Influence des modifications de l’équilibre source-puits sur les paramètres physiologiques et biochimiques chez l'olivier (Olea europaea L.), sous bioclimat semi-aride de Tunisie

La culture de l’olivier en Tunisie est restée globalement une culture traditionnelle de faible rentabilité. Les causes de cette situation sont multiples, y compris les traitements culturaux archaïques, les cultivars inadéquats et l’alternance de production. De ce fait, une bonne connaissance des aspects physiologiques et biologiques de l’arbre, est d’une importance fondamentale pour une gestion raisonnable des pratiques agronomiques et un bon développement d’une oléiculture moderne. Il est impératif de déployer les efforts pour améliorer à la fois la productivité et la qualité des produits de l’olivier aux fins de faire face aux exigences de rentabilité, de qualité et d’adaptation aux attentes du marché. La présente étude, porte sur une meilleure compréhension des relations sources-puits chez l’olivier par l’évaluation de certains paramètres physiologiques, biochimiques et agronomiques comme réponse à la manipulation de l’équilibre source-puits par l’application de l’incision annulaire et la réduction de la taille des puits fructifères (par éclaircissage) et végétatifs (par décapitation), et d’autre part, essayer d’étudier l’influence de l’incision annulaire et l’éclaircissage manuel comme techniques culturales, chez l’olivier de table. Ce travail a été conduit durant quatre années consécutives (2009 à 2012), en plein champ, sous bioclimat semi-aride de Tunisie. Les deux cultivars expérimentés sont Olea europaea ‘Picholine’ et ‘Besbessi’. L’incision annulaire, la décapitation et les rapports feuille/fruit ont été appliqués chez le cultivar Picholine. L’éclaircissage manuel a été effectué sur l’arbre entier chez le cultivar Besbessi. Durant la période expérimentale, nous avons tenté de mesurer l’assimilation photosynthétique, la répartition des assimilats au sein des feuilles, du bois et des fruits, et le développement des fruits. Le suivi de la variation saisonnière des échanges gazeux et de la teneur foliaire en carbohydrates du cultivar Picholine sous bioclimat semi-aride de Tunisie, montre une forte activité photosynthétique durant la première période de la croissance végétative. Durant la période estivale, les hautes valeurs de la température, l’important déficit de vapeur d’eau et l’irrigation déficitaire occasionnent une diminution de 56% de la photosynthèse nette, avec une augmentation de 44% de la teneur foliaire en mannitol. L’analyse de l’effet de l’incision annulaire et l’augmentation du rapport feuille/fruit, montre une diminution significative de la photosynthèse nette, la conductance stomatique et la concentration intercellulaire en CO2. L’inhibition de la photosynthèse est attribuée à un contrôle stomatique, vérifié par la réduction de la conductance stomatique, mais également cette diminution peut être liée à un rétrocontrôle biochimique, suite à l’accumulation des carbohydrates. En effet, l’accumulation de l’amidon au niveau des feuilles a été associée à une réduction de la photosynthèse. Par conséquence, il serait primordial d’étudier les mécanismes biochimiques impliqués dans la régulation de la photosynthèse suite à la forte disponibilité des assimilats. La réduction de la photosynthèse n’est pas le résultat du dommage ou inactivation du PSII. En autre, l’incision annulaire et la réduction de la taille du puits fructifère n’ont aucun effet significatif sur les paramètres de la fluorescence chlorophyllienne. La réduction de la taille du puits fructifère a un effet plus prononcé sur les paramètres des échanges gazeux par comparaison à celle du puits végétatif. Cependant, ce dernier favorise la ramification axillaire. L’application de l’incision annulaire sur une branche de premier ordre chez le cultivar Picholine amène à l’amélioration du calibre des fruits. La date de réalisation de cette technique affecte significativement l’assimilation photosynthétique, ainsi que la qualité des fruits. Nos résultats montrent que la période la plus favorable pour ce traitement coïncide avec la fin de la nouaison, tandis que, l’incision annulaire tardive (un mois avant la véraison) accélère la maturation des fruits. L’étude de l’effet de la variation de la charge en fruits au niveau de l’arbre entier, montre que l‘éclaircissage sévère (élimination de 75% de la charge initiale), entraine une croissance végétative importante, une augmentation du calibre des fruits, mais également, une amélioration de la productivité de l’année suivante

