Luis R. López-Lefebre

Resistance to cold and heat stress: accumulation of phenolic compounds in tomato and watermelon plants.

Tomato plants, Lycopersicon esculentum L. cv. Tmknvf(2), and watermelon plants, Citrullus lanatus [Thomb.] Mansf. cv. Dulce maravilla, were grown for 30 days at different temperatures (15, 25 and 35 degrees C). We analysed soluble phenolics, enzymatic activities (phenylalanine ammonia-lyase, polyphenol oxidase and peroxidase), and dry weight. The impact of the three temperatures was different in tomato and watermelon. Our results indicate that heat stress in tomato plants occurred at 35 degrees C, while chilling stress occurred in watermelon plants at 15 degrees C. Thermal stress in both plants caused: (1) decreased shoot weight; (2) accumulation of soluble phenolics; (3) highest phenylalanine ammonia-lyase activity; and (4) lowest peroxidase and polyphenol oxidase activity. These results indicate that thermal stress induces the accumulation of phenolics in the plant by activating their biosynthesis as well as inhibiting their oxidation. This could be considered an acclimation mechanism of the plant against thermal stress.

Proline metabolism and NAD kinase activity in greenbean plants subjected to cold-shock

The aim of the present work was to examine the relationship between proline metabolism and NAD kinase activity in greenbeans submitted to cold-shock. For this, 15-day-old greenbean plants were subjected to a temperature of 4 degrees C (cold shock) for 180 min. Our results indicate that the plants showed foliar accumulation of proline, with the enzymes ornithine-delta-aminotransferase (OAT) and proline dehydrogenase (PDH) appearing as determinant in this accumulation under cold-shock. Also, we found a close relationship between the Ca(2+)-CaM-dependent NAD kinase activity and proline metabolism, suggesting that the adaptive responses or acclimation of plants to cold stress are preceded by increased [Ca(2+)](cyt).

Proline metabolism in response to highest nitrogen dosages in green bean plants (Phaseolus vulgaris L. cv. Strike)

Summary The objective of the present work was to determine what impact extremely high nitrogen dosages would have on proline metabolism in order to use this amino acid as a bioindicator of N status of green bean plants (Phaseolus vulgaris L. cv. Strike). In this effort, we identified the most favourable pathway of proline synthesis under our experimental conditions. The N was applied to the nutrient solution in the form of NH4NO3 at 5.4 mmol/L (N1, optimal level), 11.6 mmol/L (N2), 17.4 mmol/L (N3), and 23.2 mmol/L (N4). Our results indicate that the application of high N dosages inPhaseolus is characterized by the accumulation of NO3−, NH4+ and proline in root and foliar organs. However, although the enzymes in charge of proline biosynthesis, ornithine-δ-aminotransferase (OAT, EC 2.6.1.13) and Δ1-pyrroline-5-carboxylate synthetase (P5CS, EC 2.7.2.11/1.2.2.41) vary in behaviour depending on the N status, in our experiment, this amino acid appears to be synthesized mainly by the enzyme ornithine-δ-aminotransferase. This suggests predominance of the ornithine pathway over the glutamine pathway. Finally, under our experimental conditions, proline can be defined as a good indicator of N excess of green bean plants.

Phenolic compounds and oxidative metabolism in green bean plants under nitrogen toxicity

The objective of the present work was to determine the effect of nitrogen toxicity on the metabolism of phenolic compounds and of oxidative stress in Phaseolus vulgaris L. cv. Strike. The nitrogen was applied to the nutrient solution as NH4NO3 at 5.4, 10.8, 16.2, 21.6 and 27 mM. The results indicate that the application of 27 mM N can be defined as toxic, as it drastically depressed growth of the green bean plants in our experiment. In addition, the abiotic stress from the application of this N dosage inhibited the enzymes polyphenol oxidase, peroxidase and cata-lase, and stimulated phenylalanine ammonia-lyase and superoxide dismutase activities. The result was foliar accumulation of phenolic compounds and hydrogen peroxide (H2O2). The accumulation of H2O2 also apparently caused a reduction in biomass production.

BORON EFFECT ON MINERAL NUTRIENTS OF TOBACCO

Information on the effect of boron (B) nutrition on nutrient uptake and concentration and biomass production in tobacco plants (Nicotiana tabacumL.) is limited. Thus the aim of this study was to analyze the response of the nutritional state and biomass in tobacco plants administered different B treatments (B1 : 5 μmol/LH3BO3, B2 :  10 μmol/LH3BO3, B3 :  20 μmol/LH3BO3). Tobacco plants were grown under controlled conditions and submitted to regular fertilization with macro- and micro-nutrients. The concentration of the elements organic nitrogen (N), phosphorus (P), potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), chloride (Cl), and B were analyzed in roots and leaves. The increase in the B application in the culture medium translated as a progressive accumulation of this element. Similarly, the macronutrients N, P, K, and Na responded positively to the dosage of B, notably increasing in concentration. Nevertheless, Mg responded negatively, declining considerably in concentration in the roots and leaves. The relationship between the elements B and Ca in our experiment can be defined as synergetic, showing a steady accumulation and translocation of Ca as the B dosage increased. The root and foliar concentrations of the micronutrients Fe and Mn also increased with the B dosage, whereas Cu and Zn to a lesser degree diminished in concentration. Finally, the positive effect that increased B application exerted on dry-material production in the roots and leaves could be explained by the general improvement in the nutritional state, particularly of the essential macronutrients N and P.

