Climate change will increase drought in many regions of the world. Besides decreasing productivity, drought also decreases the concentration (%) of nitrogen (N) and phosphorous (P) in plants. We investigated if decreases in nutrient status during drought are correlated with decreases in levels of nutrient-uptake proteins in roots, which has not been quantified. Drought-sensitive (Hordeum vulgare, Zea mays) and -tolerant grasses (Andropogon gerardii) were harvested at mid and late drought, when we measured biomass, plant %N and P, root N- and P-uptake rates, and concentrations of major nutrient-uptake proteins in roots (NRT1 for NO3, AMT1 for NH4, and PHT1 for P). Drought reduced %N and P, indicating that it reduced nutrient acquisition more than growth. Decreases in P uptake with drought were correlated with decreases in both concentration and activity of P-uptake proteins, but decreases in N uptake were weakly correlated with levels of N-uptake proteins. Nutrient-uptake proteins per gram root decreased despite increases per gram total protein, because of the larger decreases in total protein per gram. Thus, drought-related decreases in nutrient concentration, especially %P, were likely caused, at least partly, by decreases in the concentration of root nutrient-uptake proteins in both drought-sensitive and -tolerant species.

As with above-ground tissues, plant roots can be subjected to stressful high temperatures that can limit whole-plant function and decrease crop productivity. Further, with impending climate change, the frequency, duration, and severity of root heat stress will increase. In comparison to shoot function, especially photosynthesis, much less is known regarding heat stress and roots. Most previous research on roots and heat stress has been conducted on detached roots or by heating only roots or parts of root systems (but not shoots too) in intact herbaceous plants, and most past studies on intact plants have imposed chronic heat stress, with few examining effects of abrupt heat stress (e.g., heat waves). Importantly, plant responses to heat stress often differ in detached roots or when only roots are heated, compared to plants wherein both shoots and roots (or shoots only) are heated, and responses to chronic and abrupt heat stress can differ. Hence, many past results do not inform as to how natural heat stress often impacts roots in intact plants or do not inform as to the effects of heat waves on roots. Both chronic and abrupt heat stress can decrease root growth and function, including nutrient and water uptake, and studies in which both roots and shoots were heat-stressed indicate that roots are often more sensitive to heat stress than shoots. Heat stress may affect roots both directly and indirectly, and indirect effects likely involve decreases in shoot carbon provided to roots or changes in root water relations driven by increased shoot water demand, which then affect root growth and nutrient uptake. Interactive effects between heat stress and other global environmental change factors (e.g., elevated carbon dioxide, drought) on roots are likely. We conclude that further research on roots and heat stress is strongly warranted.

Atmospheric CO enrichment is expected to often benefit plant growth, despite causing global warming and nitrogen (N) dilution in plants. Most plants primarily procure N as inorganic nitrate (NO ) or ammonium (NH ), using membrane-localized transport proteins in roots, which are key targets for improving N use. Although interactive effects of elevated CO , chronic warming and N form on N relations are expected, these have not been studied. In this study, tomato (Solanum lycopersicum) plants were grown at two levels of CO (400 or 700 ppm) and two temperature regimes (30 or 37°C), with NO or NH as the N source. Elevated CO plus chronic warming severely inhibited plant growth, regardless of N form, while individually they had smaller effects on growth. Although %N in roots was similar among all treatments, elevated CO plus warming decreased (1) N-uptake rate by roots, (2) total protein concentration in roots, indicating an inhibition of N assimilation and (3) shoot %N, indicating a potential inhibition of N translocation from roots to shoots. Under elevated CO plus warming, reduced NO -uptake rate per g root was correlated with a decrease in the concentration of NO -uptake proteins per g root, reduced NH uptake was correlated with decreased activity of NH -uptake proteins and reduced N assimilation was correlated with decreased concentration of N-assimilatory proteins. These results indicate that elevated CO and chronic warming can act synergistically to decrease plant N uptake and assimilation; hence, future global warming may decrease both plant growth and food quality (%N).

