The risk of severe water scarcity events simultaneously affecting key wheat-producing areas doubles despite CO ₂ mitigation efforts.

Summary: 1. Cloud cover affects carbon exchange between biota and the atmosphere. Recent studies have demonstrated that an increase in the diffuse radiation fraction enhances the photosynthetic efficiency of canopies. Although the exact mechanism behind this effect is not clear, a more even distribution of light among leaves across the vertical profile of the canopy is considered to be the most important cause of this difference.2. To test this hypothesis, the net ecosystem production (NEP) of a Norway spruce forest (30‐year‐old) was measured under cloudy and sunny skies by the eddy covariance method. In parallel, measurements of the diurnal courses of gas exchange and chlorophyll fluorescence parameters were made in the upper sun (5th whorl; 1‐year‐old needles), middle (8th and 10th whorl; 1‐ and 2‐year‐old needles) and lower shade (15th whorl; >2‐year‐old needles) shoots.3. The higher diffuse radiation fraction during cloudy days resulted in significantly higher ecosystem carbon uptake than at corresponding incident photosynthetic photon flux density on sunny days. Our shoot‐level data show that shoots from deep within the canopy contribute substantially to the overall carbon balance during cloudy days. But, although shade‐adapted shoots had a markedly positive carbon balance over a 24‐h period on cloudy days, their performance was impaired on sunny days contributing only a marginal or even negative carbon balance from the middle and shaded parts of the canopy. The uppermost sun shoots contributed 78% of the total carbon assimilated during a sunny day, but only 43% during a cloudy day.4. In addition, afternoon depression of canopy NEP and CO₂ assimilation rates of the uppermost shoots (5th and 8th whorl) occurred in response to irradiance on sunny days, characterized by significant decreases in CO₂ uptake and apparent quantum yield; however, this depression did not occur under cloudy conditions. Stomatal and non‐stomatal regulations of carbon assimilation in the afternoon are discussed.

The number of ecometabolomic studies, which use metabolomic analyses to disentangle organisms’ metabolic responses and acclimation to a changing environment, has grown exponentially in recent years. Here, we review the results and conclusions of ecometabolomic studies on the impacts of four main drivers of global change (increasing frequencies of drought episodes, heat stress, increasing atmospheric carbon dioxide (CO₂) concentrations and increasing nitrogen (N) loads) on plant metabolism. Ecometabolomic studies of drought effects confirmed findings of previous target studies, in which most changes in metabolism are characterized by increased concentrations of soluble sugars and carbohydrate derivatives and frequently also by elevated concentrations of free amino acids. Secondary metabolites, especially flavonoids and terpenes, also commonly exhibited increased concentrations when drought intensified. Under heat and increasing N loads, soluble amino acids derived from glutamate and glutamine were the most responsive metabolites. Foliar metabolic responses to elevated atmospheric CO₂ concentrations were dominated by greater production of monosaccharides and associated synthesis of secondary metabolites, such as terpenes, rather than secondary metabolites synthesized along longer sugar pathways involving N-rich precursor molecules, such as those formed from cyclic amino acids and along the shikimate pathway. We suggest that breeding for crop genotypes tolerant to drought and heat stress should be based on their capacity to increase the concentrations of C-rich compounds more than the concentrations of smaller N-rich molecules, such as amino acids. This could facilitate rapid and efficient stress response by reducing protein catabolism without compromising enzymatic capacity or increasing the requirement for re-transcription and de novo biosynthesis of proteins.