Leaving the warmth of bed, my first stop each winter morning is a downstairs window that looks out on an old rhododendron, a pink-flowering catawba of forgotten pedigree planted years ago against the north wall of our home. “Rhody says it’s cold outside!” I announce out loud, even if only Berry the Cat is awake to hear me. Or Marjorie may be the first to pass that window and she will make the same announcement. We mean really cold, so frigid that Rhody’s leaves are curled into green pencils that point straight down. In terms of how we should dress for the morning, Rhody’s read on winter weather is as meaningful as the thermometer.
In addition to signaling the weather, this rhododendron is an important functional plant in our garden. Throughout the year, chickadees use it as a stopover in flight from tall trees at the edge of the garden to the porch sunflower feeders, often taking time to scout the branches for aphid eggs. In spring, bumblebees get tipsy on nectar from the bright pink blossoms. And this is the one plant where I can count on seeing bumblebee moths hovering at the throat of flowers to sip nectar.
Why do evergreen rhododendrons curl and droop their leaves on cold winter mornings? The first botanists to study this phenomenon focused entirely on the leaf curling, hypothesizing that it served to protect the leaf from desiccation on cold sunny days. They assumed that winter sunlight would warm the leaf surface sufficiently to crank up photosynthesis, a process that requires at least partial opening of microscopic pores, called stomata, on the underside of the leaf. It is through the stomata that gas exchange occurs, including the uptake of carbon dioxide needed in photosynthesis accompanied by loss of water vapor from the saturated surfaces of the inner leaf. By wrapping the stomata within a tightly curled leaf, sufficient carbon dioxide uptake could occur while minimizing water vapor loss, a mechanism that would be particularly effective in the face of stiff winds that pull water out of open stomata on flat leaves.
The above theory was presented without the essential evidence of stomates opening at below freezing temperatures. When instruments to directly measure gas exchange at the leaf surface were developed, plant physiologists discovered that rhododendron stomates do not open on winter days when the leaf temperature is below the freezing point. Photosynthesis would be shut down. A new theory to explain Rhody’s behavior needed to surface.
As a plant physiologist by training, I have to be careful here and not venture too far into a history of the theories proposed to explain the curling and drooping of winter leaves. The essential question should be, what does the gardener need to understand about current explanations for these phenomena. Let’s begin with work done as early as 1933 by a Japanese scientist, Y. Fukuda, who demonstrated that rhododendron leaves did not curl if covered with snow, thereby insulating them from cold air temperatures. He concluded that curling was correlated with leaf temperature, not air temperature.
Fukuda’s studies were followed by those of scientists looking at the interaction of cold temperatures and bright light. It was discovered that leaf cell membranes, particularly those of the chlorophyll-rich chloroplasts (organelles within the leaf cell where photosynthesis occurs), are susceptible to damage by intense light when they are cold. Such damage is called “photoinhibition”, loosely defined as damage to the photosynthetic mechanism by light when photosynthesis cannot proceed, as when the leaf is frozen. Leaf drooping and curling reduce the exposed surface area of the leaf, thereby reducing the quantity of light impinging on the leaf during cold temperatures, thus limiting photoinhibition.
The observant gardener will notice that curled rhododendron leaves do indeed point downward on frigid winter mornings. On sunny afternoons, should leaf temperatures rise above the freezing point, the leaves will uncurl and return to nearly horizontal positions, and some winter photosynthesis may occur, just as it may in the needles of pines, firs, and other evergreens, as long as soil water is available for uptake by the plant. If the soil water is frozen, stomata will stay closed, and photosynthesis will remain shut down, yet Rhody’s leaves will still uncurl, as long as leaf temperatures remain above freezing, and they may return to the horizontal, depending on the amount of sunlight.
As I understand current thinking about rhododendron leaf behavior in winter, leaf curling and drooping, both induced by cold temperatures, should be thought of as independent of one another. I have observed Rhody’s leaves to quickly uncurl when the temperature rises above 32 ºF, yet the leaves continue to droop downward for hours, particularly in rainy weather, an indication that light intensity is an important factor in leaf orientation.
To view garden plants as the complex organisms that they are, capable of measuring environmental conditions and reacting with precision, adds another dimension to the joy of gardening. The mechanisms for their behavior, their reactions to changes in their environment, may not be fully understood, but we know with the certainty of science that plants can measure their environment. They can detect change in the availability of water in the soil, predict a drought, and regulate their growth accordingly through complex chemical messages sent from roots to leaves. They can intercept the chemical signals of herbivores feeding on nearby plants, and construct specific chemical defenses against those herbivores. By secreting specific root substances into the soil, they can enhance populations of beneficial soil microorganisms around their roots. And they have adapted truly remarkable ways of surviving in harsh environments, like a Maine garden in winter.