Papers by Alexis Leonardo Salas Hernandez
Biografía de Juan Vucetich
Eur J Phys, 2001
An elementary mechanical example is discussed in which the appearance of the easiest inertial for... more An elementary mechanical example is discussed in which the appearance of the easiest inertial force leads naturally to two different but equivalent Lagrangians. This provides a family of simple examples to discuss gauge invariance in analytical mechanics. The physical meaning of the gauge in these examples is also analysed.
Lettere Al Nuovo Cimento Series 2, 1980
It is well known that when an extended body is at rest in an inertial frame i~* under the action ... more It is well known that when an extended body is at rest in an inertial frame i~* under the action of a system of external forces and the situation is considered from a nonproper inertial frame R a net moment of the applied forces results. This despite the fact that the body also is at equilibrium in R. Several explanations of this fact have been proposed (l'~), all of them essentially amounting to finding a mechanism that yielding a rate increase in the angular momentum of the body compensates the net external torque. The mechanism usually gives rise to a transversal momentum which is responsible for the change in the angular momentum of the body. So VoN LAu~ (1) introduced the transverse energy current to solve the Lewis-Tolman-lever ((paradox ~. This concept has been eritiziced questioning the validity of Von Laue's solution (4-7). It has been remarked that, despite the compensating calculations to which the Von Laue flux leads, it is experimentally undetectable and thus void of physical meaning. ALSlNA (4) introduced the idea of the ((additional momentum ~) as a substitute for the Von Laue flux. The calculations based on the additional momentum were first proved to yield the desired compensating effect in several particular cases of extended bodies at equilibrium (s). It has been nlentioned in two recent papers (9.~0) that the explanation of the rightangled lever paradox in terms of nonconservation of simultaneity (the additionalmomentum concept is a consequence of nonconservation of simultaneity) has been
Plant and Soil, 1999
Studies on the inhibitory effects of combined nitrogen on biological nitrogen fixation in legume ... more Studies on the inhibitory effects of combined nitrogen on biological nitrogen fixation in legume crops have been usually carried out after short-term nitrate treatments at high concentrations. As these treatments are quite different from field conditions, a study was conducted to evaluate the effects of the continuous presence of nitrate (0, 1, 5 and 10 mM) throughout three months on lucerne (Medicago sativa L.). Plants were grown in a greenhouse with perlite as substrate and were inoculated with a denitrifying Sinorhizobium meliloti strain (102-F-51) and a non-denitrifying strain (102-F-65). During the first 60 days of growth, the highest nitrate treatment resulted in a complete inhibition of the main symbiotic parameters (nodule initiation and development and specific nitrogen fixation) in plants inoculated with either strain. However, after 3 months of growth in the presence of nitrate, this inhibition was partly abolished, with a high number of new functioning nodules being formed. Acetylene reduction activity (ARA) of these plants was 70% of the control plants. As this process was observed in plants nodulated with either strain, it is concluded that this was not related to the denitrifying ability of the strain, but is an intrinsic property of the lucerne nitrogen fixing system. As legume plants usually grow under natural field conditions in the continuous presence of nitrate, the ability to use simultaneously nitrate and atmospheric nitrogen could be of adaptive and agronomic importance.
Trees-Structure and Function, 2003
The photoprotective responses to an abrupt period of exceptional cold weather were studied in sev... more The photoprotective responses to an abrupt period of exceptional cold weather were studied in several Mediterranean evergreen species with different ecological requirements. The same pattern of response was observed in most of the species with little change in hydrophilic antioxidants (ascorbate and glutathione) and the carotenoid pool, an increase in the content of a-tocopherol, and a night retention of de-epoxidised xanthophylls (antheraxanthin and zeaxanthin). The accumulation of these xanthophylls correlated with a sustained decrease in maximal photochemical efficiency (F v /F m). This reduction in the rate of electron transport would reduce the production of superoxide in photosystem I, as well as the subsequent hydrogen peroxide and hydroxyl radical. Thereby if any transitory photooxidative stress is produced under these conditions it should be due mainly to the formation of singlet oxygen by triplet excited chlorophyll within the antenna. Since a-tocopherol is the main scavenger of singlet oxygen and lipid peroxy radicals, the large increase of this antioxidant within the species could be enough to compensate for the higher degree of photooxidative stress, playing an essential role in the survival of vegetation during the incidence of exceptional cold fronts in the Mediterranean region.
