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Fluorescence and confocal laser scanning microscopy were explored to investigate the movement and localization of mineral oils in citrus. In a laboratory experiment, fluorescence microscopy observation indicated that when a 'narrow' distillation fraction of an nC23 horticultural mineral oil was applied to adaxial and opposing abaxial leaf surfaces of potted orange [Citrus x aurantium L. (Sapindales: Rutaceae)] trees, oil penetrated steadily into treated leaves and, subsequently, moved to untreated petioles of the leaves and adjacent untreated stems. In another experiment, confocal laser scanning microscopy was used to visualize the penetration into, and the subsequent cellular distribution of, an nC24 agricultural mineral oil in C. trifoliata L. seedlings. Oil droplets penetrated or diffused into plants via both stomata and the cuticle of leaves and stems, and then moved within intercellular spaces and into various cells including phloem and xylem. Oil accumulated in droplets in intercellular spaces and within cells near the cell membrane. Oil entered cells without visibly damaging membranes or causing cell death. In a field experiment with mature orange trees, droplets of an nC23 horticultural mineral oil were observed, by fluorescence microscopy, in phloem sieve elements in spring flush growth produced 4-5 months and 16-17 months after the trees were sprayed with oil. These results suggest that movement of mineral oil in plants is both apoplastic via intercellular spaces and symplastic via plasmodesmata. The putative pattern of the translocation of mineral oil in plants and its relevance to oil-induced chronic phytotoxicity are discussed.

The efficacy of pest control of three different oil formulations in multiple low-concentration spray programmes on sweet orange and pummelo were compared with unsprayed and normal farmer-treatments over three years. Phytotoxicity of sprays was assessed in terms of fruit and leafdrop, fruityield and external fruitquality. Trees sprayed with any type of oil had lower pest numbers than unsprayed trees. On sweet orange, the heaviest oil was better than the normal farm practice at controlling chaff scale on fruit and red mite and whitefly on leaves. On pummelo it provided the best control of red mite on leaves. All three oils were as effective as the normal farm practice in control of rust mite on sweet orange fruit and leaves and red mite, rust mite and red scale on pummelo fruit. On sweet orange trees there was no evidence of phytotoxicity, and the external quality of fruit generally improved over time. On pummelo trees, oil sprays were unable to improve the external fruitquality. The number of pummelo per tree was reduced in three of the oil spray schedules in 1995 and one in 1996, but the total weight of fruit harvested per tree was unaffected. [ABSTRACT FROM AUTHOR]