Alberto Muñoz-Rueda

The oxidative stress caused by salinity in two barley cultivars is mitigated by elevated CO2.

Changes in antioxidant metabolism because of the effect of salinity stress (0, 80, 160 or 240 mM NaCl) on protective enzyme activities under ambient (350 micromol mol(-1)) and elevated (700 micromol mol(-1)) CO(2) concentrations were investigated in two barley cultivars (Hordeum vulgare L., cvs Alpha and Iranis). Electrolyte leakage, peroxidation, antioxidant enzyme activities [superoxide dismutase (SOD), EC 1.15.1.1; ascorbate peroxidase (APX), EC 1.11.1.11; catalase (CAT), EC 1.11.1.6; dehydroascorbate reductase (DHAR), EC 1.8.5.1; monodehydroascorbate reductase (MDHAR), EC 1.6.5.4; glutathione reductase (GR), EC 1.6.4.2] and their isoenzymatic profiles were determined. Under salinity and ambient CO(2), upregulation of antioxidant enzymes such as SOD, APX, CAT, DHAR and GR occurred. However, this upregulation was not enough to counteract all ROS formation as both ion leakage and lipid peroxidation came into play. The higher constitutive SOD and CAT activities together with a higher contribution of Cu,Zn-SOD 1 detected in Iranis might possibly contribute and make this cultivar more salt-tolerant than Alpha. Elevated CO(2) alone had no effect on the constitutive levels of antioxidant enzymes in Iranis, whereas in Alpha it induced an increase in SOD, CAT and MDHAR together with a decrease of DHAR and GR. Under combined conditions of elevated CO(2) and salinity the oxidative damage recorded was lower, above all in Alpha, together with a lower upregulation of the antioxidant system. So it can be concluded that elevated CO(2) mitigates the oxidative stress caused by salinity, involving lower ROS generation and a better maintenance of redox homeostasis as a consequence of higher assimilation rates and lower photorespiration, being the response dependent on the cultivar analysed.

Lipoic acid and redox status in barley plants subjected to salinity and elevated CO2

Future environmental conditions will include elevated concentrations of salt in the soil and an elevated concentration of CO(2) in the atmosphere. Because these environmental changes will likely affect reactive oxygen species (ROS) formation and cellular antioxidant metabolism in opposite ways, we analyzed changes in cellular H(2)O(2) and non-enzymatic antioxidant metabolite [lipoic acid (LA), ascorbate (ASA), glutathione (GSH)] content induced by salt stress (0, 80, 160 or 240 mM NaCl) under ambient (350 micromol mol(-1)) or elevated (700 micromol mol(-1)) CO(2) concentrations in two barley cultivars (Hordeum vulgare L.) that differ in sensitivity to salinity (cv. Alpha is more sensitive than cv. Iranis). Under non-salinized conditions, elevated CO(2) increased LA content, while ASA and GSH content decreased. Under salinized conditions and ambient CO(2), ASA increased, while GSH and LA decreased. At 240 mM NaCl, H(2)O(2) increased in Alpha and decreased in Iranis. When salt stress was imposed at elevated CO(2), less oxidative stress and lower increases in ASA were detected, while LA was constitutively higher. The decrease in oxidative stress could have been because of less ROS formation or to a higher constitutive LA level, which might have improved regulation of ASA and GSH reductions. Iranis had a greater capacity to synthesize ASA de novo and had higher constitutive LA content than did Alpha. Therefore, we conclude that elevated CO(2) protects barley cultivars against oxidative damage. However, the magnitude of the positive effect is cultivar specific.

