Picture two houses in the desert.
One is built of wood, the other adobe. The wood one has air conditioning. The adobe house does not — nor does it need it.
People are like the pine house, spending lots of our molecular currency on keeping ourselves at a comfortable, optimal temperature of about 98.6 degrees. It’s called homeostasis: keeping things mostly stable no matter what's going on outside.
But there's more than just one way to regulate temperature.
Most plants, says Brian Enquist, don't spend energy like people on maintaining their body temperature. Instead, depending on the environment, they can build leaves cheaply or they can invest in careful, expensively constructed leaves that pay off in the long run.
Enquist is a professor of ecology and evolutionary biology at the University of Arizona and an external professor at the Santa Fe Institute. Using grant funding from the National Science Foundation, his lab is challenging the popular belief that plants are poikilotherms, organisms whose internal temperatures depend solely on their external environment.
The field of biology still widely asserts that plants accept the temperature of their environment. For example, if it's 100 degrees outside, the plant must be that hot, too. According to new research, that's just not so.
"Plants typically don't generate their own body heat, but they have other ways of regulating their body temperature," Enquist said. "Plants that grow at dramatically different temperatures appear to be able to partially compensate for changes in temperature."
In their recent paper in Trends in Ecology and Evolution, Enquist and his team compiled data from pre-existing literature on temperatures in plant leaves from a variety of species, including oak trees, cotton, pineapple, lettuce, tomato and pine trees. They found, overwhelmingly, that leaves of these plants can adjust their internal temperatures as mammals do. They just do it differently than we do.
Sean Michaletz, a postdoctoral fellow in Enquist's lab and the primary author of the paper, said, "Across the globe, plants are generally warmer than air in cool environments, and cooler than air in warm environments, so despite wide variation in air temperatures, plant temperatures are relatively stable."
The researchers developed mathematical models to link climate, leaf traits, leaf temperatures and photosynthesis. The models show that some leaf traits have evolved to stabilize leaf temperatures and speed up photosynthesis.
"Although air temperatures and plant traits vary widely around the world, there appears to be relatively little variation in leaf temperatures and a remarkable constancy in the total amount of carbon a leaf gains over its lifetime," Michaletz said. "So while on the surface many plants appear to be very different, fundamentally they all operate in the same ways."
Much as it does in architecture, form follows function in biology, so plants evolve with specific traits that make the most sense for their environment (think wood versus adobe in the desert). For example, the size, thickness, texture and color of leaves all affect the ability of plants to not only photosynthesize but also to regulate their temperature.
"Instead of spending energy on cooling and warming themselves, plants can change their construction to heat up slower and maintain temperatures that are much lower than the ambient temperatures around them," Enquist said.
Why might a plant care if it is too hot or cold? The answer is RuBisCO, one of the most ubiquitous enzymes on the face of the planet. It exists inside the leaves of plants.
RuBisCO catalyzes the first major step of carbon fixation, a process by which plants convert carbon dioxide in the air into energy in the form of molecules such as glucose. RuBisCO creates the precursor for sugar. In this sense, it's the difference between life and death for a plant.
But for a life-and-death enzyme, RuBisCO is pretty fussy: It will work only within a narrow range of temperatures.
"RuBisCO has this sweet spot of temperature, where it performs best," Enquist said. "There's not a lot of variation in its optimal temperature, so we suspect that there's been very strong selection to try to keep the temperature environment in the leaves as similar as possible."
The more the plant can minimize temperature variation within the leaf, the happier the RuBisCO. The happier the RuBisCO, the more it's working. And, finally, the more it's working, the more the plant can photosynthesize and grow.
Novelty aside, the Enquist lab's paper has big practical implications.
With the global population growing fast, maximizing food production is ever important. In the past, agriculturists have not considered thermal properties of plants before selecting specific varieties of crop plants for domestication. By looking at what physical traits best facilitate temperature regulation in a certain environment, food producers could potentially increase their yields.
"Our paper points to new ways to begin to address these sorts of big questions, and how we understand ecosystems," Enquist said.