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Accueil du site → Doctorat → Brésil → Fotossintese, relações hidricas e alterações bioquimicas em laranjeira ’pera’ com CVC e submetida a deficiencia hidrica

Universidade Estadual de Campinas (2002)

Fotossintese, relações hidricas e alterações bioquimicas em laranjeira ’pera’ com CVC e submetida a deficiencia hidrica

Camilo Lazaro Medina

Titre : Fotossintese, relações hidricas e alterações bioquimicas em laranjeira ’pera’ com CVC e submetida a deficiencia hidrica

Auteur : Camilo Lazaro Medina

Université de soutenance : Universidade Estadual de Campinas

Grade : Doutor em Biologia Vegetal 2002

Brazil is the largest producer of citrus ftuits in the World. However, the citrus variegated chlorosis (eVe), a disease caused by the bacteria restrict to xylem Xylella fastidiosa, is causing many impairment in Brazilian citrus industry. Although this disease is disseminated, there are some indications that water deficiency may increase the severity of eve in some regions. The leaves of eve aifected plants presents drought stress symptoms, associated with significant decrease on carbon dioxide assimilation (A) and transpiration rates (E), stomatal conductance (gs) and leaf water potential (i a). These symptoms may be related to the increased water flow resistance through xylem vessels, although there are some possibilities of others bacterial pathogenicity mechanisms are interfering on gs and photosynthesis efficiency. Oranges trees with a long time infected by X fastidiosa, increase the phenolic concentration and higher enzymes activities in leaves related to natural pathogens defenses. Although there are not studies about the presence of these reactions during the onset of disease development, it is possible that they may become an important barrier for xylem pathogen invasion and may be aifected by water stress. This work was organized in two chapters. The objective ofthe first part were were to study the development of symptoms, and to observe the variation on the gas exchanges, water relations induced by eve, and drought stress by length of time. The experiment was carried under natural conditions with sweet orange ’Pêra’ (Citrus sinensis L. Osbeck) on ’Rangpur’ lime rootstock (Citrus limonia). The plants were grown in 100 L pots and the bacteria was inoculated by spliced approach grafting. The CVC caused the decrease of ’li a, A and gs even without drought stress. Up to 9 months afier the bacteria inoculation (MAl), these reductions were limited to aftemoons and occurred during aIl day afier 22 MAL The raise of the ratio from interna ! and air CO2 concentration (C/Ca) and the decrease of water use efficiency (WUE) was modified on1y afier 22 MAl, and showed a decreased of photosynthesis efficiency. The responses of A vs. gs, and A vs. ’lia were sllnilar for CVC and healthy plants. It was concluded that lower photosynthesis efficiency from CVC plants was mainly related to the lower leaf water potential, resulting from the deficiency ofxylem hydraulic conductivity. The drought stress, applied in the beginning of disease development, increased the severity of the symptons and decreased ’li a, A and gs from CVC plants. This confirms that water deficiency increases CVC damages. The lower photosynthesis efficiency of CVC plants and the reduction of A in function ofthe raise ofthe vapor pressure deficits (VPD) may be tum unfavorable for the adaptation for CVC plants to regions with higher demand of atmospheric. This fact, associated with the problems caused by drought stress, may be a justification for a higher CVC severity in regions with a high VPD and subjected to water deficiency, as the center and the north of the São Paulo State. The second part was carried out with the objective ofmeasuring alterations on the leaf composition of the soluble sugars, free amino acids, phenolics compounds and peroxidase activity at the begging symptoms of CVC, when the plants were submitted to water stress. Until 9 MAl, the composition of soluble sugars, free aminoacids, phenolic compounds activity and electrophoresis pattem of peroxidases were not affected, although the experiment was carried out at begging of the symptoms and CVC had reduced A, E, and gs. There was reduction of the soluble sugar concentration on1y in leaves with CVC submitted to drought stress. The raise of the peroxidase activity was observed only m very symptomatic leaves. These results supports that the biochemical defenses are not mduced during the onset of the development of CVC, and that the imbalance of water relations, caused by xylem bacteria colonization, may be main patogenicity mechanism of X fastidiosa. The water deficit reduced the peroxidase activity m leaves with symptoms, suggesting that the drought stress damages plant defenses and is one more factor that justifies the higher severity of CVC m drought regions


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