Gas Exchange Dynamics in C^3 and C^4 Grasses: Consequence of Differences in Stomatal Conductance

摘要: In full sunlight (photosynthetic photon flux density, 0.4—0.7. μm, PPFD), stomatal conductance to water vapor (g) and transpiration rates (E) are usually higher, use efficiency lower, in C3 than C4 species. the grass Bromus inermis, difference between sun [PPFD] > 1500 μmol°m—2°S—1) shade (PPFD ° 400 μmol°m—2°s—1) was twice as large species Sorghastrum nutans (226.7 vs. 97.8 mmol.m—2.S—1, respectively). Similar differences were measured four additional grasses. Variability PPFD incident on leaves is very common grasslands, stomata known respond relatively rapidly fluctuations this resource. This study evaluated consequences of different responses grasses during periods variability. Photosynthesis (A, CO2 uptake) g varied substantially both B. inermis S. fluctuated levels (4—10 min intervals). Time constants derived form first—order equations describing after abrupt changes indicated that approached steady—state more inermis. However, because greater magnitude change absolute rate rapid compared (25.7 12.4 mmol°m—2°s—1°min—1 for reductions g, New achieved 12—13 a 15—19 A statistically based two—state (sun/shade) simulation model used quantify costs E nonsteady—state varied. Costs calculated output or field hypothetical gas exchange which changed instantaneously. Results suggested even though grass, short—term (2—6 min) variations much sixfold higher Lower cost variable (due inherently smaller alterations g) previously unrecognized consequence photosynthetic pathway. species, particular, often most successful high—PPFD, water—limited environments leaf—level variability occurs daily from cloud cover within—canopy shading. these environments, conserve under conditions may contribute significantly success