Responsiveness to long days for flowering is reduced in Arabidopsis by yearly variation in growing season temperatures.

Conservative flowering behaviours, such as flowering during long days in summer or late flowering at a high leaf number, are often proposed to protect against variable winter and spring temperatures which lead to frost damage if premature flowering occurs. Yet, due the many factors in natural environments relative to the number of individuals compared, assessing which climate characteristics drive these flowering traits has been difficult. We applied a multidisciplinary approach to 10 winter-annual Arabidopsis thaliana populations from a wide climactic gradient in Norway. We used a variable reduction strategy to assess which of 100 climate descriptors from their home sites correlated most to their flowering behaviours when tested for responsiveness to photoperiod after saturation of vernalization; then, assessed sequence variation of 19 known environmental-response flowering genes. Photoperiod responsiveness inversely correlated with interannual variation in timing of growing season onset. Time to flowering appeared driven by growing season length, curtailed by cold fall temperatures. The distribution of FLM, TFL2 and HOS1 haplotypes, genes involved in ambient temperature response, correlated with growing-season climate. We show that long-day responsiveness and late flowering may be driven not by risk of spring frosts, but by growing season temperature and length, perhaps to opportunistically maximize growth.

Interactions between rising CO2 and temperature drive accelerated flowering in model plants under changing conditions of the last century.

Past studies have shown that flowering times have accelerated over the last century. These responses are often attributed to rising temperature, although short-term field experiments with warming treatments have under-estimated accelerations in flowering time that have been observed in long-term field surveys. Thus, there appears to be a missing factor(s) for explaining accelerated flowering over the last century. Rising atmospheric CO2 concentration ([CO2]) is a possible candidate, and its contributions to affecting flowering time over historic periods are not well understood. This is likely because rising [CO2] is confounded with temperature in the field and preindustrial [CO2] studies are relatively rare. To address this, we tested the individual and interactive effects of rising [CO2] and temperature between preindustrial and modern periods on flowering time in the model system, Arabidopsis thaliana. We used a variety of genotypes originating from diverse locations, allowing us to test intraspecific responses to last-century climate change. We found that accelerated flowering time between the full-preindustrial and full-modern treatments was mainly driven by an interaction between rising [CO2] and temperature, rather than through the individual effects of either factor in isolation. Furthermore, accelerated flowering time was driven by enhanced plant growth rates and not through changes in plant size at flowering. Thus, the interaction between rising [CO2] and temperature may be key for explaining large accelerations in flowering times that have been observed over the last century and that could not be explained by rising temperature alone.

Does sexual dimorphism predispose dioecious riparian trees to sex ratio imbalances under climate change?

Environmental changes have resulted in significant declines in native riparian forests that are comprised largely of dioecious tree taxa, including boxelder and iconic cottonwood/willow gallery forests. Dioecious species may be especially vulnerable to the effects of climate change given that they often exhibit skewed sex ratios that are reinforced by physiological and morphological specialization of each sex to specific microhabitats. A comprehensive data synthesis suggests that male individuals of boxelder and cottonwood taxa have a higher representation on dry microhabitats than females and are less physiologically sensitive to increased aridity than co-occurring females. Consequently, extreme male-biased sex ratios are possible under future climate conditions that could reduce population fitness below a sustainable threshold. Riparian willows, on the other hand, generally do not express obvious sexual dimorphism in habitat preference or physiological sensitivity to aridity. Thus, it is unclear whether climate change will impact population structure of willows in ways that parallel other dioecious riparian tree taxa. Future riparian tree restoration programs should aim to maintain future sex ratio balance that maximizes population fitness under projected hydro-climatological conditions. Recent advances in genomics will likely provide the critical tools for early sex determination in pre-reproductive trees across riparian tree species such that sex ratio balance could be targeted during initial stages of restoration, along with adaptations for drought tolerance and other key traits that are essential for survival under future conditions.

CO2 studies remain key to understanding a future world.

