Stomatal Physics Kathryn Sweet1, Keith A. Mo2, David Peak1 1 Physics Department, 2 Biology Department, Utah State University, Logan, UT 84322

Background

• Stomata, microscopic pores on a leaf’s surface, regulate the diffusion of CO2 from, and the diffusion of water vapor to, the air. VAPOR PHASE MODEL OF STOMATAL CONDUCTANCE Experimental chamber: Leaves are placed in a gas exchange chamber that regulates • Stomata are responsible for fixing essenally all carbon in the biosphere and g = stomatal conductance (aperture) S and measures environmental condions. generang over 90% of the water vapor in the atmosphere over landmasses. g = conductance at 100% external humidity S0 A thermal imaging camera captures • Exactly how stomata respond to temperature, light intensity, and ambient Δw w = saturated water vapor (WV) concentraon inside the leaf L temperature (±0.05 ˚C) images of the leaf CO2 and humidity, is sll a maer of acve debate. gS0 −θ Δw = w –w , where w is the WV concentraon in the outside air L a a (160,000 pixels–roughly one stoma per • Most research probing this queson focuses on idenfying and unraveling wL Θ = humidity sensivity due to resistance to WV diffusion from pixel). See image to the right. complicated biochemistry. Recent invesgaons in our laboratory, however, gS = inside the leaf to the air indicate that much of stomatal behavior can be understood in terms of a 1 Z w + Δ Z = humidity sensivity due to resistance to heat transport inside simple vapor phase physical model. From the non-vein pixel temperatures the leaf Leaf heterogeneity: The (veins don’t contribute to evaporaon) temperature across a leaf is MECHANICS we can calculate an accurate average leaf rarely uniform (see image). temperature and an accurate whole leaf Measurements that do not gS = χ(Pg − mPe ) conductance. account for this (and cannot 10 to 1000 mm–2 • Stomatal aperture is proporonal to the difference in turgor pressures in guard (P ) and adjust for veins) for are g Bean-shaped “guard” surrounding epidermal (P ) cells. unlikely to accurately assess e cell pairs form stomatal • Larger epidermal cells have a mechanical advantage (m). stomatal behavior. pores on the surface of a leaf surrounded by jig- saw-puzzle-shaped THERMODYNAMICS epidermal cells • Chemical potenal of water, Ψ, determines direcon of water transport

Experiments: with Xanthium strumarium (cocklebur), to test the • Liquid Phase Ψ = P − π , π = cRT validity of the model. • This yields gS 0 = χ(π g − mπ e ) 1. Constant CO2, light, and chamber air humidity constant; vary RT w −Tw 0 • Vapor Phase Ψ = ln , w = w e T air temperature between 20 and 33 C. sat 0 0 vw wsat 2. Constant CO2, light, and leaf temperature (26 C); vary chamber air humidity. • Transport of water vapor is fast compared with hydraon of guard cells 3. Constant CO2, light, and air temperature (260 C); vary chamber air humidity. • This leads to equilibrium condions: • Liquid phase potenal in the epidermis = liquid phase Results: When fit to the potenal at the evaporaon site = vapor phase potenal data to the vapor phase immediately outside the evaporaon site model agrees with • Vapor phase potenal in the stomatal pore = liquid phase experimental observaon potenal in the guard cells with the same values for Θ • And a steady state condion: and Z and same gS0 • Vapor phase potenal in the stomatal pore = (fracon, σ) temperature dependence. vapor phase potenal in the external air + (fracon, 1–σ) The figure to the right vapor phase potenal at the evaporang site shows the gS values • Approximaons: predicted by the model vs • Liquid phase potenal at the evaporaon site << RT / vw the gS values • Temperature of evaporang site is slightly lower than that of experimentally observed epidermis for 39 different condions • These yield Z across 10 days of experiments. Schemac of the interior of a leaf: Leaves evaporate water to regulate • σ is small (< 0.1) their temperature and take in CO2 for use in photosynthesis through • This yields θ stomatal pores on the leaf’s surface. The interior of a leaf is roughly 50% humid air.

• Some important aspects of stomatal behavior can be explained through simple mechanics and vapor phase physics. This runs counter to the prevailing tradion in plant physiology that essenally all stomatal behavior is 1. Isolated stomata (removed from mesophyll cells in leaf interior) respond to biochemistry. air humidity just as they do in intact leaves.1 1. Shope, J.C., et al., Plant, Cell & Environ., DOI: 10.1111/j.1365-3040.2008.01844.x (2008) • Some metabolic component to stomatal response is contained in the 2. Isolated stomata don’t respond to light and CO2, but when brought close to, 2. Mo K.A., et al. Plant, Cell & Environ., DOI: 10.1111/j.1365-3040.2008.01845.x. (2008) temperature dependence of g . A more complete understanding of this 3. Mo, K.A., et al., Plant, Cell & Environ., DOI: 10.1111/pce.12226 (2013) S0 but not in contact with, mesophyll cells they do.2 requires addional experiments varying light and CO2, which we are now 3. Isolated stomata respond to vapor phase ions.3 conducng.