Limited evidence indicates that moderate leaf hyponasty can be induced by high temperatures or unnaturally high CO2. Here, we report that the combination of warming plus elevated CO2 (eCO2) induces severe leaf hyponasty in tomato (Solanum lycopersicum L.). To characterize this phenomenon, tomato plants were grown at two levels of CO2 (400 vs. 700 ppm) and two temperature regimes (30 vs. 37°C) for 16–18 days. Leaf hyponasty increased dramatically with warming plus eCO2 but increased only slightly with either factor alone and was slowly reversible upon transfer to control treatments. Increases in leaf angle were not correlated with leaf temperature, leaf water stress, or heat‐related damage to photosynthesis. However, steeper leaf angles were correlated with decreases in leaf area and biomass, which could be explained by decreased light interception and thus in situ photosynthesis, as leaves became more vertical. Petiole hyponasty and leaf‐blade cupping were also observed with warming + eCO2 in marigold and soybean, respectively, which are compound‐leaved species like tomato, but no such hyponasty was observed in sunflower and okra, which have simple leaves. If severe leaf hyponasty is common under eCO2 and warming, then this may have serious consequences for food production in the future.

Atmospheric carbon dioxide (CO 2) concentration is increasing, as is the frequency and duration of drought in some regions. Elevated CO 2 can decrease the effects of drought by further decreasing stomatal opening and, hence, water loss from leaves. Both elevated CO 2 and drought typically decrease plant nutrient concentration, but their interactive effects on nutrient status and uptake are little studied. We investigated whether elevated CO 2 helps negate the decrease in plant nutrient status during drought by upregulating nutrient-uptake proteins in roots. METHODS: Barley (Hordeum vulgare) was subjected to current vs. elevated CO 2 (400 or 700 ppm) and drought vs. well-watered conditions, after which we measured biomass, tissue nitrogen (N) and phosphorus (P) concentrations (%N and P), N-and P-uptake rates, and the concentration of the major N-and P-uptake proteins in roots. RESULTS: Elevated CO 2 decreased the impact of drought on biomass. In contrast, both drought and elevated CO 2 decreased %N and %P in most cases, and their effects were additive for shoots. Root N-and P-uptake rates were strongly decreased by drought, but were not significantly affected by CO 2. Averaged across treatments, both drought and high CO 2 resulted in upregulation of NRT1 (NO 3 − transporter) and AMT1 (NH 4 + transporter) per unit total root protein, while only drought increased PHT1 (P transporter). CONCLUSIONS: Elevated CO 2 exacerbated decreases in %N and %P, and hence food quality, during drought, despite increases in the concentration of nutrient-uptake proteins in roots, indicating other limitations to nutrient uptake.

Harmful algal blooms (HABs) in freshwater ecosystems, especially of cyanobacterial species, are becoming more frequent and expanding geographically, including in Lake Erie in North America. HABs are the result of complex and synergistic environmental factors, though N or P eutrophication is a leading cause. With global mean temperatures expected to increase an additional 2˚C -5˚C by 2100, cyanobacterial blooms are predicted to increase even more, given their typically-high temperature optimum for growth. We investigated how increases in temperature and nitrogen, singly or in combination, affect the growth, food quality, and herbivory of Lake Erie cyanobacteria. Algal community samples collected from Lake Erie, and isolated non-N-fixing (Microcystis aeruginosa) and N-fixing (Anabaena flos-aquae) cyanobacterial species, were cultured at 20˚C, 25˚C, or 30˚C, and at 5, 50, 150, or 250 μM N, and then analyzed for growth and (for isolates) content of total protein and non-structural carbohydrates (NSC). Temperature and N both affected algal growth, and there were temperature × N interactions, which were sometimes affected by presence/absence of zooplankton. For example, cyanobacteria (but not green algae) growth increased with both temperature and N, especially from 25˚C to 30˚C, but N and herbivore presence increased cyanobacterial growth primarily only at 30˚C. In general, temperature and N had little consistent effect on NSC, but increasing temperature and N tended to increase protein content in Microcystis and Anabaena (temperature effects mostly at higher N levels). In Anabaena, increases in N did not increase growth or protein at 20˚C or 25˚C, but did increase both at 30˚C, indicating that N fixation * Corresponding author. D. Bista et al.1140 is damaged at high temperatures and that high NO3 can overcome this damage. These results indicate that future global warming and continued eutrophication will increase cyanobacterial growth, as well influence algal herbivory and competition between N-fixing and non-N-fixing cyanobacteria.