Effects of abiotic stress on the germination of Pennisetum dichotomum

Seeds germination of Pennisetum dichotomum was tested for three alternating temperatures (10, 25, 35°C) at different hydric potentials (0, – 1, – 3, – 7, – 10 and – 16 bar) applied with different concentrations of sodium chloride (0-3-6-12 or 15g/l of NaCl). The highest germination parameters (germination rate, germination percentage) were obtained with no osmotic potential (0 bar) under 25°C. Germination rate decreased progressively with increasing NaCl dose, and the lower value of this parameter was reached with 15g/l of NaCl. Compared to control (0g/l of NaCl) P. dichotomum seeds, the radical length (RL) and the hypocotyl length (HL) of the NaCl-treated ones were severely decreased at 12 and 15g/l concentrations. In another way, seedling growth of P. dichotomum was shown more sensitive than germination seeds to salinity stress.Chaffei Haouari Chiraz, Hajjaji Nasraoui Afef, Gouia Houda, Haouari Ali. Effects of abiotic stress on the germination of Pennisetum dichotomum. In: Ecologia mediterranea, tome 38 n°2, 2012. pp. 17-22

Variations in α-, β-amylase and α-glycosidase activities in two genotypes of wheat under NaCl salinity stress

Two wheat differing in salt sensitivity, was examined for osmolyte contents and activities of α-amylase, β-amylase and α-glucosidase enzymes involved in seeds germination, in absence as well as in presence of  100, 150, 200 and 300 mM NaCl. The inhibitory effects of NaCl differed, depending on the species tested. In wild wheat specie (Triticum monococcum), with reduced germination percentage and lower relative water  content, the   increase in NaCl concentration resulted in the decrease in endogenous level of proline, total soluble sugars and activities of the main enzymes involved in the germination process. In contrast, cultivated  wheat specie (Triticum aestivum) seed in response to salt stress accumulated higher proline and total soluble  carbohydrate concentrations  which improved  their  water  status and the enzyme  activities  involved  in the germination  process. Differential response of the different species of wheat to salt stress is governed by the accumulation of osmolytes in seeds

Long-term salt stress responsive growth, carbohydrate metabolism, proline and anti-stress enzymes in Nicotiana tabaccum

We investigated the long-term responses of tobacco tissues to salt stress, with a particular interest for growth parameters, proline (Pro) accumulation, and carbohydrate metabolism. Exposure of 17-day-oldtobacco plants to 0.2 M NaCl was followed by a higher decrease in dry matter in roots than shoots with a decrease of both Chlorophyll a and b. Analysis of potassium, sodium and chloral accumulation showed dramatic effect to K+ accumulation especially in shoots, whereas Na+ and Cl- accumulation were strongly increased in shoots with NaCl treatment, but with minor effects in the roots. Salinity stress conditions induced a strong increase in sugar free content in the shoot and root and Pro and 2-oxoglutarate biosynthesis in tobacco tissues. However, salt-stress resulted in significant increase in the activity of NADPH and NADH malate dehydrogenase dependency, particularly in the roots, and this effect was reversed with NADP+ and NAD+-dependent malate dehydrogenase, but the activity of these enzymes was also maintained significantly higher in the absence of NaCl. Significant decreases in NADPH-depend isocitrate dehydrogenase was also observed after long-term salt treatment in tobacco plants. In addition, a gradual increase was observed in the in vitro aminating GDH activity, but the in vitro deaminating activity remained unchanged under salinity stress. These results confirmed thehypothesis that the salinity-generated MDH and GDH induction act as anti-stress enzymes in ammonia detoxification and production of Glu for Pro synthesis.Keywords: Tobacco, carbohydrate metabolism, salinity stres

Exogenous application of salt (NaCl) inhibit antioxidative system in Cd-treated watercress (Nasturtium officinale R. Br.)

