Errors in how scientists account for water loss from leaves may be skewing estimates of how much energy plants make through photosynthesis and cause errors in the models of how individual leaves function and even of the global climate. The errors are particularly pronounced when a plant’s water supply is limited — a condition of increasing interest as plant breeders and climate scientists grapple with the effects of global warming.
Researchers have long assumed that the main way that plants lose water is through leaf pores called stomata. When water is abundant, the stomata open wide to let carbon dioxide flow in — maximizing photosynthesis, but allowing water to exit. Plants also lose moisture through a leaf’s waxy outer surface, or cuticle, but this effect has been considered negligible.
This understanding, in turn, has shaped how scientists extrapolate the flow of CO2 into a leaf. Measuring CO2 inside a leaf requires cumbersome, custom-made equipment, so researchers in the field often use measures of water loss and other factors to calculate the concentration of CO2 inside. Once they have estimated the internal CO2 concentration, researchers can calculate how efficiently the plant is converting the gas into food — a component of primary productivity, a measure that is an important factor in some climate models.
Calculations are based on water loss through stomata, and disregard the water vapor that passes directly through the cuticle. Hanson’s experiments suggest that this is a workable approximation when water is plentiful — but when it is scarce, and the stomata close, a greater proportion of moisture is lost through the cuticle. Failing to adjust for this could throw off calculations of how well plants convert CO2 to sugars during photosynthesis, Hanson says. “While stomata are closed, this small (15%) error is now a massive (600%) error,” he adds.
Hanson first became aware of this in 2015, when plant physiologist John Boyer of the University of Missouri in Columbia approached him after a seminar. Hanson had just presented data showing his attempts to explain how properties of leaf cells can limit CO2 capture. Boyer offered Hanson an alternative explanation — water loss through the cuticle — and described data that his lab had collected in the 1980s, but that had garnered little attention.
Boyer’s team had found that water loss through the cuticle skewed calculations of CO2 concentrations inside sunflower leaves to be 15% too high when water was abundant and stomata were open. And when stomata were closed, predicted CO2 concentrations were six times higher than direct measurements taken inside the leaf.