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Potassium (K) is a major nutrient known to help plants resist drought. In the context of climate change,quantifying the role of K on maize physiological acclimation to reduced precipitations is essential to betterpredict future productivity. Maize (Zea mays L.) plants grown under controlled or field conditions weresubmitted to different K and water levels. Plant growth (shoot and root biomass, grain yield) as well as plantwater status (transpiration, stomatal conductance, water potential) and ecophysiological mechanisms of Carbonmetabolism (photosynthesis, sugar transport) were studied. Regardless of the water regime and experimentalconditions, K nutrition increased growth and whole-plant development and improved grain yield. The effect ofwater stress on stomatal regulation was not straightforward and depended on the level of K fertilization. Theeffects of water or K deficit tend to decrease photosynthesis. Drought or K nutrition affected more leafphotosynthesis in old than in young leaves, and sugar transport did not seem to be a growth limiting factor. Ourresults demonstrated a strong effect of K on biomass production and a higher water use efficiency with less of animpact on leaf-level physiology. This better water use was mainly the consequence of the positive effect of leafarea on yield, and not due to a reduce water use.; Le potassium (K) est un élément majeur connu pour contribuer à la résistance des plantes à la sècheresse. L'étudede son influence sur la réponse physiologique du maïs (Zea mays L.) sous contrainte hydrique est essentiellepour prédire la future productivité dans un contexte de changements climatiques, en particulier de la diminutiondes précipitations. Des modalités d'apports en K et en eau ont été croisées et soumises à des plants de maïs,élevés en condition contrôlées ou cultivés au champ. La croissance (biomasses aériennes et racinaires,rendements en grain) ainsi que les mécanismes écophysiologiques du métabolisme carboné (photosynthèse,transport des sucres) et du statut hydrique (transpiration, conductance stomatique, potentiels hydriques) ont étéétudiés. L'apport de K a contribué à l'augmentation de la croissance, le développement et le rendement grain quelque soit le régime hydrique imposé au maïs et les conditions d'expérimentation. Les résultats attendus sur lameilleure régulation stomatique en cas de déficit hydrique sont moins évidents. L'effet du stress hydrique ou dela déficience en K tendent à diminuer la photosynthèse. Cependant, ces effets ressortent plus sur les feuillesâgées que sur les feuilles jeunes. Dans ces mêmes conditions, le transport des sucres ne semble pas être unélément limitant de la croissance. Plusieurs résultats convergent pour attribuer au K un rôle dans la maîtrise despertes en eau (par unité de surface foliaire) et sur la meilleure efficience d'utilisation de l'eau. Néanmoins, cetteefficience est imputée à des meilleurs rendements, liés à une surface foliaire plus importante, et non pas à unemoindre consommation de l'eau.