International audienceThis research simulates conditions found in metal-contaminated sites also affected by a high concentration of salt. For these reasons, several exogenous doses of NaCl were used to treat Cd-stressed Nasturtium officinale R. Br. We have chosen Nasturtium officinale R. Br which is a hyper accumulator plant of metals (based on literature). Data suggested induction of oxidative stress under Cd treatment and demonstrated watercress's capacity to upregulate its antioxidative defense. Superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and peroxidase (GPX) activities were stimulated by Cd. Supplementation of (100, 200, 300 mM NaCl) simultaneously with 1 MCd to six weeks aged Nasturtium officinale R. Br caused growth inhibition, reduction of photosynthetic pigment contents and a significant increase of Malonyldialdehyde (MDA) and hydrogen peroxide (H2O2) ones in leaves. Increase of NaCl dose reduced Cd tenor in Nasturtium officinale leaves. Moreover, a dramatic lost of antioxidant system efficiency was detected in these plants. Rise of salinity concentration significantly inhibited SOD, CAT, APX and GPX activities in Cd-treated Nasturtium officinale R. Br

Effect of leaf-to-fruit ratio and girdling on gas exchanges, fruit growth and carbohydrate contents at different stages of fruit development of Olea europaea L. 'Picholine'

The effects of different source-sink ratios on fruit quality were studied for the table olive cultivar ‘Picholine’. This cultivar represents 15% of the production of table olives in Tunisia. Fruit size is an important quality parameter for table olives as small fruits have lower economical value. Three tertiary branches/tree were selected on 27-years-old olive trees (10 trees in total) at the start of the growing season 2009. After fruit set in early May four fruit-to-leaf ratios were imposed on 1-year-old wood (no fruits, 1:1, 1:2, 1:3) of each branch. Apexes of all shoots were removed in order to reduce the vegetative sinks. In mid-July five trees were randomly selected and all branches were girdled. The absence of fruits or girdling decreased leaf net photosynthetic rate (An) and increased leaf carbohydrate contents. Girdling stimulated fruit growth, while the effect of leaf-to-fruit ratio was less pronounced

Fruit thinning affects photosynthetic activity, carbohydrate levels, and shoot and fruit development of olive trees grown under semiarid conditions

Olive (Olea europaea L.) production is marked by annual oscillations as trees alternate from high to low crop loads in successive years. Gas exchanges and carbohydrate content of leaves and fruits in olive tree (O. europaea cv. Besbassi) were monitored at pit hardening and fruit ripening. After fruit set, three crop loads were applied (100%, 50% and 25% of the initial fruit load) by manual thinning. Severe fruit thinning reduced photosynthesis, stomatal conductance and intercellular CO2 concentration. Crop load had no significant effect on chlorophyll fluorescence parameters. The reduction of 75% of the initial crop load favoured the accumulation of starch in leaves and soluble sugars in leaves and fruits. The reduction in initial fruit load had a significant positive effect on the current year's shoot elongation and on inflorescence number the following spring. To increase the fruit size, a strong thinning (75%) was necessary, which coincided with the highest shoot vigour. Moderate thinning (50%) hardly affected leaf carbohydrate content and fruit size, but photosynthetic capacity was only limited at fruit ripening

Fruit thinning affects photosynthetic activity, carbohydrate levels, and shoot and fruit development of olive trees grown under semiarid conditions

Olive (Olea europaea L.) production is marked by annual oscillations as trees alternate from high to low crop loads in successive years. Gas exchanges and carbohydrate content of leaves and fruits in olive tree (O. europaea cv. Besbassi) were monitored at pit hardening and fruit ripening. After fruit set, three crop loads were applied (100%, 50% and 25% of the initial fruit load) by manual thinning. Severe fruit thinning reduced photosynthesis, stomatal conductance and intercellular CO2 concentration. Crop load had no significant effect on chlorophyll fluorescence parameters. The reduction of 75% of the initial crop load favoured the accumulation of starch in leaves and soluble sugars in leaves and fruits. The reduction in initial fruit load had a significant positive effect on the current year's shoot elongation and on inflorescence number the following spring. To increase the fruit size, a strong thinning (75%) was necessary, which coincided with the highest shoot vigour. Moderate thinning (50%) hardly affected leaf carbohydrate content and fruit size, but photosynthetic capacity was only limited at fruit ripening