Effect of phosphorus deficiency induced in calcareous soil on plant growth, phosphorus use efficiency and acid phosphatase activity of Medicago truncatula.

Calcareous soils, characterized by a higher pH, are frequent in the North West of Tunisia. Large concentrations of calcium carbonate in calcareous soils result in accumulation of high levels of bicarbonate ions, which complex with phosphate, resulting in phosphorus deficiency (induced P deficiency) for plants. The impact of calcareous soil on plant growth, photosynthetic activity and acid phosphatase activity was explored in two lines of Medicago truncatula: TN6.18 and Jemalong. Calcareous soil significantly restricted shoot growth only in Jemalong (-45 % of the control). When grown on calcareous soil, root length was stimulated, this effect being more pronounced in TN6.18. Under calcareous soil, net CO2 assimilation declined more in Jemalong (-40 % of the control) than in TN6.18 (-20 % of the control). CO2 accumulation was increased in Jemalong (+35% of the control) plants grown in calcareous soil. The acid phosphatase activity was higher in plants cultivated under calcareous soil. This increased phosphatase activity was more pronounced in TN6.18, which showed higher accumulation of Pi in shoots and roots than Jemalolng.  In the light of these results, the present study proposes acid phosphatase as a useful candidate for improving Pi acquisition and utilization under calcareous soil

Natural variation in physiological responses of tunisian hedysarum carnosum under iron deficiency

Iron (Fe) is an essential element for plant growth and development. The cultivation of leguminous plants has generated strong interest because of their growth even on poor soils. Calcareous and saline soils with poor mineral availability are wide-spread in Tunisia. In an attempt to select better forage crops adapted to Tunisian soils, we characterized Fe deficiency responses of three different isolates of Hedysarum carnosum, an endemic Tunisian extremophile species growing in native stands in salt and calcareous soil conditions. H. carnosum is a non-model crop. The three isolates, named according to their habitats Karkar, Thelja, and Douiret, differed in the expression of Fe deficiency symptoms like morphology, leaf chlorosis with compromised leaf chlorophyll content and photosynthetic capacity and leaf metal contents. Across these parameters Thelja was found to be tolerant, while Karkar and Douiret were susceptible to Fe deficiency stress. The three physiological and molecular indicators of the iron deficiency response in roots, Fe reductase activity, growth medium acidification and induction of the IRON-REGULATED TRANSPORTER1 homolog, indicated that all lines responded to -Fe, however, varied in the strength of the different responses. We conclude that the individual lines have distinct adaptation capacities to react to iron deficiency, presumably involving mechanisms of whole-plant iron homeostasis and internal metal distribution. The Fe deficiency tolerance of Thelja might be linked with adaptation to its natural habitat on calcareous soil

Effect of iron deficiency on the localization of phosphoenolpyruvate carboxylase in common bean nodules

Phosphoenolpyruvate carboxylase (PEPC) plays an important role in nodules, when there is an increase in the demand for energy. This enzyme provides carbon skeletons to sustain amino acid synthesis and malate to support energy required to fix nitrogen. Since PEPC is important for nodules, and there is lack of information about the effect of some nutrient deficiency in the expression and localization of this enzyme in legume nodules, this work focused on the localization of PEPC in nodules under iron deficiency of two common bean cultivars: Flamingo tolerant and Coco blanc sensitive to iron (Fe) deficiency. The results of immunolocalization using polyclonal antibody showed that this enzyme was detected in all regions of nodule sections; but the signal intensity was increased in Fe-deficient nodules as compared to Fe-sufficient ones in the tolerant cultivar, whereas the intensity was less pronounced in nodules of Fe-deficient plants than in those of Fe-sufficient plants for the sensitive cultivar Coco blanc. This work showed that the symbiotic tolerance of Flamingo to iron deficiency was linked to the increase of PEPC enzymes expression. However, the activity of these enzymes supported the energy required in bacteroids to maintain the nitrogenase activity.Keywords: Common bean, immunolocalization, iron deficiency, nodules, phosphoenol pyruvate carboxylas

Effect of phosphorus deficiency induced in calcareous soil on plant growth, phosphorus use efficiency and acid phosphatase activity of Medicago truncatula.

Calcareous soils, characterized by a higher pH, are frequent in the North West of Tunisia. Large concentrations of calcium carbonate in calcareous soils result in accumulation of high levels of bicarbonate ions, which complex with phosphate, resulting in phosphorus deficiency (induced P deficiency) for plants. The impact of calcareous soil on plant growth, photosynthetic activity and acid phosphatase activity was explored in two lines of Medicago truncatula: TN6.18 and Jemalong. Calcareous soil significantly restricted shoot growth only in Jemalong (-45 % of the control). When grown on calcareous soil, root length was stimulated, this effect being more pronounced in TN6.18. Under calcareous soil, net CO2 assimilation declined more in Jemalong (-40 % of the control) than in TN6.18 (-20 % of the control). CO2 accumulation was increased in Jemalong (+35% of the control) plants grown in calcareous soil. The acid phosphatase activity was higher in plants cultivated under calcareous soil. This increased phosphatase activity was more pronounced in TN6.18, which showed higher accumulation of Pi in shoots and roots than Jemalolng.  In the light of these results, the present study proposes acid phosphatase as a useful candidate for improving Pi acquisition and utilization under calcareous soil

