How a Tiny Pit Decides a Pine Tree’s Fate


Wanna know your future? If you’re a ponderosa pine, barring unfortunate axe-related incidents, you actually can. Yet you or I couldn’t tell by looking. The fortune-telling body part is a microscopic manhole cover.

In trees, water is essential for producing food from sunlight and carbon dioxide, a process called photosynthesis, but also for maintaining pressure in plant cells, a concept called turgor. For both these reasons, the more water a tree transports, the faster it grows.

Trees accomplish this by exploiting two physical properties of water: evaporation and the capillary effect. Openings in needles or leaves called stomata permit gas exchange by the plant, an important ability in its own right (plants must breathe just as we do). But stomata also permit evaporation. This might seem like a bad thing, but under normal circumstances it is all part of the plant’s plan.

As water molecules evaporate from these openings, those right behind them advance to take their place, attracted by the charges of the surfaces near the opening. But as they do, a whole chain of water molecules —tightly bound to each other by hydrogen bonds — advances behind. This is called the capillary effect, and it it is why water magically ascends capillary tubes or a paper towel that even briefly grazes a spill below it. In a tree, water is hoisted through the water-conducting tissues in the trunk with zero effort on the plant’s part.

The water pipes in pine trees are not like the plumbing in your house, though. Instead of long, uninterrupted tubes, they are constructed of many short tubes called tracheids with angled ends. These shorter tubes connect to each other along their sides via structures called bordered pits, which look a lot like speakers.

Bordered pits line tracheids in Norway spruce wood. Credit: Dr. Michael Rosenthal, Technische Universität Dresden, Wikimedia (CC-by-SA 3.0)

In cross section the bordered pit takes on more of a donut shape. The pit is made of three parts: the aperture, the torus, and the margo. The aperture is the little hole through which water enters the pit. The torus is the donut hole, which is suspended in the middle of the pit by the margo, a porous barrier made of the tracheid’s primary cell wall  (the much thicker secondary cell wall makes up the torus and surrounds the pit).

Bordered pit in cross section (left) and transverse (right) with diameters of aperture, torus, and margo indicated. Credit: Roskilly et al. 2019

If during a drought air starts to creep into the tracheids from the roots, like a kid slurping up the dregs of a drink through a straw, the torus is pulled up against the aperture. The flexible margo bends to permit the movement. As the torus is wider than the aperture, it seals the door of the pit, blocking the air from traveling further into the wood.

It is a tiny variation in the structure of these microscopic bordered pits seems to be a crucial factor determining a tree’s longevity. A team of scientists from Montana, New York, and France examined two populations of ponderosa pine in Idaho and collected two core samples from the trunk of each tree sampled. They measured and compared the rings of the trees over time. Thicker rings indicate faster growth and thinner rings vice versa. They published their findings this June in the journal PNAS.

The oldest trees the scientists studied had slower average growth rates throughout their lives than the young trees they sampled. Even when they were young these old trees had grown slower. And, crucially, these older trees had larger torus overlap — the width of the pit border covered by the torus — than younger trees. The difference was largely the result of having smaller apertures.

Trees with smaller apertures resist drought better because when the torus is drawn up against them, they seal better. But smaller apertures also means water travels more slowly through the tree when the doors are open, slowing growth.

So there’s the rub: grow fast — get big, compete for light better, reproduce faster, and increase your chances of early survival — but make it easier for drought to kill you. Or make your drought-tight doors smaller and stronger, slow growth and prolong success, but increase the chance you’ll live to see your grandkids make cones of their own.

These sorts of trade-offs are well known in the natural world. What is surprising and unusual about this one is that it seems to come down to a single, microscopic trait. No other trait the scientists tested, such as wood density or tracheid diameter, affected growth rate or longevity.

It is also surprising in light of the fact that so many other factors could be at play: pathway length, conduit size and pit density, for instance. But since the scientists focused on a single species of tree at similar trunk sizes, they may have been able to unmask the effect of the predictive pit.

It is also possible that tracheid diameters and wood density are constrained by factors like freezing and dryness in a way that pits are not. For instance, smaller tracheid diameters are favored in cold places because skinnier cells reduce the incidence of tube-clogging air bubbles popping up in the pipes as a result of freeze-thaw cycle.

Based on these data, if climate change leads to drier conditions in already dry environments as most scientists forecast, we can expect conifer forest productivity to decline as slower growing trees are favored over speedier whippersnappers. That will not be pleasing to those who harvest trees for pulp or wood, but what can you do? It’s the pits.

Reference

Roskilly, Beth, Eric Keeling, Sharon Hood, Arnaud Giuggiola, and Anna Sala. “Conflicting functional effects of xylem pit structure relate to the growth-longevity trade-off in a conifer species.” Proceedings of the National Academy of Sciences(2019): 201900734.



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