Effect of shoot removal on remobilization of carbon and nitrogen during regrowth of nitrogen-fixing alfalfa

The contribution of carbon and nitrogen reserves to regrowth following shoot removal and the processes involved in the reduction of nodule functioning were studied in alfalfa plants (Medicago sativa L.). To do so, isotopic labelling was conducted at root and canopy level with both 15N2 and 13C-depleted CO2 on exclusively nitrogen fixing alfalfa plants. Our results indicate that the roots were the main sink organs before shoot removal as expected. Seven days after regrowth the carbon and nitrogen stored in the roots was invested in shoot biomass formation and partitioned to the nodules in order to sustain respiratory activity. However, this carbon and nitrogen derived from the root did not overcome carbon and nitrogen limitation in nodules and leaves. Together with the limited carbohydrate availability, the up-regulation of nodule peroxidases indicates the involvement of oxidative stress in a worse nodule performance. Fourteen days after shoot removal, leaf and nodule performance were completely reestablished

Short-Term Exposure to High Atmospheric Vapor Pressure Deficit (VPD) Severely Impacts Durum Wheat Carbon and Nitrogen Metabolism in the Absence of Edaphic Water Stress

Low atmospheric relative humidity (RH) accompanied by elevated air temperature and decreased precipitation are environmental challenges that wheat production will face in future decades. These changes to the atmosphere are causing increases in air vapor pressure deficit (VPD) and low soil water availability during certain periods of the wheat-growing season. The main objective of this study was to analyze the physiological, metabolic, and transcriptional response of carbon (C) and nitrogen (N) metabolism of wheat (Triticum durum cv. Sula) to increases in VPD and soil water stress conditions, either alone or in combination. Plants were first grown in well-watered conditions and near-ambient temperature and RH in temperature-gradient greenhouses until anthesis, and they were then subjected to two different water regimes well-watered (WW) and water-stressed (WS), i.e., watered at 50% of the control for one week, followed by two VPD levels (low, 1.01/0.36 KPa and high, 2.27/0.62 KPa; day/night) for five additional days. Both VPD and soil water content had an important impact on water status and the plant physiological apparatus. While high VPD and water stress-induced stomatal closure affected photosynthetic rates, in the case of plants watered at 50%, high VPD also caused a direct impairment of the RuBisCO large subunit, RuBisCO activase and the electron transport rate. Regarding N metabolism, the gene expression, nitrite reductase (NIR) and transport levels detected in young leaves, as well as determinations of the δ15N and amino acid profiles (arginine, leucine, tryptophan, aspartic acid, and serine) indicated activation of N metabolism and final transport of nitrate to leaves and photosynthesizing cells. On the other hand, under low VPD conditions, a positive effect was only observed on gene expression related to the final step of nitrate supply to photosynthesizing cells, whereas the amount of 15N supplied to the roots that reached the leaves decreased. Such an effect would suggest an impaired N remobilization from other organs to young leaves under water stress conditions and low VPD.This work was supported by the Spanish Innovation and Universities Ministry (AGL2016-79868-R; 427 PCIN-2017-007) and the Basque Country Government consolidated group program (IT-932-16)

Accelerated aging test of solar reflectors according to the new AENOR standard – results of a round Robin test

Durability tests of reflector materials for concentrating solar applications are crucial to guarantee the profitability of the plants and to ensure a proper efficiency during their lifetime. A standard including a set of five accelerated aging tests is close to be published by the Spanish standardization entity AENOR, within the sub-committee AEN/CTN 206/SC117. Under the framework of the STAGE-STE project, a Round Robin Test was organized by six partners to evaluate the comparability of results obtained in their respective laboratories after performing these durability tests. According to the results, in general a good agreement among the partners was found, with negligible to slight reflectance losses. In addition, it was noticed that the reference standard used in the reflectance measurements is of high importance. The conclusions of this work will help to improve the standard in future versions.The research leading to this work has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 609837 (STAGE-STE). The authors would like to thank the participants of the working group for solar reflectors of WG 2 of the Spanish sub-committee AEN/CTN 206/SC 117

Carbon balance, partitioning and photosynthetic acclimation in fruit-bearing grapevine (Vitis vinifera L. cv. Tempranillo) grown under simulated climate change (elevated CO2, elevated temperature and moderate drought) scenarios in temperature gradient

Although plant performance under elevated CO2 has been extensively studied in the past little is known about photosynthetic performance changing simultaneously CO2, water availability and temperature conditions. Moreover, despite of its relevancy in crop responsiveness to elevated CO2 conditions, plant level C balance is a topic that, comparatively, has received little attention. In order to test responsiveness of grapevine photosynthetic apparatus to predicted climate change conditions, grapevine (Vitis vinifera L. cv. Tempranillo) fruit-bearing cuttings were exposed to different CO2 (elevated, 700 ppm vs. ambient, ca. 400 ppm), temperature (ambient vs. elevated, ambient +4 °C) and irrigation levels (partial vs. full irrigation). Carbon balance was followed monitoring net photosynthesis (AN, C gain), respiration (RD) and photorespiration (RL) (C losses). Modification of environment 13C isotopic composition (δ13C) under elevated CO2 (from −10.30 to −24.93‰) enabled the further characterization of C partitioning into roots, cuttings, shoots, petioles, leaves, rachides and berries. Irrespective of irrigation level and temperature, exposure to elevated CO2 induced photosynthetic acclimation of plants. C/N imbalance reflected the inability of plants grown at 700 ppm CO2 to develop strong C sinks. Partitioning of labeled C to storage organs (main stem and roots) did not avoid accumulation of labeled photoassimilates in leaves, affecting negatively Rubisco carboxylation activity. The study also revealed that, after 20 days of treatment, no oxidative damage to chlorophylls or carotenoids was observed, suggesting a protective role of CO2 either at current or elevated temperatures against the adverse effect of water stress

Plant physiology and proteomics reveals the leaf response to drought in alfalfa (Medicago sativa L.)