Le potassium (K) est un élément majeur connu pour contribuer à la résistance des plantes à la sècheresse. L'étudede son influence sur la réponse physiologique du maïs (Zea mays L.) sous contrainte hydrique est essentiellepour prédire la future productivité dans un contexte de changements climatiques, en particulier de la diminutiondes précipitations. Des modalités d'apports en K et en eau ont été croisées et soumises à des plants de maïs,élevés en condition contrôlées ou cultivés au champ. La croissance (biomasses aériennes et racinaires,rendements en grain) ainsi que les mécanismes écophysiologiques du métabolisme carboné (photosynthèse,transport des sucres) et du statut hydrique (transpiration, conductance stomatique, potentiels hydriques) ont étéétudiés. L'apport de K a contribué à l'augmentation de la croissance, le développement et le rendement grain quelque soit le régime hydrique imposé au maïs et les conditions d'expérimentation. Les résultats attendus sur lameilleure régulation stomatique en cas de déficit hydrique sont moins évidents. L'effet du stress hydrique ou dela déficience en K tendent à diminuer la photosynthèse. Cependant, ces effets ressortent plus sur les feuillesâgées que sur les feuilles jeunes. Dans ces mêmes conditions, le transport des sucres ne semble pas être unélément limitant de la croissance. Plusieurs résultats convergent pour attribuer au K un rôle dans la maîtrise despertes en eau (par unité de surface foliaire) et sur la meilleure efficience d'utilisation de l'eau. Néanmoins, cetteefficience est imputée à des meilleurs rendements, liés à une surface foliaire plus importante, et non pas à unemoindre consommation de l'eau., Potassium (K) is a major nutrient known to help plants resist drought. In the context of climate change,quantifying the role of K on maize physiological acclimation to reduced precipitations is essential to betterpredict future productivity. Maize (Zea mays L.) plants grown under controlled or field conditions weresubmitted to different K and water levels. Plant growth (shoot and root biomass, grain yield) as well as plantwater status (transpiration, stomatal conductance, water potential) and ecophysiological mechanisms of Carbonmetabolism (photosynthesis, sugar transport) were studied. Regardless of the water regime and experimentalconditions, K nutrition increased growth and whole-plant development and improved grain yield. The effect ofwater stress on stomatal regulation was not straightforward and depended on the level of K fertilization. Theeffects of water or K deficit tend to decrease photosynthesis. Drought or K nutrition affected more leafphotosynthesis in old than in young leaves, and sugar transport did not seem to be a growth limiting factor. Ourresults demonstrated a strong effect of K on biomass production and a higher water use efficiency with less of animpact on leaf-level physiology. This better water use was mainly the consequence of the positive effect of leafarea on yield, and not due to a reduce water use.

International audience; Deep root water uptake and hydraulic redistribution (HR) play a major role in forest ecosystems during drought, but little is known about the impact of climate change on root-zone processes influencing HR and its consequences on water and carbon fluxes. Using data from two old growth sites in the western USA, two mature sites in the eastern USA, one site in southern Brazil, and simulations with the process-based model MuSICA, our objectives were to show that HR can 1) mitigate the effects of soil drying on root functioning, and 2) have important implications for carbon uptake and net ecosystem exchange (NEE). In a dry, old-growth ponderosa pine (USA) and a eucalyptus stand (Brazil) both characterized by deep sandy soils, HR limited the decline in root hydraulic conductivity and increased dry season tree transpiration (T) by up to 30%, which impacted NEE through major increases in gross primary productivity (GPP). The presence of deep-rooted trees did not necessarily imply high rates of HR unless soil texture allowed large water potential gradients to occur, as was the case in the wet old-growth Douglas-fir/mixed conifer stand. At the Duke mixed hardwood forest characterized by a shallow clay-loam soil, modeled HR was low but not negligible, representing annually up to 10% of T, and maintaining root conductance high. At this site, in the absence of HR, it was predicted that annual GPP would have been diminished by 7-19%. At the coastal loblolly pine plantation, characterized by deep organic soil, HR limited the decline in shallow root conductivity by more than 50% and increased dry season T by up to 40%, which increased net carbon gain by the ecosystem by about 400 gC m-2 yr-1, demonstrating the significance of HR in maintaining the stomatal conductance and assimilation capacity of the whole ecosystem. Under future climate conditions (elevated atmospheric [CO2] and temperature), HR is predicted to be reduced by up to 50%; reducing the resilience of trees to droughts. Under future conditions, T is predicted to stay the same at the Duke mixed hardwood forest, but to decline slightly at the coastal loblolly pine plantation and slightly increase at the old-growth ponderosa pine stand and the eucalyptus plantation. As a consequence, water use efficiency in all sites was predicted to improve dramatically under future climate conditions. Our simulations also showed that the negative effect of drier nights on HR would be greater under future climate conditions. Assuming no increase in stomatal control with increasing drier nights, increased vapor pressure deficit at night under future conditions was sufficient to drive significant nighttime T at all sites , which reduced HR, because the plant and the atmosphere became a sink for hydraulically redistributed water . We concluded that the predicted reductions in HR under future climate conditions are expected to play an important regulatory role in land–atmosphere interactions by affecting whole ecosystem carbon and water balance. We suggest that root distribution should be treated dynamically in response to climate change and that HR and its interactions with rooting depth and soil texture should be implemented in soil–vegetation–atmosphere transfer models.

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