Contents 34 I. 34 II. 36 III. 37 IV. 37 V. 38 38 References 38 SUMMARY: Characterizing plant responses to past, present and future changes in atmospheric carbon dioxide concentration ([CO2 ]) is critical for understanding and predicting the consequences of global change over evolutionary and ecological timescales. Previous CO2 studies have provided great insights into the effects of rising [CO2 ] on leaf-level gas exchange, carbohydrate dynamics and plant growth. However, scaling CO2 effects across biological levels, especially in field settings, has proved challenging. Moreover, many questions remain about the fundamental molecular mechanisms driving plant responses to [CO2 ] and other global change factors. Here we discuss three examples of topics in which significant questions in CO2 research remain unresolved: (1) mechanisms of CO2 effects on plant developmental transitions; (2) implications of rising [CO2 ] for integrated plant-water dynamics and drought tolerance; and (3) CO2 effects on symbiotic interactions and eco-evolutionary feedbacks. Addressing these and other key questions in CO2 research will require collaborations across scientific disciplines and new approaches that link molecular mechanisms to complex physiological and ecological interactions across spatiotemporal scales.

Host Plant Physiology and Mycorrhizal Functioning Shift across a Glacial through Future [CO2] Gradient.

Rising atmospheric carbon dioxide concentration ([CO2]) may modulate the functioning of mycorrhizal associations by altering the relative degree of nutrient and carbohydrate limitations in plants. To test this, we grew Taraxacum ceratophorum and Taraxacum officinale (native and exotic dandelions) with and without mycorrhizal fungi across a broad [CO2] gradient (180-1,000 µL L-1). Differential plant growth rates and vegetative plasticity were hypothesized to drive species-specific responses to [CO2] and arbuscular mycorrhizal fungi. To evaluate [CO2] effects on mycorrhizal functioning, we calculated response ratios based on the relative biomass of mycorrhizal (MBio) and nonmycorrhizal (NMBio) plants (RBio = [MBio - NMBio]/NMBio). We then assessed linkages between RBio and host physiology, fungal growth, and biomass allocation using structural equation modeling. For T. officinale, RBio increased with rising [CO2], shifting from negative to positive values at 700 µL L-1 [CO2] and mycorrhizal effects on photosynthesis and leaf growth rates drove shifts in RBio in this species. For T. ceratophorum, RBio increased from 180 to 390 µL L-1 and further increases in [CO2] caused RBio to shift from positive to negative values. [CO2] and fungal effects on plant growth and carbon sink strength were correlated with shifts in RBio in this species. Overall, we show that rising [CO2] significantly altered the functioning of mycorrhizal associations. These symbioses became more beneficial with rising [CO2], but nonlinear effects may limit plant responses to mycorrhizal fungi under future [CO2]. The magnitude and mechanisms driving mycorrhizal-CO2 responses reflected species-specific differences in growth rate and vegetative plasticity, indicating that these traits may provide a framework for predicting mycorrhizal responses to global change.

Warmest extreme year in U.S. history alters thermal requirements for tree phenology.

The frequency of extreme warm years is increasing across the majority of the planet. Shifts in plant phenology in response to extreme years can influence plant survival, productivity, and synchrony with pollinators/herbivores. Despite extensive work on plant phenological responses to climate change, little is known about responses to extreme warm years, particularly at the intraspecific level. Here we investigate 43 populations of white ash trees (Fraxinus americana) from throughout the species range that were all grown in a common garden. We compared the timing of leaf emergence during the warmest year in U.S. history (2012) with relatively non-extreme years. We show that (a) leaf emergence among white ash populations was accelerated by 21 days on average during the extreme warm year of 2012 relative to non-extreme years; (b) rank order for the timing of leaf emergence was maintained among populations across extreme and non-extreme years, with southern populations emerging earlier than northern populations; (c) greater amounts of warming units accumulated prior to leaf emergence during the extreme warm year relative to non-extreme years, and this constrained the potential for even earlier leaf emergence by an average of 9 days among populations; and (d) the extreme warm year reduced the reliability of a relevant phenological model for white ash by producing a consistent bias toward earlier predicted leaf emergence relative to observations. These results demonstrate a critical need to better understand how extreme warm years will impact tree phenology, particularly at the intraspecific level.

