Figure 2.

Guard cell pressure drives stomatal aperture and stomatal conductance. (a) Stomatal aperture, a and, consequently, (b) stomatal conductance to CO₂ diffusion, g_c, increase with guard cell pressure P_g in a saturation fashion. Stomatal aperture control is achieved through osmotic regulation of P_g. The operational g_c, g_c(op), is typically regulated via P_g within the high sensitivity region of the curve (double arrow). Maximum conductance, g_c(max), is determined by the number and size of stomata (see equation (2.1)). Data are for Tradescantia virginiana, adapted from Franks & Farquhar [17]. (c) Illustrating how altering g_c(max) keeps g_c(op) in the high sensitivity region of the P_g versus g_c curve. Starting at the initial operating point (point a), a sustained change in c_a from ambient (amb.; solid line) to elevated CO₂ concentration results in adjustment of g_c to a lower g_c(op) (point b). The plant then changes stomatal size and density in new leaves, altering the g_c versus P_g curve (dashed line, elevated CO₂; elev.), to return g_c(op) to its optimal position (point c). Similarly, following a sustained shift from ambient to subambient c_a, the plant adjusts g_c to a higher g_c(op) (point d). New leaves are produced with altered stomatal size and density, shifting to a new g_c versus P_g curve (dotted line, subambient CO₂; sub.) and increasing g_c(max) to return g_c(op) to its optimal position (point e). Curves in (c) are representative, based on (b).