INFLUENCE OF DIFFERENT SEED PRIMING METHODS FOR IMPROVING SALT STRESS TOLERANCE IN LETTUCE PLANTS

Seeds of lettuce, variety Romaine were subjected to different priming treatments such as water, potassium nitrate (KNO₃) and gibberellic acid (GA₃). Seedlings obtained from primed (Pr) and nonprimed (NP) seeds were grown in a hydroponic culture system supplemented with 0, 100 or 200 mM sodium chloride (NaCl). The different physiological and biochemical responses were studied 15 days after treatment. Under NaCl, the dry weight was higher in plants derived from hydro-primed (HP) seeds when compared to NP, osmoprimed (KNO₃P), and hormonal primed (GA₃P) ones. Under control and 100 mM NaCl treatment, malondialdehyde (MDA) content and EL did not show any correlation with activities of gaiacol peroxidase (GPX) and catalase (CAT), but did with the increase in reduced ascorbate (AsA) and total ascorbate contents. The results indicated that plants derived from HP seeds exhibited higher adaptive potential under salinity stress. Our findings suggest that a hydropriming technique can be used as a simple commercial approach to alleviate the effects of NaCl induced stress in lettuce plants.     

Virulence spectra and geographical distribution of Mal Secco disease of citrus caused by Phoma tracheiphila in the Mediterranean countries: Tunisia and Italy

This work aimed to find out patterns of virulence variability of a Phoma tracheiphila population of 51 isolates, to determine geographic distribution of Mal Secco disease in citrus orchards of six Mediterranean countries and also to establish correlation between geographic distribution and pathotypic distance of P. tr population structure over our sampling spatial scale. Based on unweighted pair-group method with arithmetic averaging clustering and mean disease rating scores, three distinct virulence groups were identified. The 51 isolates were classified into 20 pathotypes. Extensive virulence variability was detected in 51 isolates of P. tr causing MSD of citrus in the Mediterranean basin. Regression plot between pairwise virulence and geographical distance showed that virulence is independent of the geographical origin and that isolates collected from the same country have different degrees of virulence. The lack of significant correlation between virulence and geographic structure confirmed the absence of isolation-by-distance pattern, suggesting non-regular and non-gradual dispersal of the pathogen over this spatial scale.     

Optimization of a Novel Vegetable Nursery Substrate Using Date Palm Wastes Peat and Indigenous Arbuscular Mycorrhizal Fungi

Quality of vegetable seedlings is affected by nursery management practices, essentially growing media. For sustainable greenhouse horticulture, it is important to characterize biologically based approaches to improve plantlet quality. Here, we investigate the possibility of developing a biotized growth substrate for nursery production using date palm wastes (DPWs) peat as an alternative to commercial sphagnum peat and indigenous arbuscular mycorrhizal fungi (AMF). A greenhouse experiment was established using different rates of DPWs peat to substitute commercial peat in both conventional and organic nursery production systems. Lettuce seedlings were inoculated or not with native AMF isolated from plots with conventional or organic production system (CM and OM respectively). When lettuce plantlets attained commercial size, a microscopic observation revealed that the establishment of mycorrhizal association was successful in all mixtures of growing media. Significant increase in growth parameters (height shoot, number and area of leaves, shoot and root biomass) were observed on plantlets inoculated with indigenous AMF isolated from organic plot compared to those isolated from conventional plot and to control (without AMF). Commercial peat partial substitution with DPWs peat in proportion of 25–50% improves better lettuce performance than complete-peat use. Consequently, these mixtures seem to be an appropriate formulation for sustainable nursery production.     