Denitrification losses from a natural grassland in the Basque Country under organic and inorganic fertilization

Denitrification losses from a poorly drained clayey loamy soil under natural pasture were measured over a two-year period using the acetylene inhibition technique. Plots received two different applications of fertilizer as calcium ammonium nitrate or cow slurry (a total of 145–290 kg N ha−1 in 1991 and 120–240 kg in 1992). In the first year, N losses in the mineral treatments were about 4 times greater than losses in the slurry treatments. In the second year losses in the slurry treatments increased in such a way that losses in the higher slurry application became similar to those for the two mineral treatments. Soil nitrate was the factor producing differences between treatments. In this way, N mineralization in periods between fertilizations coinciding with high soil water contents was responsible in the second year for the increase in N losses in the slurry treatments. Denitrification rates greater than 0.1 kg N ha−1 day−1 occurred at soil water contents > 33 % (air filled porosity < 26 %) and soil nitrate contents > 1 mg N kg−1 dry soil. Spring and autumn were the seasons of highest risk of denitrification because of N fertilizations coinciding with periods of soil saturation with water. Winter losses were low, but this is a period when there is a risk of denitrification in wetter seasons, particularly for a slurry application management.

Temporal study of the effect of phosphinothricin on the activity of glutamine synthetase, glutamate dehydrogenase and nitrate reductase in Medicago sativa L.

Summary In a preliminary work we showed that when plants were sprayed with several doses of glufosinate, GS activity of the leaf was reduced by 50% after 48 h of application of 250 µM herbicide. Enzyme inhibition was accompained by a dramatic accumulation of ammonia. In this paper we analyze the time-course effect of phosphinothricin on nitrogen metabolism. Our results show that GS activity is the first process affected; more than 50% activity reduction is observed after 2 h of a 1,000 µM treatment, whereby ammonia values 400% higher than control were reached. NRase did not change until 24 h of assay, whereas protein content was reduced after 48 h. GDH activity was enhanced, but only 24 h after starting the treatment. These results indicate that the main target of PPT action is on GS activity, and other processes are long-term modified. GDH could, perhaps, reassimilate some of the ammonia produced but its activity is not able to completely prevent, the injury caused by phosphinothricin.

Atmospheric CO2 concentration influences the contributions of osmolyte accumulation and cell wall elasticity to salt tolerance in barley cultivars.

Future environmental conditions will include elevated concentrations of salt in the soils and elevated concentrations of CO(2) in the atmosphere. Soil salinization inhibits crop growth due to osmotic and ionic stress. However, plants possess salt tolerance mechanisms, such as osmotic and elastic adjustment, to maintain water status. These mechanisms, which enhance the uptake and accumulation of ions and the synthesis of compatible solutes, require substantial energy expenditure. Under elevated CO(2), the carbon and energy supplies are usually higher, which could facilitate the energetically expensive salt tolerance mechanisms. To test this hypothesis, the factors involved in osmotic and elastic adjustments in two barley cultivars (Hordeum vulgare cv. Alpha and cv. Iranis) grown under several salt concentrations and at ambient or elevated [CO(2)] were evaluated. Under ambient [CO(2)] and salt stress, both cultivars (1) decreased the volumetric elasticity modulus (epsilon) of their cell walls, and (2) adjusted osmotically by accumulating ions (Na(+) and Cl(-)) from the soil, confirming barley as an includer species. The contributions of sugars and other unidentified osmolytes also increased, while the contribution of organic acids decreased. Under elevated [CO(2)] and salt stress, epsilon decreased less and osmotic adjustment (OA) was greater than at ambient [CO(2)]. In fact, the greater OA under elevated [CO(2)] was positively correlated with the contributions of sugars and other unidentified compounds. These results indicate that barley is likely to be successful in more salinized soils due to its capacity for OA under elevated [CO(2)].

