Abstract: Seasonal changes in Mg and B contents in leaves of two navel orange varieties，Newhall and Skaggs Bonanza（Citrus sinensis Osbeck）on trifoliate orange [Poncirus trifoliata（L.）] rootstocks，growing on red soil in the middle subtropical monsoon climate zone, were monitored to study the effects of such changes on chlorophyll contents and leaf yellowing. Mg content decreased as leaves grew older and the most pronounced decrease occurred in June when the fruit began expanding. The decrease in Mg paralleled with the progress of leaf yellowing. In contrast, leaf B content increased or stabilized as leaf age increased. A significant positive correlation（P < 0.01）was found between Mg contents and chlorophyll contents as showed by high correlation coefficients of 0.9082 and 0.9063 in leaves of Newhall and Skaggs Bonanza trees，respectively. No significant correlation was shown between B contents and chlorophyll contents in the leaves of the two cultivars. Therefore，Mg deficiency was the reason for yellowing of leaves on navel orange trees.

Key words: navel orange, leaf, yellowing, magnesium, boron, seasonal change