Lettere al Nuovo Cimento, 1978
Plant Biology, 2000
ABSTRACT The role of antioxidants and regulation of photosynthesis as photoprotective adaptations... more ABSTRACT The role of antioxidants and regulation of photosynthesis as photoprotective adaptations of Mediterranean vegetation to winter stress have been studied. These mechanisms are influenced by the degree of environmental stress (studied at different altitudes) and the taxonomic origin of each species. Thus, antioxidant composition was species-dependent, but not apparently affected by altitude; while the reductions in photochemical efficiency, associated with the presence of zeaxanthin and a lower chlorophyll content, were common among species with increasing altitude. Three different strategies to cope with winter stress were observed in Mediterranean evergreens. First, in Buxus sempervirens antioxidant concentration was very high and the photochemical efficiency was much reduced, with a large accumulation of zeaxanthin. Second, in other Mediterranean sclerophylls, reversible reductions of PSII efficiency related to activity of the xanthophyll cycle allowed day to day adaptation of photosynthetic rates. Third, the summer semideciduous Cistus species maintained a high photochemical efficiency and accumulated little antioxidant, and this strategy could be justified by the occurrence two leaf populations with different physiological tolerances to environmental stress.
Il Nuovo Cimento B, 1977
A form-invariant synchronous formulation of relativistic statics is proposed in which one does no... more A form-invariant synchronous formulation of relativistic statics is proposed in which one does not deal with a hidden momentunl such as the one associated with the yon Laue flux. This momentum is unphysical. It is also shown that the line of reasoning that has been used in the theory of the additional momentum always yields a compensating result for the net moment of the external forces. The new synchronous formulation is established after specifying the concept of equilibriunl of an extended body. It describes f.he equilibrium in the inertial frame in which the cquilibriunl conditions '~.re written in contradistinction to the asynchronous formulation of Cavalleri and Salgarelli, in which the equilibrium described is always the one relative to the proper frame of the body. The present formulation requires the relevant 4-torque to be constant, and further establishes the relationships that the components of the 4-torque must fulfil if equilibrium is given. 1.-Introduction.
Plant, Cell & Environment, 2003
The present study investigated the interaction of growth irradiance (Q int) with leaf capacity fo... more The present study investigated the interaction of growth irradiance (Q int) with leaf capacity for and kinetics of adjustment of the pool size of xanthophyll cycle carotenoids (sum of violaxanthin, antheraxanthin and zeaxanthin; VAZ) and photosynthetic electron transport rate (J max) after changes in leaf light environment. Individual leaves of lower-canopy/lower photosynthetic capacity species Tilia cordata Mill. and upper canopy/higher photosynthetic capacity species Populus tremula L. were either illuminated by additional light of 500-800 m m m m mol m-2 s-1 for 12 h photoperiod or enclosed in shade bags. The extra irradiance increased the total amount of light intercepted by twofold for the upper and 10-15-fold for the lower canopy leaves, whereas the shade bags transmitted 45% of incident irradiance. In control leaves, VAZ/area, VAZ/Chl and J max were positively associated with leaf growth irradiance (Q int). After 11 d extra illumination, VAZ/Chl increased in all cases due to a strong reduction in foliar chlorophyll, but VAZ/area increased in the upper canopy leaves of both species, and remained constant or decreased in the lower canopy leaves of T. cordata. The slope for VAZ/area changes with cumulative extra irradiance was positively associated with Q int only in T. cordata , but not in P. tremula. Nevertheless, all leaves of P. tremula increased VAZ/ area more than the most responsive leaves of T. cordata. Shading reduced VAZ content only in P. tremula , but not in T. cordata , again demonstrating that P. tremula is a more responsive species. Compatible with the hypothesis of the role of VAZ in photoprotection, the rates of photosynthetic electron transport declined less in P. tremula than in T. cordata after the extra irradiance treatment. However, foliar chlorophyll contents of the exposed leaves declined significantly more in the upper canopy of P. tremula , which is not consistent with the suggestion that the leaves with the highest VAZ content are more resistant to photoinhibition. This study demonstrates that previous leaf light environment may significantly affect the adaptation capacity of foliage to altered light environment, and also that species differences in photosynthetic capacity and acclimation potentials importantly alter this interaction.
Plant Biology, 2004
The red leaf coloration of several plant species during autumn and winter is due to the synthesis... more The red leaf coloration of several plant species during autumn and winter is due to the synthesis of phenolic compounds such as anthocyanins or red carotenoids. The latter occur very rarely and are non-ubiquitous and taxonomically restricted compounds. The present study shows that the leaves of common box ( Buxus sempervirens L.) accumulate red carotenoids (eschscholtzxanthin, monoanhydroeschscholtzxanthin, anhydroeschscholtzxanthin) as a response to photoinhibitory conditions during winter acclimation. These compounds are produced in a coordinated manner with the operation of other photoprotective systems: accumulation and sustained deepoxidation of VAZ pigments with a concomitant decrease in maximal photochemical efficiency, accumulation of alpha-tocopherol and a gradual decrease on chlorophyll content. All these processes were reversed when the photosynthetic tissues were transferred from photoinhibitory winter conditions to room temperature for 9 days. Buxus leaves showed a large degree of phenotype variation in the degree of reddening, ranging from green to orange. The differences in colour pattern were mainly due to differences in the accumulation of red carotenoids and xanthophyll esters. Red pigments were mainly anhydroeschscholtzxanthin and esters of eschscholtzxanthin. Conversely to fruit or petal chromoplasts, the plastids of red leaves in this species are not the terminal differentiated state but are able to redifferentiate again to chloroplasts. Their photoprotective role during winter as a light screen system or as antioxidants, in a similar way to other red pigments, and their implications on the wide ecological tolerance of this evergreen species are discussed.