Influence of water stress on photosynthetic characteristics in barley plants under ambient and elevated CO2 concentrations

We evaluated the combined effects of elevated CO2 and water availability on photosynthesis in barley. Soil and plant water content decreased with water stress, but less under elevated CO2 concentration (EC) compared with ambient CO2 concentration (AC). During water stress, stomatal conductance, carboxylation rate, RuBP regeneration, and the rate of triose phosphate utilisation (TPU) were decreased but less when plants grew under EC. Drought treatments caused only a slight effect on maximum photochemical efficiency (variable to maximum fluorescence ratio, Fv/Fm), whereas the actual quantum yield (ΦPS2), maximum electron transport rate (Jmax) and photochemical quenching (qP) were decreased and the non photochemical quenching (NPQ) was enhanced. Under water deficit, the allocation of electrons to CO2 assimilation was diminished by 49 % at AC and by 26 % at EC while the allocation to O2 reduction was increased by 15 % at AC and by 12 % at EC.

Effect of Phosphinothricin (Glufosinate) on Activities of Glutamine Synthetase and Glutamate Dehydrogenase in Medicago sativa L.

Summary The effect of the glutamate analogue phosphinothricin on enzymes of nitrogen assimilation was studied as well as the accumulation of ammonia and nitrate after treatment of lucerne plants with this herbicide. Glutamine synthetase (GS) was assayed with several phosphinothricin concentrations. When plants were sprayed with 250, 500 and 1,000 μM of the herbicide, GS activity of the leaf was reduced by 50% at 250 μM. Phosphinothricin caused a high ammonia accumulation in the tissues, reaching values of 70 μmol ammonia/g dry weight after treatment with 1,000 μM phosphinothricin. Glutamate dehydrogenase (GDH) activity increased ca. 40 % and nitrate reductase activity (NRase) was strongly decreased. Neither nitrate content nor proteolytic activity were altered.

Lettuce production and antioxidant capacity are differentially modified by salt stress and light intensity under ambient and elevated CO2.

As a consequence of the increasing importance of vegetables in the human diet, there is an interest in enhancing both the productivity and quality of vegetables. A number of factors, including plant genotype and environmental growing conditions, can impact the production and quality of vegetables. The objective of this study was to determine whether elevated CO2, salinity, or high light treatments assayed individually, or salinity or high light in combination with elevated CO2, increased biomass production and antioxidant capacity in two lettuce cultivars. Elevated CO2 and its combination with salinity or high light increased biomass production in both cultivars, while high light treatment alone increased production in green-leaf lettuce but not in red-leaf lettuce. On the other hand, elevated CO2 and its combination with salinity or high light increased the antioxidant capacity of both cultivars, while high light treatment alone increased the antioxidant capacity of red-leaf lettuce, but not of green-leaf lettuce.

Glycolate accumulation causes a decrease of photosynthesis by inhibiting RUBISCO activity in maize

Summary The consequences of interruption of the photorespiratory cycle by phosphinothricin (PPT) and amino-oxyacetic acid (AOA) in several aspects of maize photosynthesis were investigated. Both compounds inhibited the photorespiratory pathway, causing glycolate accumulation and photosynthesis inhibition. PEPCase activity was not affected by PPT or by AOA, but NADP-ME activity was slightly increased. The inhibition of RUBISCO activity was not due to depletion of its substrates, neither CO 2 nor RuBP. An inverse relationship was found between glycolate accumulation and RUBISCO activity when photorespiration is blocked by PPT or AOA. We suggest that the accumulation of the photorespiratory metabolite, glycolate, provokes the inhibition of RUBISCO activity, and consequendy diminution of CO 2 assimilation.

Comparative study of the inhibition of photosynthesis caused by aminooxyacetic acid and phosphinothricin in Zea mays

Summary Maize plants fed with PPT (a glutamine synthetase inhibitor) accumulate ammonia. Approximately 50 % of the ammonia accumulated seems to come from the photorespiratory pathway, while the nonphotorespiratory ammonia is derived from nitrate reductase activity. The ammonia accumulated, however, is not sufficient to increase GDH activity. We also carried out a comparative study of the effect of PPT and AOA on photosynthesis. Both compounds inhibit the photorespiratory pathway, causing glycolate accumulation and diminishing photosynthesis.