Eco-physiological responses and symbiotic nitrogen fixation capacity of salt-exposed Hedysarum carnosum plants

Nitrogen nutrition of Hedysarum carnosum, a pastoral legume common in Tunisian central and southern rangelands, largely depends on atmospheric nitrogen fixation. Yet, this process is greatly affected by environmental factors such as salinity. This study aimed to characterize the tolerance limits and the physiological responses of H. carnosum to salinity under symbiotic nitrogen fixation. Salt treatment was imposed by adding NaCl at different concentrations (0, 50, 100 and 200 mM) to the nutrient solution. Na+ content generally increased in the plant organs with increasing salinity in the culture medium. Especially, an excess accumulation of this cation was observed in leaves. Despite the fact that Na+ accumulation decreased plant growth, both nodulation and symbiotic nitrogen fixation capacity of H. carnosum appeared to be relatively salt-tolerant, owing to the plant capacity to maintain tissue hydration, control Na+ accumulation in shoots, and to conserve nodule efficiency to fix N2. Taken together, our findings indicate that H. carnosum is a glycophyte that can tolerate moderate salinity (100 mM), suggesting its possible utilization (i) in the improvement of soil fertility and (ii) in saline pastures, where the survival of other fodder species is critical.Key words: Hedysarum carnosum, nodulation, salinity, symbiotic nitrogen fixation

Natural Variation in Physiological Responses of Tunisian Hedysarum carnosum Under Iron Deficiency

Iron (Fe) is an essential element for plant growth and development. The cultivation of leguminous plants has generated strong interest because of their growth even on poor soils. Calcareous and saline soils with poor mineral availability are wide-spread in Tunisia. In an attempt to select better forage crops adapted to Tunisian soils, we characterized Fe deficiency responses of three different isolates of Hedysarum carnosum, an endemic Tunisian extremophile species growing in native stands in salt and calcareous soil conditions. H. carnosum is a non-model crop. The three isolates, named according to their habitats Karkar, Thelja, and Douiret, differed in the expression of Fe deficiency symptoms like morphology, leaf chlorosis with compromised leaf chlorophyll content and photosynthetic capacity and leaf metal contents. Across these parameters Thelja was found to be tolerant, while Karkar and Douiret were susceptible to Fe deficiency stress. The three physiological and molecular indicators of the iron deficiency response in roots, Fe reductase activity, growth medium acidification and induction of the IRON-REGULATED TRANSPORTER1 homolog, indicated that all lines responded to -Fe, however, varied in the strength of the different responses. We conclude that the individual lines have distinct adaptation capacities to react to iron deficiency, presumably involving mechanisms of whole-plant iron homeostasis and internal metal distribution. The Fe deficiency tolerance of Thelja might be linked with adaptation to its natural habitat on calcareous soil

Silicon (Si) mitigates the negative effects of iron deficiency in common bean (Phaseolus vulgaris L.) by improving photosystem activities and nutritional status

Silicon (Si) is the second most abundant element in the Earth's crust after oxygen. Its beneficial impact on crop development and yield, particularly under stressful conditions such as iron (Fe) deficiency, has been well documented. Fe deficiency is a critical constraint that limits crop production globally. The objective of this study was to investigate the effects of silicon (Na2SiO3) on common bean (Phaseolus vulgaris L. ‘Coco Rose’ variety) under iron-deficient conditions. The common bean plants were subjected to six treatments, which included three sufficient iron treatments (50 μM Fe) each paired with three varying silicon concentrations (0, 0.25, and 0.5 mM Si), and three iron-deficient treatments (0.1 μM Fe) each associated with the same silicon concentrations (0, 0.25, and 0.5 mM Si). The results indicate that iron deficiency had a negative impact on almost all the measured parameters. However, under silicon treatments, especially with 0.5 mM Si, the depressive effects of iron deficiency were significantly mitigated. The addition of 0.5 mM Si alleviated leaf chlorosis and improved biomass production, nutritional status, photosynthetic pigment content, photosynthetic gas exchange, and photosystem (PSI and PSII) activities. Interestingly, a greater beneficial effect of silicon was observed on PSII compared to PSI. This was accompanied by a significant augmentation in leaf iron concentration by 42%. Therefore, by enhancing the photosystem activities and nutritional status, among other mechanisms, silicon is capable of mitigating the adverse effects of iron-deficient conditions, making it a successful and effective solution to cope with this nutritional stressThis work was funded by the Tunisian Ministry of Higher Education and Scientific Research under grant LR15CBBC02. A portion of the work was also supported by the PHC Maghreb 2019 network (Partenariat Hubert CURIEN Maghreb 2019 no. 19MAG41) and the Young Researchers Encouragement Program (Programme d’Encouragement des Jeunes Chercheurs - 21PEJCD1P22 for Tarek SLATNI and 21PEJCD4P1 for Walid ZORRIG