April 14, 2005
Plants need to chill out before they spring into spring
WEST LAFAYETTE, Ind. - April showers bringing May flowers is only half the story. Trees and many other plants actually keep a careful record of how much time they spend at temperatures close to freezing and bloom only when they've met a quota of cold.
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Plants must meet this "chilling requirement" before they can blossom in the spring, said Peter Hirst, associate professor of horticulture at Purdue University.
The chilling requirement, which is the length of time that must be spent at near-freezing temperatures before a plant can bloom, differs among species and is longer for plants growing in higher latitudes.
"The chilling requirement explains why we don't have apricots growing in Indiana," Hirst said. "They have such a short chilling requirement that they bloom too early in the spring to survive here."
Hirst said the chilling requirement protects plants from blooming on unseasonably warm days in the middle of winter.
"It's of great evolutionary importance," he said. "Plants don't want their flowers to open too early in the year because there's too great a risk their flowers will freeze, and if their flowers are damaged, they can't reproduce."
A fairly high chilling requirement ensures plants won't bloom until early spring, when the risk of freezing is lower.
But the functional aspects of the chilling requirement remain a mystery.
"Nobody knows how plants keep track of the amount of chilling they've received in a season," Hirst said. "The patterns are well-documented, but we still don't know how a plant records chill or how it tells itself it's met the quota."
After enough days of chilling, a warm day is all it takes for spring to burst forth, said Robert Joly, a Purdue horticulture professor.
"Once the chilling requirement has been met, warm temperatures will bring a tree out of dormancy," Joly said. "Those buds are ready to emerge and are just waiting for the proper temperature to speed things along. The enzymes that control flower opening function faster at higher temperatures, which is why trees seem to blossom overnight after the first warm day in the spring."
This phenomenon raises questions about the general topic of how plants keep time and sense changes in their surroundings, Joly said.
"How plants perceive environmental factors is something you would think we'd know by 2005. Although we know a great deal about how plants respond to their environment, we don't yet have a clear understanding of how plants sense their environment at the cellular or molecular level," he said.
"We don't understand how plants 'know' it's getting hot or cold, how they can tell the days are getting longer or shorter, or how they know when they've met their chilling requirement. These are some of the most interesting and important questions in plant biology today."
Writer: Jennifer Cutraro, (765) 496-2050, jcutraro@purdue.edu
Sources: Peter Hirst, (765) 494-1323, hirst@purdue.edu
Robert Joly, (765) 494-6997, joly@purdue.edu
Ag Communications: (765) 494-2722;
Beth Forbes, forbes@purdue.edu
Agriculture News Page
Related Web site:
Purdue Department of Horticulture and Landscape Architecture
PHOTO CAPTION:
This star magnolia has literally had enough of the cold. Many shrubs, trees and other plants do not bloom until they've accumulated a set length of time at near-freezing temperatures, an adaptation that prevents them opening up too early in the spring when there is still a risk of freezing. (Purdue University photo)
A publication-quality photo is available at http://news.uns.purdue.edu/images/+2005/star-magnolia.jpg
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