Does apoplastic ascorbic acid enhance manganese tolerance of Vigna unguiculata and Phaseolus vulgaris

In cowpea (Vigna unguiculata), the development of manganese (Mn) toxicity is considered to be accompanied by the formation of reactive oxygen species, oxidized Mn, and phenoxy radicals in the leaf apoplast. Ascorbic acid (AA) is a common antioxidant in plants, and the oxidation of AA, particularly in the leaf apoplast, contributes to the first line of defence against several biotic and abiotic stress factors. The objective of the present study was to contribute to a better understanding of the role of AA in Mn leaf-tissue tolerance of cowpea and common bean (Phaseolus vulgaris). Five cowpea cultivars (cvs.) differed greatly in Mn tolerance, which was expressed in differences in numbers of brown spots on leaves and in peroxidase (POD) activity in the apoplastic washing fluid (AWF). In a Mn-sensitive cv., after 3 d of Mn treatment, brown spots were formed, and POD activities were increased, accompanied by a release of proteins into the apoplast. In the AWF, the concentration of AA and the ratio of AA : (AA+DHA) decreased already after 1 d, and to only 2% after 3 d. In the leaf tissue, the ratio was nearly unaffected, and the total AA+DHA content in the leaf tissue was even increased with advanced expression of Mn toxicity. The application of AA solutions in the range of 5–10 μM via the petiole slightly enhanced Mn tolerance as indicated by the reduction of brown spots (however inconsistently) and POD activity (consistently) in the AWF. Common bean cultivars differing in ozone tolerance, which has been reported to be due to a high AA availability in the leaf apoplast, were studied for their Mn tolerance. Clear differences in Mn tolerance between the cultivars existed, however, these differences were not related to their ozone tolerance. From these results, we conclude that the maintenance of sufficient AA levels in the leaf apoplast contribute to Mn tolerance, but does not fully explain genotypic differences in Mn tolerance in cowpea and common bean. Beeinflusst apoplastische Ascorbinsaure die Mangantoleranz von Vigna unguiculata und Phaseolus vulgaris? Fruhere Arbeiten weisen darauf hin, dass Mangan (Mn)-Toxizitat bei Cowpea (Vigna unguiculata) mit der Bildung von reaktiven Sauerstoffradikalen, oxidiertem Mn und Phenoxyradikalen im Blattapoplasten verbunden ist. Ascorbinsaure (AA) ist ein weit verbreitetes Antioxidationsmittel in Pflanzen. Die Oxidation von AA, insbesondere im Blattapoplasten, tragt zur primaren Abwehrreaktion von Pflanzen gegen verschiedene biotische und abiotische Stressfaktoren bei. Ziel dieser Untersuchung war es, zum besseren Verstandnis der Rolle von AA bei der Mn-Blattgewebetoleranz von Cowpea und Buschbohne (Phaseolus vulgaris) beizutragen. Funf Cowpea-Genotypen unterschieden sich stark in der Mn-Toleranz, was sich in Unterschieden in der Zahl brauner Punkte auf den Blattern und der Peroxidase (POD)-Aktivitat in der apoplastischen Waschflussigkeit (AWF) widerspiegelte. Bei einem Mn-empfindlichen Genotyp traten braune Punkte und eine erhohte POD-Aktivitat verbunden mit einer verstarkten Abgabe von Proteinen in den Apoplasten nach 3 Tagen Mn-Behandlung auf. In der AWF sanken die Konzentration von AA und das Verhaltnis AA : (AA+DHA) bereits nach einem Tag und bis auf 2 % nach 3 Tagen. Im Blattgewebe blieb dieses Verhaltnis praktisch unbeeinflusst und der Gesamt-(AA+DHA)-Gehalt stieg sogar mit zunehmender Auspragung von Mn-Toxizitat an. Die Applikation von AA-Losungen im Bereich von 5–10 μM uber den Blattstiel erhohte die Mn-Toleranz leicht, gemessen an der Verringerung des Auftretens von braunen Punkten (jedoch nicht konsistent) und der POD-Aktivitat in der AWF (konsistent). Buschbohnen-Genotypen mit unterschiedlicher Ozontoleranz, die auf einer unterschiedlichen AA-Verfugbarkeit im Blattapoplasten zuruckgefuhrt worden war, wurden auf ihre Mn-Toleranz untersucht. Zwischen den Genotypen bestanden deutliche Unterschiede in der Mn-Toleranz. Diese Unterschiede standen aber in keiner Beziehung zur ihrer Ozon-Toleranz. Auf der Basis der Ergebnisse schlussfolgern wir, dass die Aufrechterhaltung eines ausreichenden AA-Gehaltes zur Mn-Toleranz beitragt. Dies reicht aber nicht aus, um die genotypischen Unterschiede in der Mn-Toleranz bei Cowpea und Buschbohne hinreichend zu erklaren.

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