2018 ASHS Annual Conference

Oxidative metabolism is an integral component of plant defense against both biotic and abiotic stresses. Current work showed decline in oxidative defense in HLB-affected fibrous root orders. One-year-old ‘Valencia’ sweet orange (Citrus sinensis L. Osbeck) trees on Swingle citrumelo were budded with HLB-infected budwood to determine the HLB-induced oxidative damage in fibrous root orders. Three independent experiments (2011-2013) were conducted in a randomized complete block design with seven blocks, and one plant of each treatment per block in the greenhouse. Seven trees were used for each treatment. The means per plant was determined and subjected to analysis of variance (SAS Institute, Cary, NC) and separated using a protected least significant difference (LSD) at P < 0.05. The standard error of the mean was also calculated. Four fibrous root orders were observed in one-year-old healthy ‘Valencia’ sweet orange (Citrus sinensis L. Osbeck) trees on rootstock Swingle citrumelo. First order (1^st) roots were subtending on second order (2^nd) and second order roots on third order (3^rd) and third order on fourth order roots (4^th), which are finally spanned on thick rudimentary taproot. The thickness of roots within various root orders declined from 4^th order to 1^st order. The concentration of hydrogen peroxide (H₂O₂) was higher in HLB-affected root orders in comparison to healthy controls. Similarly, the steady state levels of H₂O₂ concentration also varied in different root orders of healthy trees. However, these levels were low in comparison to HLB-affected roots orders. The specific activity of total superoxide dismutase (SOD), Cu-Zn-SOD, peroxidase, and catalase was higher in HLB-affected fibrous root orders. However, the specific activity of ascorbate peroxidase (APOD) declined in HLB-affected fibrous root orders. APOD is an essential part of H₂O₂ catabolism machinery in plant cells. Severe declined in APOD activity limits the normal functioning of ascorbate-glutathione cycle and concomitant higher levels of H₂O₂ caused the severe decline in fibrous root orders.