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

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 progressivedrought 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 (Δ 13C) in shoots was also investigated as a function of water stress intensity. Mild drought (7 days of water withholding) decreased the net CO2exchange (A), leaf conductance to water (g) and transpiration in EU leading to an increased WUEph. Δ 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 Δ 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 Δ 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 Δ 13C, supporting the view that WUEp is a more integrative parameter than WUEph

Photosynthesis, N2 fixation and taproot reserves during the cutting regrowth cycle of alfalfa under elevated CO2 and temperature

Future climatic conditions, including rising atmospheric CO2 and temperature may increase photosynthesis and, consequently, plant production. A larger knowledge of legume performance under the predicted growth conditions will be crucial for safeguarding crop management and extending the area under cultivation with these plants in the near future. N2fixation is a key process conditioning plant responsiveness to varying growth conditions. Moreover, it is likely to increase under future environments, due to the higher photosynthate availability, as a consequence of the higher growth rate underelevated CO2. However, as described in the literature, photosynthesis performance is frequently down-regulated (acclimated) under long-term exposure to CO2, especially when affected by stressful temperature and water availability conditions. As growth responses to elevated CO2 are dependent on sink-source status, it is generally accepted that down-regulation occurs in situations with insufficient plant C sink capacity. Alfalfa management involves the cutting of shoots, which alters the source–sink relationship and thus the photosynthetic behaviour. As the growth rate decreases at the end of the pre-cut vegetative growth period, nodulated alfalfa plants show photosynthetic down-regulation, but during regrowth following defoliation, acclimation to elevated CO2 disappears. The shoot harvest also leads to a drop in mineral N uptake and C translocation to the roots, resulting in a reduction in N2fixation due to the dependence on photosynthate supply to support nodule function. Therefore, the production of new shoots during the first days following cutting requires the utilization of reduced C and N compounds that have been stored previously in reserve organs. The stored reserves are mediated by phytohormones such as methyl jasmonate and abscisic acid and in situations where water stress reduces shoot production this potentially enables the enhancement of taproot protein levels in nodulated alfalfa, which may lead to these plants being in better condition in the following cut/regrowthcycle. Furthering our knowledge of legume performance under predicted climate change conditions will be crucial for the development of varieties with better adaptation that will achieve greater and more efficient production values. Furthermore, for this purpose it will be necessary to improve existing methodologies and create new ones for phenotype characterization. Such knowledge will provide key information for future plant breeding programs

Photosynthesis, N2 fixation and taproot reserves during the cutting regrowth cycle of alfalfa under elevated CO2 and temperature

Future climatic conditions, including rising atmospheric CO2 and temperature may increase photosynthesis and, consequently, plant production. A larger knowledge of legume performance under the predicted growth conditions will be crucial for safeguarding crop management and extending the area under cultivation with these plants in the near future. N2fixation is a key process conditioning plant responsiveness to varying growth conditions. Moreover, it is likely to increase under future environments, due to the higher photosynthate availability, as a consequence of the higher growth rate underelevated CO2. However, as described in the literature, photosynthesis performance is frequently down-regulated (acclimated) under long-term exposure to CO2, especially when affected by stressful temperature and water availability conditions. As growth responses to elevated CO2 are dependent on sink-source status, it is generally accepted that down-regulation occurs in situations with insufficient plant C sink capacity. Alfalfa management involves the cutting of shoots, which alters the source–sink relationship and thus the photosynthetic behaviour. As the growth rate decreases at the end of the pre-cut vegetative growth period, nodulated alfalfa plants show photosynthetic down-regulation, but during regrowth following defoliation, acclimation to elevated CO2 disappears. The shoot harvest also leads to a drop in mineral N uptake and C translocation to the roots, resulting in a reduction in N2fixation due to the dependence on photosynthate supply to support nodule function. Therefore, the production of new shoots during the first days following cutting requires the utilization of reduced C and N compounds that have been stored previously in reserve organs. The stored reserves are mediated by phytohormones such as methyl jasmonate and abscisic acid and in situations where water stress reduces shoot production this potentially enables the enhancement of taproot protein levels in nodulated alfalfa, which may lead to these plants being in better condition in the following cut/regrowthcycle. Furthering our knowledge of legume performance under predicted climate change conditions will be crucial for the development of varieties with better adaptation that will achieve greater and more efficient production values. Furthermore, for this purpose it will be necessary to improve existing methodologies and create new ones for phenotype characterization. Such knowledge will provide key information for future plant breeding programs