Despite its relevance, protein regulation, metabolic adjustment, and the physiological status of plants under drought is not well understood in relation to the role of nitrogen fixation in nodules. In this study, nodulated alfalfa plants were exposed to drought conditions. The study determined the physiological, metabolic, and proteomic processes involved in photosynthetic inhibition in relation to the decrease in nitrogenase (Nase) activity. The deleterious effect of drought on alfalfa performance was targeted towards photosynthesis and Nase activity. At the leaf level, photosynthetic inhibition was mainly caused by the inhibition of Rubisco. The proteomic profile and physiological measurements revealed that the reduced carboxylation capacity of droughted plants was related to limitations in Rubisco protein content, activation state, and RuBP regeneration. Drought also decreased amino acid content such as asparagine, and glutamic acid, and Rubisco protein content indicating that N availability limitations were caused by Nase activity inhibition. In this context, drought induced the decrease in Rubisco binding protein content at the leaf level and proteases were up-regulated so as to degrade Rubisco protein. This degradation enabled the reallocation of the Rubisco-derived N to the synthesis of amino acids with osmoregulant capacity. Rubisco degradation under drought conditions was induced so as to remobilize Rubisco-derived N to compensate for the decrease in N associated with Nase inhibition. Metabolic analyses showed that droughted plants increased amino acid (proline, a major compound involved in osmotic regulation) and soluble sugar (D-pinitol) levels to contribute towards the decrease in osmotic potential (Ψs). At the nodule level, drought had an inhibitory effect on Nase activity. This decrease in Nase activity was not induced by substrate shortage, as reflected by an increase in total soluble sugars (TSS) in the nodules. Proline accumulation in the nodule could also be associated with an osmoregulatory response to drought and might function as a protective agent against ROS. In droughted nodules, the decrease in N2 fixation was caused by an increase in oxygen resistance that was induced in the nodule. This was a mechanism to avoid oxidative damage associated with reduced respiration activity and the consequent increase in oxygen content. This study highlighted that even though drought had a direct effect on leaves, the deleterious effects of drought on nodules also conditioned leaf responsiveness

Water use efficiency, transpiration and net CO2 exchange of four alfalfa genotypes submitted to progressive drought and subsequent recovery

Gorka Erice...et al.The predicted worldwide increase in arid areas and water stress episodes will strongly affect crop production. Plants have developed a wide diversity of physiological mechanisms for drought tolerance. A decline in photosynthesis and thus yield production is a common response to drought, as are increases in the water use efficiency of photosynthesis (WUEph) and productivity (WUEp). The aim of our study was to determine the physiological effects (especially WUEph and WUEp) of progressive drought and subsequent recovery in three cultivars adapted to a Mediterranean climate [Tafilalet (TA), Tierra de Campos (TC), and Moapa (MO)], and another representative from an oceanic climate (Europe (EU)). The accuracy of the relationships between WUEph or WUEp and carbon isotope discrimination ( 13C) in shoots was also investigated as a function of water stress intensity. Mild drought (7 days of water withholding) decreased the net CO2 exchange (A), leaf conductance to water (g) and transpiration in EU leading to an increased WUEph. 13C was correlated with WUEp but not with WUEph, probably due to a late decrease in g. After moderate drought (14 days), A and g decreased in all cultivars, increasing WUEph. In this period WUEp also increased. Both WUE parameters were correlated with 13C, which may indicate that the g value at the end of moderate water stress was representative of the growing period. After 21 days, TA was the most productive cultivar but under severe drought conditions there was no difference in DM accumulation among cultivars. After the recovery period, leaf area was increased but not total DM, showing that leaves were the most responsive organs to rewatering. Severe water stress did not decrease WUEph or WUEp, and 13C did not increase after recovery. This absence of a response to severe drought may indicate significant effects on the photosynthetic apparatus after 21 days of withholding water. As for mild drought, WUEp but not WUEph was correlated with 13C, supporting the view that WUEp is a more integrative parameter than WUEphThis work was partially supported by a PHC PRAD program (08- 0316872QC). Dr. Gorka Erice was the recipient of a research grant from the Basque Government (BFI-0691).Peer reviewe

Harvest index, combined with impaired N availability constrains the responsiveness of durum wheat to elevated CO 2 concentration and terminal water stress.

Despite its relevance, few studies to date have analysed the role of harvest index (HI) in the responsiveness of wheat (Triticum spp.) to elevated CO2 concentration ([CO2]) under limited water availability. The goal of the present work was to characterise the role of HI in the physiological responsiveness of durum wheat (Triticum durum Desf.) exposed to elevated [CO2] and terminal (i.e. during grain filling) water stress. For this purpose, the performance of wheat plants with high versus low HI (cvv. Sula and Blanqueta, respectively) was assessed under elevated [CO2](700m molmol-1 vs 400 m molmol-1 CO2) and terminal water stress (imposed after ear emergence) in CO2 greenhouses. Leaf carbohydrate build-up combined with limitations in CO2 diffusion (in droughted plants) limited the responsiveness to elevated [CO2] in both cultivars. Elevated [CO2] only increased wheat yield in fully watered Sula plants, where its larger HI prevented an elevated accumulation of total nonstructural carbohydrates. It is likely that the putative shortened grain filling period in plants exposed to water stress also limited the responsiveness of plants to elevated [CO2]. In summary, our study showed that even under optimal water availability conditions, only plants with a high HI responded to elevated [CO2] with increased plant growth, and that terminal drought constrained the responsiveness of wheat plants to elevated [CO2Peer Reviewe