Plant Biology, 2007
Summer 2003 was unusually hot in Western Europe, with local droughts and an intense heatwave, tha... more Summer 2003 was unusually hot in Western Europe, with local droughts and an intense heatwave, that led to a massive damage in vegetation. Since high temperatures are supposed to generate photooxidative stress, we analysed photoprotective responses in leaves of the evergreen boxtree (Buxus sempervirens L.) during summer 2003. All the photoprotective compounds analysed (alpha-tocopherol, beta-carotene, and xanthophylls cycle pigments) were simultaneously induced in parallel with a reduction in photochemical efficiency (Fv/Fm). To characterise these responses, we compared these data with other data obtained during cold stress periods (2003, 2005) and with an unstressful summer (2002). Photoprotective responses observed during the heatwave were also induced by low temperature stress, and in both situations, this effect was exacerbated by light. In parallel with such induction the accumulation of red retro-carotenoids and xanthophyll esters was also observed under unfavourable conditions, suggesting a photoprotective role for both groups of carotenoids. This is the first report showing that in any species (Buxus sempervirens L.), the same retro-carotenoids can be induced in response to winter and summer stress. Present results demonstrate that the same mechanisms are induced as response to sub- and supraoptimal temperatures and the plasticity of such responses plays a critical role in plant acclimation to extreme temperatures, an ability that is specially important in the context of any future climate warming.
Plant Biology, 2003
... In yellowing leaves of B. alba and P. tremula xanthophylls accumulated, esterified with fatty... more ... In yellowing leaves of B. alba and P. tremula xanthophylls accumulated, esterified with fatty acids. ... A. Hernández received a postdoctoral fellowship from the Department of Education of the Basque ... autumnal leaf senescence.áPlant Physiol.á(1990);á93:á1184-1190 del Arco et al ...
Plant and Soil, 1993
JI GARCIA-PLAZAOLA, JM BECERR1L 2, C. ARRESE-IGOR l, A. HERNANDEZ, C. GONZALEZ-MURUA and PM APARI... more JI GARCIA-PLAZAOLA, JM BECERR1L 2, C. ARRESE-IGOR l, A. HERNANDEZ, C. GONZALEZ-MURUA and PM APARICIO-TEJO ~ Department of Plant Biology and Ecology, University of the Basque Country/EHU, Aptd. 644, E-48080 Bilbao, Spain. 1Department of ...
Physiologia Plantarum, 1990
... References Arrese-lgor, C , Estavillo, JM, Peiia, JI, Gonzalez-Murua, C. & Aparic... more ... References Arrese-lgor, C , Estavillo, JM, Peiia, JI, Gonzalez-Murua, C. & Aparicio-Tejo, PM 1989. Effect of low nitrate supply on nitrogen fixation in alfalfa root nodules induced by Rhizobium meliioti strains with varied nitrate reductase activity. ... Daniel, RM & Gray, J. 1976. ...
Journal of Physics A: Mathematical and General, 1997
We discuss the phenomenon of pre-acceleration in the light of a method of successive approximatio... more We discuss the phenomenon of pre-acceleration in the light of a method of successive approximations used to construct the physical order reduction of a large class of singular equations. A simple but illustrative physical example is analysed to gain more insight into the convergence properties of the method.
European Journal of Physics, 1981
We propose a simpler model in order to facilitate calculations of the Feynman paradox concerning ... more We propose a simpler model in order to facilitate calculations of the Feynman paradox concerning the angular momentum of a static electromagnetic field. When an angular momentum is attached to the static electromagnetic field the paradox disappears. The storage of the angular momentum in the field during the assembling process is also analysed,
European Journal of Physics, 1982
A non-relativistic Lewis-Tolman-like paradox is proposed. It is checked by direct calculation tha... more A non-relativistic Lewis-Tolman-like paradox is proposed. It is checked by direct calculation that the paradox disappears if linear and angular momenta are attached to the static electromagnetic field. The storage of linear momentum in the electromagnetic field during the assembling process is also analysed. Finally a naive model of the electron suggested by this system is proposed.
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Papers by Alexis Leonardo Salas Hernandez