The Physiology of Flowering Plants - KHAM PHA MOI

20 FLOW OF ENERGY AND CARBON THROUGH THE PLANTP ¼ CO 2R(2:2)R comprises components contributed respectively by the stomata, R s ; the cuticle,R c ; the boundary air layer, R a; and by the mesophyll tissue, R m . (Some authors referto conductances rather than resistances; conductance is the reciprocal of resistance,1/R.) Because the stomatal and cuticular resistances act in parallel rather than inseries, the mathematical relationship between them is1R ðsþcÞ¼ 1 R sþ 1 R c(2:3)But since values of R c are 500–1000 times higher than values of R s, 1/R c is negligiblecompared with 1/R s and is consequently often omitted in calculations. The boundarylayer, stomatal and mesophyll resistances all act in series and consequently can beaddeduptomakeR. If cuticular resistance is ignored, we can now expand Equation 2.2 :P ¼ CO 2R a þ R s þ R m(2:4)Usually there is sufficient air movement to keep R a low relative to R s and R m , andvariation in wind speed, once above a minimum, does not have much effect on CO 2uptake. As stated in the text, mesophyll resistance R m does not vary once growthhas ceased. Hence R s , the stomatal resistance, becomes the critical one.StomataMost of the entry of CO 2 into photosynthetic tissues occurs throughthe stomata (singular: stoma). These are minute structures in theepidermis, consisting of two highly specialized elongate guard cellsenclosing a pore between them (Fig. 2.6). The guard cells are oftenflanked by a few subsidiary (accessory) cells differing morphologicallyfrom the remaining epidermal cells. The shape of the guard cellsand the arrangement of their cell-wall thickenings ensure that whenthe guard cells are more turgid than the subsidiary cells, the guardcells bulge outwards into the subsidiary cells and separate in themiddle, opening the pore. When the guard cell turgor equals or isless than that of the adjacent cells, the guard cells shrink togetherand the pore closes. All intermediate stages between maximal opening,as permitted by the elasticity of the walls, and complete closureare possible. At full opening, the stomatal apertures of Phaseolusvulgaris measure only 37 mm, while fully open stomata of Zebrinapendula reach pore sizes of 1231 mm. In the grass family, Poaceae,stomata are very elongate; a fully open stomatal pore of Avena sativa(oat) measures 838 mm. The stomatal frequency per cm 2 of leafsurface usually ranges from c. 1000 to 200 000. The apertures are sosmall that at the most 3% of the total leaf surface is occupied by thepores. Yet an illuminated leaf absorbs CO 2 from the atmosphere withgreat efficiency. A leaf can maintain a steep diffusion gradient for thegas, and many small pores have a large amount of edge in relation totheir surface area. Gas diffusion through a hole is more rapid round