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

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, and water use efficiency lower, in C 3 than C 4 species. In the C 3 grass Bromus inermis, the difference in conductance between full sun (photosynthetic photon flux [PPFD] > 1500 μmol°m — 2 °S — 1 ) and shade (PPFD ° 400 μmol°m — 2 °s — 1 ) was twice as large as in the C 4 species Sorghastrum nutans (226.7 vs. 97.8 mmol.m — 2 .S — 1 , respectively). Similar differences were measured in four additional C 3 and C 4 grasses. Variability in PPFD incident on leaves is very common in grasslands, and stomata are known to respond relatively rapidly to fluctuations in this resource. This study evaluated the consequences of different responses in stomatal conductance between C 3 and C 4 grasses during periods of sunlight variability. Photosynthesis (A, measured as CO 2 uptake) and g varied substantially in both B. inermis and S. nutans as PPFD fluctuated between sun and shade levels (4—10 min intervals). Time constants derived form first—order equations describing responses in g after abrupt changes in PPFD indicated that g approached steady—state levels more rapidly in S. nutans than in B. inermis. However, because of the greater magnitude of change in g between sun and shade PPFD in B. inermis, the absolute rate of change in g in this C 3 grass was twice as rapid compared to S. nutans (25.7 vs. 12.4 mmol°m — 2 °s — 1 °min — 1 for reductions in g, respectively). New steady—state levels in g were achieved in 12—13 min after a change in PPFD in S. nutans and 15—19 min in B. inermis. A statistically based two—state (sun/shade) simulation model was used to quantify costs in A and E of the nonsteady—state responses measured for both species as PPFD varied. Costs were calculated as the difference between model output of A or E based on field responses and model output based on hypothetical gas exchange responses in which A and g changed instantaneously. Results suggested that even though the absolute rate of change in g changed was greater in the C 3 grass, costs in E during periods of short—term (2—6 min) variations in PPFD were as much as sixfold higher than in the C 4 species. Lower cost in E in C 4 species during periods of variable PPFD (due to inherently smaller alterations in g) is a previously unrecognized consequence of the C 4 photosynthetic pathway. C 4 species, and grasses in particular, often are most successful in high—PPFD, water—limited environments in which leaf—level variability in PPFD occurs daily from cloud cover and within—canopy shading. In these environments, stomatal responses that conserve water under nonsteady—state conditions may contribute significantly to the success of C 4 species.