Response of oxidative metabolism to the application of carbendazim plus boron in tobacco

In view of the essential role of oxidative process in the development of pathogen resistance in plants, the aim of the present study was to determine the individual effect of a fungicide, as well as to determine the combined effect of the fungicide and boron (B) on superoxide dismutase (SOD), guaiacol peroxidase (GPX), catalase (CAT), and ascorbate peroxidase (APX) activities and H 2 O 2 levels in tobacco plants (Nicotiana tabacum L. cv. Tennessee 86). The fungicide applied was carbendazim (carb) at a concentration of 2.6 mM . Boron was applied as H 3 BO 3 at: 1.6 mM (B1), 4 mM (B2), 8 mM (B3), 16 mM (B4), 32 mM (B5), and 64 mM (B6). The results indicated that foliar application of carbendazim by itself does not increase SOD, GPX, CAT or APX activities or H2 O2 foliar accumulation. The combined application of carbendazim and B increased SOD, GPX, CAT, and APX activities, especially in carb-B3 and carb-B4 . This effect may signify an additional tolerance mechanism to pathogenic infection, given the participation of these enzymes in the early phases of the plant–pathogen interactions.

Response of oxidative metabolism in watermelon plants subjected to cold stress

The objective of the present work was to determine the effect of thermal stress on oxidative metabolism in Citrullus lanatus [Thomb.] Mansf. cv. Dulce maravilla. Plants were grown for 30 d at two temperatures (10 and 35˚C), at which time we measured the leaf concentration of antioxidant compounds (ascorbate, dehydroascorbate, reduced glutathione, oxidized glutathione) and enzymatic activities [superoxide dismutase (SOD), catalase, guaiacol peroxidase, ascorbate peroxidase, dehydroascorbate reductase and glutathione reductase], as well as total hydrogen peroxide (H2O2) concentration and shoot dry weight. Our results indicate that chilling stress occurred in watermelon plants at 10˚C, while 35˚C is the optimal temperature for this plant. Low temperature stress caused: (i) decreased shoot weight; (ii) accumulation of H2O2; (iii) increased SOD activity; and (iv) decreased enzyme activities associated with detoxifying H2O2. The novelty of this study centres on the fact that so few cold-sensitive species have been examined to date - additional cold-sensitive species need to be studied to determine if there are shared characteristics in terms of how they respond to cold stress. Most studies have examined single antioxidant responses, whereas we conducted a comprehensive examination of many antioxidant responses.

Proline metabolism in response to nitrogen deficiency in French Bean plants (Phaseolus vulgaris L. cv Strike)

The objective of the present work was to determine the impact ofnitrogen deficiency on proline metabolism in French Bean plants(Phaseolus vulgaris L. cv. Strike). The nitrogen wasapplied to the nutrient solution in the form of NH4NO3 at1.45 mM (N1), 2.90mM (N2) and 5.80mM (N3, optimal level). Our results indicateNdeficiency is characterised by a decline in proline accumulation both in theroot and leaves, fundamentally because proline degradation is encouraged by thestimulation of the enzyme proline dehydrogenase. By contrast, under conditionsof adequate N (N3), proline levels rise due to the action of ornithine,suggesting predominance of the ornithine pathway over the glutamine pathway, inaddition to the inhibition of proline dehydrogenase activity.

Role of CaCl2 in Ammonium Assimilation in Roots of Tobacco Plants (Nicotiana tabacum L.)

Summary The aim of this study was to determine the response of NH 4 + assimilation in roots to different CaCl 2 application (Ca1: 1.25 mmol/L CaCl 2 · 2H 2 O, Ca2: 2.5 mmol/L CaCl 2 · 2H 2 O and Ca3: 5 mmol/L CaCh · 2H 2 O). Tobacco plants ( Nicotiana tabacum cv. Tennessee 86) were grown under controlled conditions and submitted to regular fertilization with macro- and micronutrients. The contents of Ca 2+ , Cl - and NH 4 + , the activity of enzymes related to the process of NH 4 + assimilation (GS: glutamine synthetase, EC 6.3.1.2.; GOGAT: glutamate synthase, EC 1.4.1.14; GDH, glutamate dehydrogenase EC 1.4.1.2.), and the end products of this process (amino acids and proteins) were analysed in roots. Our results indicate that the utilization of NH 4 + in the plant was influenced by the different treatments. In the Ca3 treatment the NH 4 + content was lower than in Ca1 and Ca2, apparently due to the higher GS, GOGAT and GDH activities registered in Ca3. However, we found that the concentration of amino acids and protein as well as biomass production in the root was more important in the Ca1 and Ca2 treatments than in Ca3. This is the result of the greater translocation of these products to the shoot, stimulated by the high Ca 2+ dosage applied (5 mmol/L) to the cultivation medium in the Ca3 treatment.

Supplemental boron stimulates ammonium assimilation in leaves of tobacco plants (Nicotiana tabacum L.)

The aim of this work was to analyse the response ofNH4+ assimilation in leaves of tobacco plants(Nicotiana tabacum L. cv. Tennessee 86), to different Bapplications (B1, 5 μM H3BO3; B2, 10μM H3BO3; B3, 20 μMH3BO3). The plants were grown under controlledenvironmental conditions and received a complete nutrient solution. In thisexperiment, we analysed the foliar concentrations of B andNH4+, as well as the corresponding enzymaticactivities: GS,GOGAT, GDH, PEPC; the end products of this assimilation, aminoacids and proteins; and finally the concentration of non-structural sugars. Ourresults indicated that the different B treatments influenced the utilization ofNH4+ by tobacco leaves. The B3 treatment registeredthe lowest NH4+ concentration, and B1 the highest,due probably to the higher GS, GOGAT and GDH activities registered at B3.Conversely, a decline in the concentration of non-structural sugars wasrecordedat B3. In addition, the high assimilation rate caused a progressiveaccumulationof amino acids as well as proteins, and boosted biomass production in theleaves.