Factors affecting rind oil content of lemon [Citrus limon (L.) Burm. f.]
Among the controllable and non-controllable factors affecting rind oil content of lemons, genetic (scion, rootstock) and environmental (growing conditions, canopy microclimate) factors play the most important role. 'Limoneira 8A', followed by 'Cicily', 'Lisbon' and 'Genoa' had the highest rind oil content, whereas 'Villafranca', 'Messina' and 'Yen Ben Lisbon' had the lowest. Rind oil content of 'Eureka' lemon was disappointingly low. Seedless cultivars, 'Eureka SL' and 'Lisbon SL', had ∼18.0% higher rind oil content than the seeded cultivars from which they were derived. Fruit from lemon trees budded on non-invigorating rootstocks, e.g. X639 (a Cleopatra mandarin × Trifoliate orange hybrid), had the highest rind oil content, whereas rind oil content was low on invigorating rootstocks, e.g. rough lemon. South Africa has a diverse climate, and rind oil content from fruit produced in a hot, arid growing region (Upington) had the highest rind oil content, whereas fruit from relatively warm regions (Malelane and Marble Hall) ranked second, and rind oil content of fruit from a cooler region (Karino) was intermediate. Rind oil content from a cold growing region (Vaalharts) was the lowest. Regression analysis between rind oil content and cumulative heat units revealed a positive linear relationship, and in general, rind oil content increased with increasing heat unit accumulation. Following the sampling of fruit from different positions in a tree's canopy, light exposure was found to affect rind oil content of 'Eureka' lemon fruit. Fruit borne on the outside of trees, higher in the tree, north-facing or not within the hedgerow had the highest rind oil content. PAR data supports the hypothesis that rind oil content is correlated with light exposure. Therefore, the choice of scion cultivar and rootstock in a given growing region, together with judicious pruning to optimise light penetration into a tree's canopy, contribute to enhanced rind oil yields.