Effects of Whitening Agents and Frozen Storage on the Quality of Sardine (Sardina pilchardus) Surimi: Physicochemical and Mechanical Properties

This work is focused on the effect of using whitening agents (WA) during a process followed by frozen storage (4 months at ?18°C) on the whiteness, quality parameters, and mechanical properties of sardine surimi. The whiteness of surimi exhibited a significant improvement when whitening agents (calcium carbonate and peroxide hydrogen) were used (48.75 to 60.24). Dynamic rheological measurement showed that storage moduli, G′, was considerably higher than loss moduli, G″, for both surimi with WA and control; however, the highest viscoelastic moduli were found in the treated surimi. In addition, the microstructure of surimi with WA showed more compact matrix and higher water holding capacity. On the other hand, most tested indexes showed quality losses throughout the frozen storage in both samples. Proteins underwent denaturation as indicated by the reduction in band intensities, especially myosin heavy chain (MHC) bands. Peroxide value (PV) and thiobarbituric acid reactive substances (TBARS) measurements showed that lipid oxidation took place during storage; however, the degree of lipid oxidation was not relevant. Therefore, it was proved that the addition of whitening agents had a marked effect on the production of surimi with better functional attributes including whiteness, water holding capacity, and mechanical properties.     

Phaseolus vulgaris-rhizobia symbiosis increases the phosphorus uptake and symbiotic N2 fixation under insoluble phosphorus

Five north Tunisian native rhizobia were tested in symbiosis with ‘Coco-blanc’ variety under two phosphorus (P) supplies [P-sufficient and insoluble P (IP)] at 250 μmol P plant^?1. At flowering stage, responses against IP were assessed and indicated that inoculation with P.Ps.09 produced a high nodule biomass (0.12g). Similarly, P.Tb.09 produced 0.08 g nodule biomass under both P and P.Bj.09 that significantly increased nodule number under IP. Shoot biomass exhibited high dry weight with P.Tb.09 and CIAT899 (1.6 g plant^?1). Inoculation with CIAT899 produced a high root biomass under both P. Analysis of a halo diameter of clear zone rounding each colony under IP indicated that P.Tb.09 has developed high halo Ø (1.7cm). These variations were associated under IP with a decrease of pH by P.Ar.09 and with increase in shoot phosphatase activity by P.Ar.09, P.Bj.09, P.OM.09 and P.Ps.09. The use of P-efficient rhizobia may constitute an adaptive mechanism against P-deficiency tolerance.     

Fatty acid, triacylglycerol, and phytosterol composition in six Tunisian olive varieties

The physicochemical and stability properties as well as the fatty acid, triacylglycerol, sterol, and triterpenic dialcohol compositions of Tunisian olive oil varieties were analyzed. On the basis of our results, we classified all of the monovarietal oils into the extra virgin category. Oleic and linoleic acids were the most useful fatty acids to discriminate three cultivars, Neb Jmel, Chétoui, and Ain Jarboua, from the others. Of the six monovarietal virgin olive oils analyzed, the main triacylglycerols were OOO, POO, PLO plus SLL, and OLO, which was expected given the high oleic acid and low linoleic and linolenic acids content observed in total fatty acids. In total, these accounted for more than 80% of the total HPLC chromatogram peak area. The main sterols found were beta-sitosterol, Delta5-avenasterol, and campesterol. The statistical analysis showed significant differences between oil samples, and the obtained results showed a great variability in the oil composition between cultivars, which is influenced exclusively by genetic factors.     

The variation of phosphorous content,grain yield,and rhizosphere microbial biomass among durum wheat cultivars under salinity stress

ABSTRACT

The effectiveness of plant–soil synergies is largely modulated by interaction between cultivar and rhizosphere microbiome. We evaluated the agronomic performance of six durum wheat cultivars, in two semi-arid locations in Tunisia that differed in their irrigation water salinity: S1 (6 dS m^?1) and S2 (12 dS m^?1). The two-consecutive-year field experiments assessed the effects of the microbial biomass carbon (MBC), leaf phosphorus (LP) and rhizosphere phosphorus (P) on the grain yield (GY) and yield components at tillering and flowering stages. Overall, in saline conditions, cultivars differed in above- and below-ground traits, particularly, with tolerant cultivars presenting relatively greater MBC, P and LP. Furthermore, in S2, GY positively correlated with MBC (r = 0.69), LP (r = 0.80) and P (r = 0.79). Additionally, in S2, MBC positively correlated with P (r = 0.87) and LP (r = 0.85) at flowering. This result was further confirmed by multiple linear regression (step-wise) analysis, which revealed that MBC and LP were the determinant components of GY variability under S2. The present study demonstrates that LP and soil P are mandatory for improving the management of durum wheat. Salinity tolerance was largely affected by the cultivars’ rhizosphere MBC.     

