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Source and sink tissue of Catharanthus roseus (periwinkle) infected with the grapevine yellows (GY), apple proliferation (AP), or ash yellows (ASHY) phytoplasma and of Nicotiana tabacum (tobacco) infected with the AP phytoplasma, were examined for concentrations of glucose, fructose, sucrose, starch and total amino acids. Phytoplasmal infection caused growth inhibition and severe yellowing in both plants, the GY phytoplasma being more virulent than the AP and ASHY agents in periwinkle. Leaf size in periwinkle was greatly reduced by the ASHY phytoplasma with little or no effect caused by either the AP or GY agents. Both phytoplasma strain and host plant had an influence on the observed results. In source leaves of periwinkle and tobacco, infection mostly led to a marked increase in soluble carbohydrates and starch. Increase was smallest in periwinkle affected by ASHY. During a dark period of 42 h, carbohydrate levels tended to decrease in healthy plants whereas they remained high in diseased plants. In sink leaves of periwinkle, infections by the GY and AP phytoplasmas led to a marked decrease in sugar whereas starch levels were either only slightly or not affected at all. Infection by the ASHY phytoplasma led to a smaller decrease of sugars and an increase in starch. In tobacco sink leaves, sugars remained largely unchanged whereas starch increased after infection with the AP phytoplasma. In periwinkle roots, soluble carbohydrates were not markedly altered but starch was considerably reduced, in the GY and AP phytoplasmas in particular. In tobacco roots, there was a general decrease in soluble carbohydrates and starch following infection by the AP phytoplasma. The amino acid concentrations were not altered by infection, except that source and sink leaves of ASHY-infected periwinkles and source leaves of AP-infected tobacco showed higher levels of amino acids than healthy plants. The results are discussed with respect to the effect of phytoplasmas on phloem transport, source leaf phenotype and sink growth. |
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