CASSAVA BROWN STREAK DISEASE EFFECTS ON LEAF METABOLITES AND PIGMENT ACCUMULATION

Abstract

Cassava Brown Streak Disease (CBSD) is a threat to productivity and product quality in East Africa. The objective of this study was to understand the effect of CBSD on the primary photosynthetic apparatus of cassava ( Manihot esculenta Crantz). Three cassava varieties with varying levels of reaction to infection by CBSD were chosen and subjected to field disease pressure. Disease progression and the resulting effects on leaf morphology, metabolite and pigment accumulation were assessed over a period of five months, beginning from 3 months after planting. Slight increments in leaf dry matter were observed up to 4 MAP, and there after a drop was registered. A significant (P<0.05) reduction in photosynthetic pigments occured with a fall in the Chla:Chlb and Chla:Carotenoid ratio, indicative of specific reductions in chlorophyll a (-80%) compared to Chlorophyll b (-41 to -62%) and Carotenoid (-11 to -18%). Total reducing sugar and starch content also dropped significantly (-30 and -60%, respectively), much as NASE 14 maintained a relatively higher amount of carbohydrates. Leaf protein levels were significantly reduced at a rate of 0.07 ug g-1 leaf per month in diseased treatments. Significant reductions in primary metabolites show altered leaf photosynthetic and growth metabolism, resulting into a compromised plant system that cannot perform optimally.La Maladie Stri\ue9e Brune du Manioc (CBSD) constitue une menace pour la productivit\ue9 et la qualit\ue9 des produits du manioc en Afrique de l\u2019Est. L\u2019objectif de cette \ue9tude \ue9tait de comprendre l\u2019effet de CBSD sur l\u2019appareil photosynth\ue9tique primaire du manioc ( Manihot esculenta Crantz). Trois vari\ue9t\ue9s de manioc poss\ue9dant differents niveaux de r\ue9sistance \ue0 CBSD ont \ue9t\ue9 soumises \ue0 la pression dans les conditions de plein champs. La progression de la maladie et son effet sur la morphologie foliaire, les metabolites et accumulations de pigments ont \ue9t\ue9 \ue9valu\ue9s sur une p\ue9riode de cinq mois, \ue0 compter du troisi\ue8me mois apr\ue8s semis. Il a \ue9t\ue9 observ\ue9e une l\ue9g\ue8re augmentation de mati\ue8re s\ue8che foliaire jusqu\u2019\ue0 4 MAP, suivie d\u2019une chute. Une r\ue9duction significative (P<0,05) des pigments photosynth\ue9tiques a \ue9t\ue9 observ\ue9e avec une chute dans le Chla:Chlb et Chla:ratio de Carotenoide, indiquant des r\ue9ductions sp\ue9cifique sen chlorophyllea (-80%) en comparaison avec la Chlorophylleb (-41 \ue0 -62%) et les Carotenoides (-11 \ue0 -18%). La r\ue9duction du sucre et d\u2019amidona connu aussi une diminution significative (-30 et -60%, respectivement), de m\ueame NASE 14 a maintenuun equantit\ue9 relativement\ue9 lev\ue9e d\u2019hydrates de carbone. Les niveaux de proteines foliaires \ue9taient significativement r\ue9duits \ue0 0,07 ug g-1 de feuille malade par mois. Des r\ue9ductions significatives des metabolites primaires sont \ue0 l\u2019origne de l\u2019alt\ue9ration du m\ue9tabolisme photosynth\ue9tique et de croissance, resultant \ue0 cycle v\ue9g\ue9tatif compromis qui ne peut permettre un d\ue9veloppement optimal de la plante