First serological investigation of peste-des-petits-ruminants and Rift Valley fever in Tunisia

This study, carried out between September 2006 and January 2007, is the first cross-sectional serological investigation of peste-des-petits-ruminants (PPR) and Rift Valley fever (RVF) in Tunisia. The objective was to assess the potential need to develop a dual, recombinant PPR–RVF vaccine and how such a vaccine might be utilised in Tunisia. An overall PPR seroprevalence of 7.45% was determined, a finding supported by the high specificity (99.4%) and sensitivity (94.5%) of the ELISA used. On assessment of the diversity and density of mosquitoes in the sampling area, four species of RVF-vectors of the genus Aedes and Culex were identified. However, no serological evidence of RVF was found despite the use of a highly sensitive ELISA (99–100%). Larger scale investigations are underway to confirm these findings and the continuation of the emergency vaccination program against these two diseases remains valid.     

Inoculation with Phosphate Solubilizing Mesorhizobium Strains Improves the Performance of Chickpea (Cicer aritenium L.) Under Phosphorus Deficiency

The use of phosphate-solubilising bacteria as inoculants increases plant phosphorus (P) uptake and thus crop yield. Strains from the genus Mesorhizobium are among the most powerful phosphate solubilizing microorganisms. In order to study efficiency in P uptake and N₂ fixation in chickpea (Cicer aritenium), forty-two rhizobia strains natively from Tunisian soils were studied in symbiosis with the chickpea variety “Béja1” which is frequently cultivated in Tunisia. Plants were inoculated separately with these strains under controlled conditions in perlite under two sources of P i.e. soluble (KH₂PO₄) and insoluble P (Ca₂HPO₄). At flowering stage, growth, nodulation, P uptake and N₂ fixation were assessed in all symbiotic combinations. The results showed that the S27 strain efficiently mobilized P into plants, observed as a significant increase of plant P content when insoluble P (Ca₂HPO₄) was supplied to the soil. This was associated with a significant increase in plant biomass, nodule number and N content under insoluble P conditions. Additionally, inoculation with the Mesorhizobium strain S27 significantly increased the root acid phosphatase activity under insoluble P. This study also shows significant correlations found between plant P content and acid phosphatase activity under low P conditions which may highlight the contribution of acid phosphatases in increasing P use efficiency. A field experiment also showed that most of the chickpea analyzed parameters were improved when plants inoculated with two selected rhizobia strains (S26 and S27) and supplied with P2O5. Overall, these findings postulate that rhizobial inoculation should not only be based on the effectiveness of strains regarding N fixation, but also to other traits such as P solubilisation potential.     

<Emphasis Type="Italic">Carica papaya</Emphasis> Lipase: A Naturally Immobilized Enzyme with Interesting Biochemical Properties

Triacylglycerol (TAG) lipases have been thoroughly characterized in mammals and microorganisms, whereas very little is known about plant TAG lipases. The lipolytic activity occurring in all the laticies is known to be associated with sedimentable particles, and all attempts to solubilize the lipolytic activity of Carica papaya latex have been unsuccessful so far. However, some of the biochemical properties of the lipase from Carica papaya latex (CPL) were determined from the insoluble fraction of the latex. The activity was optimum at a temperature of 37°C and a pH of 9.0, and the specific activities of CPL were found to be 2,000 ± 185 and 256 ± 8 U/g when tributyrin and olive oil were used as substrates, respectively. CPL was found to be active in the absence of any detergent, whereas many lipases require detergent to prevent the occurrence of interfacial denaturation. CPL was inactive in the presence of micellar concentrations of Triton X-100, sodium dodecyl sulfate (SDS) and tetradecyl trimethylammonium bromide (TTAB), and still showed high levels of activity in the presence of sodium taurodeoxycholate (NaTDC) and the zwitterionic Chaps detergent. The effects of various proteases on the lipolytic activity of CPL were studied, and CPL was found to be resistant to treatment with various enzymes, except in the presence of trypsin. All these properties suggest that CPL may be a good candidate for various biotechnological applications.