A dynamic leaf gas-exchange strategy is conserved in woody plants under changing ambient CO2 : evidence from carbon isotope discrimination in paleo and CO2 enrichment studies.

Rising atmospheric [CO2 ], ca , is expected to affect stomatal regulation of leaf gas-exchange of woody plants, thus influencing energy fluxes as well as carbon (C), water, and nutrient cycling of forests. Researchers have proposed various strategies for stomatal regulation of leaf gas-exchange that include maintaining a constant leaf internal [CO2 ], ci , a constant drawdown in CO2 (ca  - ci ), and a constant ci /ca . These strategies can result in drastically different consequences for leaf gas-exchange. The accuracy of Earth systems models depends in part on assumptions about generalizable patterns in leaf gas-exchange responses to varying ca . The concept of optimal stomatal behavior, exemplified by woody plants shifting along a continuum of these strategies, provides a unifying framework for understanding leaf gas-exchange responses to ca . To assess leaf gas-exchange regulation strategies, we analyzed patterns in ci inferred from studies reporting C stable isotope ratios (δ(13) C) or photosynthetic discrimination (∆) in woody angiosperms and gymnosperms that grew across a range of ca spanning at least 100 ppm. Our results suggest that much of the ca -induced changes in ci /ca occurred across ca spanning 200 to 400 ppm. These patterns imply that ca  - ci will eventually approach a constant level at high ca because assimilation rates will reach a maximum and stomatal conductance of each species should be constrained to some minimum level. These analyses are not consistent with canalization toward any single strategy, particularly maintaining a constant ci . Rather, the results are consistent with the existence of a broadly conserved pattern of stomatal optimization in woody angiosperms and gymnosperms. This results in trees being profligate water users at low ca , when additional water loss is small for each unit of C gain, and increasingly water-conservative at high ca , when photosystems are saturated and water loss is large for each unit C gain.

Evolutionary history underlies plant physiological responses to global change since the last glacial maximum.

Assessing family- and species-level variation in physiological responses to global change across geologic time is critical for understanding factors that underlie changes in species distributions and community composition. Here, we used stable carbon isotopes, leaf nitrogen content and stomatal measurements to assess changes in leaf-level physiology in a mixed conifer community that underwent significant changes in composition since the last glacial maximum (LGM) (21 kyr BP). Our results indicate that most plant taxa decreased stomatal conductance and/or maximum photosynthetic capacity in response to changing conditions since the LGM. However, plant families and species differed in the timing and magnitude of these physiological responses, and responses were more similar within families than within co-occurring species assemblages. This suggests that adaptation at the level of leaf physiology may not be the main determinant of shifts in community composition, and that plant evolutionary history may drive physiological adaptation to global change over recent geologic time.

Responses of high-elevation herbaceous plant assemblages to low glacial CO2 concentrations revealed by fossil marmot (Marmota) teeth.

Atmospheric CO2 cycles of the Quaternary likely imposed major constraints on the physiology and growth of C3 plants worldwide. However, the measured record of this remains both geographically and taxonomically sparse. We present the first reconstruction of physiological responses in a late Quaternary high-elevation herbaceous plant community from the Southern Rocky Mountains, USA. We used a novel proxy-fossilized tooth enamel of yellow-bellied marmots (Marmota flaviventris)-which we developed using detailed isotopic analysis of modern individuals. Calculated C isotopic discrimination (Δ) of alpine plants was nearly 2 ‰ lower prior to the Last Glacial Maximum than at present, a response almost identical to that of nonherbaceous taxa from lower elevations. However, initial shifts in Δ aligned most closely with the onset of the late Pleistocene bipolar temperature "seesaw" rather than CO2 increase, indicating unique limitations on glacial-age high-elevation plants may have existed due to both low temperatures and low CO2. Further development of system-specific faunal proxies can help to clarify this and other plant- and ecosystem-level responses to past environmental change.

