English abstract
Acid lime (Citrus aurantifolia, Swingle) is one of the major fruit crops in Oman. Other citrus species grown in Oman, albeit at smaller scale, include orange, grapefruit, sweet lime, and either seedlings or grafted on a variety of experimental rootstocks that include acid lime, macrophylla, rangpur lime, and rough lemon. The production of acid lime, also called Omani lime, has been significantly reduced in the last three decades due to the presence of witches' broom disease of lime caused by oplasma (Candidatus Phytoplasma aurantifolia). The disease which also infects other citrus species grown in Oman has significant economic impact on local citrus growers. The aim of this study was to determine the relationship between phytoplasmal infection and the phytochemistry of the leaves of citrus species. Eight citrus species were selected for this study, based on their cultivation area and potential value as disease resistant rootstocks. These species including; acid lime (C. aurantifolia), grapefruit (C. paradisi), alemow (C. macrophylla), rough lemon (C. jambhiri), African shaddock (C. maxima [Burm.]), borneo rangpur (C.limonia), navel orange (C. sinensis) and sweet lime (C. limettioides). Phytoplasmal infection was confirmed by a polymerase chain reaction (PCR) assay. Essential oil was extracted from the different citrus species using steam distillation method. The phytochemical composition of essential oil was analyzed using gas chromatography-mass spectrometry (GC-MS). Acid lime (Citrus aurantifolia) was the only species which showed witches' broom symptoms and positively reacted in PCR tests. One tree of grapefruit (C. paradisi) reacted positively to the PCR test but showed no witches' broom symptoms. The results indicated that there was a relationship between the concentration of essential oil compounds in the leaves and the phytoplasma infection. There was an increase in the concentration of hydrocarbons and decrease in the concentration of aldehyde and ester of the symptomatic acid lime leaf sample compared to asymptomatic acid lime leaf sample. Also, the results showed several differences in chemical compositions and concentrations between the infected acid lime and the other citrus which didn't show witches' broom symptoms. These differences were in Sabinene, Citronellol acetate, citronellal, z-citral, e-citral and others. Further studies are needed to determine the role of these compounds and the relationship between them and the susceptibility of various citrus species to phytoplasmal infection and the presence of witches' Broom symptoms on infected trees.
