Objectives The study investigated the symptoms of micronutrients deficiency of navel orange Citrus sinensis (L.) Osb. CV. Newhall in early and later stages. The diurnal change of photosynthesis of leaves was also determined in the early stage of micronutrients deficiency, which would help understanding the symptom of deficiency.

Methods A sand culture pot experiment was conducted using the rootstock of trifoliate orange Poncirus trifoliata (L.) Raf. as the tested material. For the complete nutrition control, the nutrition solution was composed of 1/2 Hoagland nutrition solution and whole Arnon solution. For the treatments of –Fe, –Mn, –B, –Zn, and –Cu, the components of Fe-EDTA, MnCl₂, H₃BO₃, ZnSO₄ and CuSO₄ were removed off from the Hoagland nutrition solution, respectively. Starting from the emergence of sprout of new branches, the navel orange seedlings were continuously treated for 6 months in nutrient deficient solution, and the early deficiency symptoms were observed. After another 4 months' treatment (a total of 10 months of growth period), the late deficient symposiums were observed. The chlorophyll contents and diurnal change of photosynthetic characteristic were measured during the observation of deficiency.

Results At the 6th month, the new leaves under Fe deficiency treatment were yellowish green with green reticulated veins; at 10th month, the old leaves were pale yellow and veins were also white. The new leaves suffering from Mn deficiency had irregular light colored band, but the old leaves were yellow or gray white, with opaque brown spots in interveinals. For B deficiency, the veins of old leaves slightly protruded, and the new leaves were significantly thickened and hardened and the new buds were clustered; at the 10th month, corky split veins and slight yellowing were observed in the old leaves. For –Zn treatment, the new leaves were mottled at the 6 months, and became narrower, smaller and even malformed at the 10 month. At the 6 month of –Cu treatment, the new leaves were uneven, and at the 10th month, the new leaves were bent into shape of bow and the branches were slender and twisted. The total chlorophyll contents in new leaves under –Fe, –Mn and –Zn treatments were all decreased significantly, and the decline was in the order of –Fe > –Mn > –Zn, with decrement of 74.7%, 31.9% and 14.8%, respectively. The diurnal change of net photosynthetic rate (P_n), transpiration rate (T_r), intercellular CO₂ concentration (C_i) and stomatal conductance (G_s) were significantly affected in the new leaves under –Fe and –Zn treatments and in the old leaves of –B treatment.

Conclusions The symptoms of microelements (Fe, Mn, B, Zn and Cu) deficiency of ‘Newhall’ navel orange seedlings leaves were different in the early and late stages; the Fe, Mn and Zn deficiency changed the diurnal change of photosynthesis of new leaves and old leaves.