COS 39-6 - Re-evaluating the adaptive significance of grass stomatal responses

Abstract: Grasses evolved unique, dumbbell-shaped stomata that, in partnership with subsidiary cells, operate in a manner different to the standard kidney-shaped stomata of dicots. Previous work has shown that grass stomata open faster than other stomatal types in response to environmental stimuli, notably changes in light and leaf-to-air vapor-pressure deficit (VPD_L ). It has been widely suggested that this fast stomatal response allowed grasslands to expand globally due to increased aridity and seasonality 30 – 40 million years ago. Wild grasses represent a large proportion of the terrestrial vegetation, yet almost all prior work on stomatal responses to environmental stimuli were performed on domesticated grasses (crops). Therefore, it is pertinent to both our knowledge of grassland evolution and for predictions of grassland responses to rising VPD_L, to examine stomatal responses in wild grasses.

We examined both rapid and steady-state responses to changes in VPD_L and light in two closely-related clusters of wild grasses containing both C₃ and C₄ photosynthetic pathways (eight species total). A LI-6400 photosynthesis system and dew-point generator were used to control the environment in the chamber and to examine each factor independently. The response to changing VPD_L was determined across a range of 0.8 to 2.5 kPa with as many as 6 different steps. The light regime (PPFD) was altered in the sequence of 1000, 300, 1000, and 0 µmol m^-2 s^-1.

Stomatal rates of change to all three light perturbations were significantly higher in C₃ versus C₄ grasses. The stomatal steady-state sensitivies to VPD_L were also higher in C₃ than C₄ grasses (83.19 ± 14.28 and 30.10 ± 5.12 mmol m^-2 s^-1 ln(VPD_L)^-1, respectively). The percentage of stomatal changes from the prior steady-state were generally smaller than the values reported for dicot species (< 10% in most of the species).

Compared to non-grass species, grass stomata (C₃ and C₄) were less sensitive and slower to respond when facing changes in VPD_L and light, but both non-grass species and our grasses were less sensitive and slower than domesticated grasses. We conclude that prior hypotheses concerning the adaptive significance of grass stomata may have been biased by focusing on crop species and not wild grasses.