Avocado tree pollination still mystery to scientists

July 14, 2006|By JIM LANDERS | JIM LANDERS,MCCLATCHY-TRIBUNE

REHOVOT, Israel -- To the many mysteries of love, add avocados.

Scientists have spent decades trying to figure out how avocado trees have sex. A tree can sprout as many as a million blossoms in spring, but only 150 to 500 will bear fruit.

Some avocado flowers bloom as females in the morning, then appear the next afternoon as males. Other avocado species do the opposite.

Honeybees are not impressed. Bees that break-dance with excitement in the hive after discovering a rich source of citrus blossoms do a slow shuffle when bringing news of avocado.

"The joke of nature is that most citrus trees don't need pollination ... and on the other hand, the avocado trees don't get any fruit without the pollination of the honeybees," said Ohad Afik, a doctoral candidate at the Hebrew University of Jerusalem's bee research center.

Others, however, argue that avocados have sex in the wind. Bees aren't needed, asserts the University of Florida's Thomas Davenport. He's done experiments with avocado trees wrapped in bee-proof nets that show lots of pollination occurs when the wind moves pollen from one flower to another.

"A controversial view," said Mary Lu Arpaia of the botany department at the University of California-Riverside.

Researchers at the two universities report "a strong correlation" between bee activity and orchard fruit.

It's more than academic.

"We average about 7,300 pounds per acre per year in California," said Guy Witney of the California Avocado Commission. "We should be able to produce about 30,000 pounds per acre."

It's an important crop for growers in Mexico, Southern California, Israel, Chile, Argentina and South Africa. Mexico is far and away the largest producer, with annual harvests of more than 2 billion pounds.

Avocados - whether in salads, soups or guacamole - are gracing more tables across America. Annual consumption has more than doubled in the past six years, to an estimated 980 million pounds, according to the California Avocado Commission. Americans ate 47 million pounds of avocados on Cinco de Mayo.

So, if avocados are so popular with the human palate, what's with the bees?

Bees and avocados are not natural partners. Avocados are native to Central America. Christopher Columbus brought the first bees to the Western Hemisphere, said entomologist Sharoni Shafir.

Another clue to the avocado mystery sits on an end table of Shafir's lab. He offers a tiny spoon and two goblets of honey - one dark and fluid, the other golden and thick. Bees make the darker honey from avocado nectar. The golden, sweeter honey comes from grapefruit nectar.

"The avocado honey is good for cooking, but a bit bitter," Shafir said. "And it's extremely rich in minerals."

Growers bring beehives to their orchards to move pollen from one flower to another, but it's an easily distracted work force. The most common type of honeybee, the Italian, would rather spend a sunny morning flitting among orange blossoms or wildflowers.

"They'll go to avocado, but if a citrus tree is blooming nearby, bees will make a beeline to citrus and not to avocado," Arpaia said.

What seems to discourage the bees is the mineral composition of avocado nectar. Afik found high concentrations of minerals, particularly potassium and phosphorus, in avocado honey. You might guess this is related to fertilizers, but horticulturalist Amon Dag tested for that.

What's next?

Shafir says their experiments show that bees can be bred to develop a taste for avocado, but it will be up to someone in the avocado business to do that work.

"Until today, there is no commercial bee line for pollination," he said.

So your average avocado tree will just have to get by with a sex life that's either on the whim of a bee or blowing in the wind.

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