Increasing atmospheric [CO2] from glacial to future concentrations affects drought tolerance via impacts on leaves, xylem and their integrated function.

Changes in atmospheric carbon dioxide concentration ([CO2]) affect plant carbon/water tradeoffs, with implications for drought tolerance. Leaf-level studies often indicate that drought tolerance may increase with rising [CO2], but integrated leaf and xylem responses are not well understood in this respect. In addition, the influence of the low [CO2] of the last glacial period on drought tolerance and xylem properties is not well understood. We investigated the interactive effects of a broad range of [CO2] and plant water potentials on leaf function, xylem structure and function and the integration of leaf and xylem function in Phaseolus vulgaris. Elevated [CO2] decreased vessel implosion strength, reduced conduit-specific hydraulic conductance, and compromised leaf-specific xylem hydraulic conductance under moderate drought. By contrast, at glacial [CO2], transpiration was maintained under moderate drought via greater conduit-specific and leaf-specific hydraulic conductance in association with increased vessel implosion strength. Our study involving the integration of leaf and xylem responses suggests that increasing [CO2] does not improve drought tolerance. We show that, under glacial conditions, changes in leaf and xylem properties could increase drought tolerance, while under future conditions, greater productivity may only occur when higher water use can be accommodated.

FLOWERING LOCUS C drives delayed flowering in Arabidopsis grown and selected at elevated CO 2.

Altered flowering time at elevated [CO 2 ] is well documented, although mechanisms are not well understood. An Arabidopsis genotype previously selected for high fitness at elevated [CO 2 ] (SG) showed delayed flowering and larger size at flowering when grown at elevated (700 ppm) versus current (380 ppm) [CO 2 ]. This response was correlated with prolonged expression of FLOWERING LOCUS C ( FLC ), a vernalization-responsive floral repressor gene. To determine if FLC directly delays flowering at elevated [CO 2 ] in SG, we used vernalization (extended cold) to downregulate FLC expression. We hypothesized that vernalization would eliminate delayed flowering at elevated [CO 2 ] through the direct reduction of FLC expression, eliminating differences in flowering time between current and elevated [CO 2 ]. We found that with downregulation of FLC expression via vernalization, SG plants grown at elevated [CO 2 ] no longer delayed flowering compared to current [CO 2 ]. Thus, vernalization returned the earlier flowering phenotype, counteracting effects of elevated [CO 2 ] on flowering. This study indicates that elevated [CO 2 ] can delay flowering directly through FLC , and downregulation of FLC under elevated [CO 2 ] reverses this effect. Moreover, this study demonstrates that increasing [CO 2 ] may potentially drive major changes in development through FLC .

Oligosaccharide production and signaling correlate with delayed flowering in an Arabidopsis genotype grown and selected in high [CO2].

