-size anthers loaded with bright yellow pollen. Nothing magical or mysterious, just petunia genitalia.
twenty-four
The Abominable Mystery
Why flowers, anyhow? Plants began to conquer the land more than 400 million years ago and ruled over it for more than 250 million years without producing a single blossom. Why should they have? Flowers are expensive. Sepals, petals, pigments for color, organic compounds for scent: Creating those fancy clothes and complex perfumes takes a lot of stored energy. Instead of manufacturing flowers, a plant could have used those carbohydrates to make more seeds or grow taller, both proven strategies in the competition for survival. Besides, there seems to be nothing in a gymnosperm that corresponds to flowers. Flowers seem to have arisen out of nothing, sui generis. Nonetheless, blossoms—from the oak’s minuscule brown nubs to the green spikelets of rice to the multi-petaled splendor of the rose—appear on at least 75 percent of all the world’s plant species.
The why and how of angiosperms, Darwin wrote in 1879, is “an abominable mystery.” It is a mystery that still has not been fully solved. Part of the difficulty is that flowers have always been fragile and when they die, they fall apart into easily scattered and perishable pieces. The fossil record of early flowers is therefore exceedingly scant. In recent years, however, evolutionary botanists have come to think the living Amborella trichopoda will help solve the puzzle.
You will never find anyone who will sell you a bouquet of amborella. For one, the cream-colored, dime-sized flowers on this knee-high shrub are not much to look at, and will set no lover’s heart aflutter. If you should want to buy an amborella plant (say, you want to write about it), you’ll also be out of luck. The shrub is rare, and grows naturally only in the cloud forests of the island of New Caledonia in the South Pacific. Only a few American conservatories cultivate amborella, which is notoriously difficult to sustain and flowers unpredictably in captivity. Nonetheless, unprepossessing and persnickety as it is, the plant has attracted a great deal of scrutiny recently. It is likely the closest living relative to the first flowering plant, and according to Harvard professor William Friedman, “a critical missing link between angiosperms and gymnosperms.”
A female amborella flower. The pollen sacs on the female’s sterile staminodes (and the male’s stamens) are related to the pollen sacs on the scale tips of conifers’ cones.
About 140 million years ago, at the beginning of the Cretaceous era, a new type of plant evolved from one of the seed-bearing, nonflowering gymnosperms—mostly conifers—that covered much of the landscape. This first angiosperm was the founder of a ne
w line, the Amborellales. Sometime later, a sister angiosperm line emerged, the Nymphaeales, which evolved to become the modern water lilies. A third line emerged. These, the Austrobaileyales, evolved rapidly and diversified profusely to become almost all of today’s 250,000 to 400,000 (depending on who is counting) flowering species, from cucumbers to pansies to elms. The Amborellales, on the other hand, are today represented by a single species, Amborella trichopoda, the modern flowering plant least changed from its gymnosperm ancestor. It is a living relic from the age of the dinosaurs.
So, what does Amborella have to say about why angiosperms evolved and conquered? As Ping might say, talking about angiosperms is talking about sex. Most gymnosperms are monoecious, although gingko trees, which are one of the few nonconiferous gymnosperms, bear either male or female organs. Among conifers, male and female cones can be on separate individual trees but are more often segregated on either the lower or upper branches of a single tree. (The male cones are smaller and drop soon after they release their pollen. Female cones grow larger after pollination, taking anywhere from a few months to two years before falling.) On the other hand, most angiosperms are hermaphrodites, with stamens and carpels inside the same organ.