The private lives of C4 grasses: drought responses across biochemical subtypes

Abstract

With changes in global climate, drought events have shifted towards more frequent and more severe regimes. Quantifying plant responses to changes in these drought conditions is important for understanding future ecosystem changes and forecasting the impacts on human populations. Droughts can elicit acute temporary environmental changes such as increased leaf thermal stress, high atmospheric moisture demand and reduced soil moisture. However, long-lasting consequences in ecosystem structure and function often coincide with prolonged intensity and duration of drought events. While we understand general trends of changes in drought events on plant function, we still require empirical evidence that translates species-level responses in an altered climate, particularly in C₄-dominated grasslands. Grasslands dominate terrestrial biomes and provide a wide array of ecosystem services. C₄ grasses are the dominant species in many tropical grasslands worldwide, as well as in temperate regions including the tallgrass prairie of the Flint Hills ecoregion. C₄ grasses play essential roles in mediating carbon, nitrogen and hydrological cycles. Nearly half of all grass species worldwide use C₄ photosynthesis, a modified anatomical and biochemical form of C₃ photosynthesis that provides an ecological advantage during hot, dry conditions. Over the past 24 – 35 million years, parallel evolution of C₄ photosynthesis has occurred over 24 times in the grass family, Poaceae, and over 66 times in all flowering plants. As a result, the evolutionary lineage and the biochemistry of C₄ grasses influence the photosynthetic and water use strategies we observe in modern C₄ lineages. Species are classified into three distinct subtypes of C₄ photosynthesis based on unique carbon-fixation biochemistry, and are each adapted to the climatic conditions in which they evolved (wet-adapted NADP-ME subtype, intermediate PCK subtype, and dry-adapted NAD-ME subtype). All C₄ grasses are presumed to have roughly similar physiological responses to drought events, and yet these processes are rarely compared across species or distinct evolutionary lineages. While the physiological C₄ grass responses to soil moisture declines have been studied extensively, similar investigation into changes in atmospheric drought are lacking. Vapor pressure deficit (VPD), a proxy for atmospheric drought, is a dynamic, yet critical regulator of ecosystem processes. For most herbaceous species, VPD reduces photosynthetic capacity while accelerating terrestrial water loss. In my thesis, I measured physiological, structural, and anatomical traits of the three C₄ biochemical subtypes related to water use safety/efficiency and framed my hypotheses around the role of ancestry and biochemistry driving responses to changes in VPD. Using a suite of traits measured across multiple temporal scales, I assessed how structural and anatomical traits drive physiological responses of C₄ subtypes during drought. In Chapter 2, I generated 87 twelve-hour response curves to quantify the gas exchange responses of 15 species of C₄ grass. I found that all C₄ subtypes showed relatively low stomatal sensitivity to changes in VPD, but no recovery in gas exchange rates were observed over the experimental time scale. The wet-adapted NADP-ME subtype showed the lowest tolerance to high VPD across the 12-hour period because these species allocated resources towards anatomical traits that maximize photosynthetic capacity during ideal conditions. The dry-adapted NAD-ME subtype maintained photosynthetic stability by prioritizing drought-safe anatomical traits, despite downregulating stomatal conductance under high VPD. The intermediate PCK subtype had the highest initial gas exchange rates but showed the steepest decline across the VPD response curve. Both the dry-adapted and intermediate subtypes exhibited anatomical traits that prioritize safety during droughts, but differences in leaf [delta]13C, stomatal traits and gas exchange responses indicate the coordination between anatomical and physiological traits vary across PCK and NAD-ME groups. In Chapter 3, I tested whole-plant responses of 6 species for 10 consecutive days in a modified drying oven that simulated high, temperature-driven VPD. I found that C₄ subtype gas exchange responses did not vary under isolated VPD driven by high temperatures, but subtype responses diverged under combined VPD and soil water deficits. Differences in microanatomical traits across C₄ subtypes were consistent with Chapter 2 results, and we further identified subtype-specific tradeoffs in specialized leaf water-holding cells. This short-term experiment was paired with a season-long study of the same species under ambient environmental conditions at Konza Prairie. Throughout the growing season, water loss strategies of C₄ subtypes varied, and showed the largest differences on the warmest sampling day. Physiological and structural traits indicated that the C₄ subtypes occupied different seasonal optima with distinct nitrogen use strategies and integrated stomatal behaviors. In both chapters, coordination between anatomical traits and physiological function was evident, with distinction by photosynthetic subtype. Thus, a combination of physiological, anatomical, and structural traits was required to illustrate the covert differences in water loss strategies across C₄ subtypes. Together, these results demonstrate that atmospheric and soil droughts interact to shape plant community structure and function in grassland ecosystems, with subtype-specific responses linked to physiological and anatomical trait variation. As rising VPD continues to reshape the terrestrial plant water balance, understanding these responses will be critical for predicting grassland responses to droughts and informing ecosystem management under a shifting climate.