Changes in the leaf NH3-NH4 + content were monitored during floral induction in Citrus. Five-year-old rooted cuttings of ‘Washington’ navel orange (Citrus sinensis) were induced to flower by subjecting the trees to 8 weeks of low temperature, 8-h day (500 μE m−2 s−1) at 15–18°C/16-h night at 10–13°C and by transferring them afterwards to 12-h day (500 μE m−2 s−1) at 24°C/12-h night at 19°C. Maximum bloom occurred 4 weeks after the transfer to the warmer temperature. The number of flowers and the NH3–NH4 + content of leaves increased significantly (p < 0.05 and P < 0.01, respectively) with the length of the low temperature treatment. Sixteen-year-old ‘Frost Lisbon’ lemon trees (Citrus limon) on ‘Troyer’ citrange rootstocks (C. sinensis × Poncirus trifoliata) under commercial production were subjected to water-deficit stress of increasing severity.

The intensity of flowering and the leaf NH3–NH4 + content increased with the severity of the stress. The foliar application of low biuret urea to trees subjected to less than 8 weeks of low temperature stress or to moderate water-deficit stress, i.e., to treatments resulting in a low degree of flowering, increased the NH3-NH4 + content of the leaves and doubled the intensity of flowering.

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Carol J. Lovatt, Yusheng Zheng & Kater D. Hake
Israel Journal of Botany