Since industrialization began, atmospheric CO2 ([CO2]) has increased from 270 to 415 ppm and is projected to reach 800-1000 ppm this century. Some Arabidopsis thaliana (Arabidopsis) genotypes delayed flowering in elevated [CO2] relative to current [CO2], while others showed no change or accelerations. To predict genotype-specific flowering behaviors, we must understand the mechanisms driving flowering response to rising [CO2]. [CO2] changes alter photosynthesis and carbohydrates in plants. Plants sense carbohydrate levels, and exogenous carbohydrate application influences flowering time and flowering transcript levels. We asked how organismal changes in carbohydrates and transcription correlate with changes in flowering time under elevated [CO2]. We used a genotype (SG) of Arabidopsis that was selected for high fitness at elevated [CO2] (700 ppm). SG delays flowering under elevated [CO2] (700 ppm) relative to current [CO2] (400 ppm). We compared SG to a closely related control genotype (CG) that shows no [CO2]-induced flowering change. We compared metabolomic and transcriptomic profiles in these genotypes at current and elevated [CO2] to assess correlations with flowering in these conditions. While both genotypes altered carbohydrates in response to elevated [CO2], SG had higher levels of sucrose than CG and showed a stronger increase in glucose and fructose in elevated [CO2]. Both genotypes demonstrated transcriptional changes, with CG increasing genes related to fructose 1,6-bisphosphate breakdown, amino acid synthesis, and secondary metabolites; and SG decreasing genes related to starch and sugar metabolism, but increasing genes involved in oligosaccharide production and sugar modifications. Genes associated with flowering regulation within the photoperiod, vernalization, and meristem identity pathways were altered in these genotypes. Elevated [CO2] may alter carbohydrates to influence transcription in both genotypes and delayed flowering in SG. Changes in the oligosaccharide pool may contribute to delayed flowering in SG. This work extends the literature exploring genotypic-specific flowering responses to elevated [CO2].

Glacial trees from the La Brea tar pits show physiological constraints of low CO2.

• While studies of modern plants indicate negative responses to low [CO2] that occurred during the last glacial period, studies with glacial plant material that incorporate evolutionary responses are rare. In this study, physiological responses to changing [CO2] were compared between glacial (La Brea tar pits) and modern Juniperus trees from southern California. • Carbon isotopes were measured on annual rings of glacial and modern Juniperus. The intercellular:atmospheric [CO2] ratio (c(i) /c(a) ) and intercellular [CO2] (c(i) ) were then calculated on an annual basis and compared through geologic time. • Juniperus showed constant mean c(i) /c(a) between the last glacial period and modern times, spanning 50,000 yr. Interannual variation in physiology was greatly dampened during the last glacial period relative to the present, indicating constraints of low [CO2] that reduced responses to other climatic factors. Furthermore, glacial Juniperus exhibited low c(i) that rarely occurs in modern trees, further suggesting limiting [CO2] in glacial plants. • This study provides some of the first direct evidence that glacial plants remained near their lower carbon limit until the beginning of the glacial-interglacial transition. Our results also suggest that environmental factors that dominate carbon-uptake physiology vary across geologic time, resulting in major alterations in physiological response patterns through time.

Removing systemic barriers to equity, diversity, and inclusion: Report of the 2019 Plant Science Research Network workshop "Inclusivity in the Plant Sciences".

A future in which scientific discoveries are valued and trusted by the general public cannot be achieved without greater inclusion and participation of diverse communities. To envision a path towards this future, in January 2019 a diverse group of researchers, educators, students, and administrators gathered to hear and share personal perspectives on equity, diversity, and inclusion (EDI) in the plant sciences. From these broad perspectives, the group developed strategies and identified tactics to facilitate and support EDI within and beyond the plant science community. The workshop leveraged scenario planning and the richness of its participants to develop recommendations aimed at promoting systemic change at the institutional level through the actions of scientific societies, universities, and individuals and through new funding models to support research and training. While these initiatives were formulated specifically for the plant science community, they can also serve as a model to advance EDI in other disciplines. The proposed actions are thematically broad, integrating into discovery, applied and translational science, requiring and embracing multidisciplinarity, and giving voice to previously unheard perspectives. We offer a vision of barrier-free access to participation in science, and a plant science community that reflects the diversity of our rapidly changing nation, and supports and invests in the training and well-being of all its members. The relevance and robustness of our recommendations has been tested by dramatic and global events since the workshop. The time to act upon them is now.

Flowering Times of Wild Arabidopsis Accessions From Across Norway Correlate With Expression Levels of FT, CO, and FLC Genes.

Temperate species often require or flower most rapidly in the long daylengths, or photoperiods, experienced in summer or after prolonged periods of cold temperatures, referred to as vernalization. Yet, even within species, plants vary in the degree of responsiveness to these cues. In Arabidopsis thaliana, CONSTANS (CO) and FLOWERING LOCUS C (FLC) genes are key to photoperiod and vernalization perception and antagonistically regulate FLOWERING LOCUS T (FT) to influence the flowering time of the plants. However, it is still an open question as to how these genes vary in their interactions among wild accessions with different flowering behaviors and adapted to different microclimates, yet this knowledge could improve our ability to predict plant responses in variable natural conditions. To assess the relationships among these genes and to flowering time, we exposed 10 winter-annual Arabidopsis accessions from throughout Norway, ranging from early to late flowering, along with two summer-annual accessions to 14 weeks of vernalization and either 8- or 19-h photoperiods to mimic Norwegian climate conditions, then assessed gene expression levels 3-, 5-, and 8-days post vernalization. CO and FLC explained both FT levels and flowering time (days) but not rosette leaf number at flowering. The correlation between FT and flowering time increased over time. Although vernalization suppresses FLC, FLC was high in the late-flowering accessions. Across accessions, FT was expressed only at low FLC levels and did not respond to CO in the late-flowering accessions. We proposed that FT may only be expressed below a threshold value of FLC and demonstrated that these three genes correlated to flowering times across genetically distinct accessions of Arabidopsis.

Plant responses to low [CO2] of the past.

During the Last Glacial Maximum (LGM; 18,000-20,000 yr ago) and previous glacial periods, atmospheric [CO(2)] dropped to 180-190 ppm, which is among the lowest concentrations that occurred during the evolution of land plants. Modern atmospheric CO(2) concentrations ([CO(2)]) are more than twice those of the LGM and 45% higher than pre-industrial concentrations. Since CO(2) is the carbon source for photosynthesis, lower carbon availability during glacial periods likely had a major impact on plant productivity and evolution. From the studies highlighted here, it is clear that the influence of low [CO(2)] transcends several scales, ranging from physiological effects on individual plants to changes in ecosystem functioning, and may have even influenced the development of early human cultures (via the timing of agriculture). Through low-[CO(2)] studies, we have determined a baseline for plant response to minimal [CO(2)] that occurred during the evolution of land plants. Moreover, an increased understanding of plant responses to low [CO(2)] contributes to our knowledge of how natural global change factors in the past may continue to influence plant responses to future anthropogenic changes. Future work, however, should focus more on the evolutionary responses of plants to changing [CO(2)] in order to account for the potentially large effects of genetic change.

Phosphorus supply drives nonlinear responses of cottonwood (Populus deltoides) to increases in CO2 concentration from glacial to future concentrations.

SUMMARY: *Despite the importance of nutrient availability in determining plant responses to climate change, few studies have addressed the interactive effects of phosphorus (P) supply and rising atmospheric CO(2) concentration ([CO(2)]) from glacial to modern and future concentrations on tree seedling growth. *The objective of our study was to examine interactive effects across a range of P supply (six concentrations from 0.004 to 0.5 mM) and [CO(2)] (200 (glacial), 350 (modern) and 700 (future) ppm) on growth, dry mass allocation, and light-saturated photosynthesis (A(sat)) in Populus deltoides (cottonwood) seedlings grown in well-watered conditions. *Increasing [CO(2)] from glacial to modern concentrations increased growth by 25% across P treatments, reflecting reduced [CO(2)] limitations to photosynthesis and increased A(sat). Conversely, the growth response to future [CO(2)] was very sensitive to P supply. Future [CO(2)] increased growth by 80% in the highest P supply but only by 7% in the lowest P supply, reflecting P limitations to A(sat), leaf area and leaf area ratio (LAR), compared with modern [CO(2)]. *Our results suggest that future [CO(2)] will minimally increase cottonwood growth in low-P soils, but in high-P soils may stimulate production to a greater extent than predicted based on responses to past increases in [CO(2)]. Our results indicate that the capacity for [CO(2)] stimulation of cottonwood growth does not decline as [CO(2)] rises from glacial to future concentrations.