The effects of soil moisture and N placement on root growth characteristics of seedlings of two citrus rootstocks were studied. Root systems of four-month-old Swingle citrumelo (Citrus paradisi Macf. × Poncirus trifoliata (L.) Raf.) and Volkamer lemon (C. volkameriana Ten. & Pasq.) seedlings were cut into two equal parts and the split-root systems were grown for approximately 10 months in two adjacent, square citripots filled with a Candler fine sand. The soil in the right compartment was kept moist ( 8 cbar), whereas the soil in the left compartment was kept either wet ( 4 cbar), moist ( 8 cbar) or dry ( 16 cbar). Soil moisture sensors (Watermarks) were used for irrigation control. Nitrogen solution was applied to either a wet, moist or dry compartment, depending on the treatment. For each irrigation treatment, two plants did not receive any fertilizer N (controls). Growth allocation to roots was highest under N-limiting conditions and for the drier soil treatments. Fine roots accounted for 51 and 40% of the root mass and 97 and 98% of the total root length for Swingle and Volk, respectively. Corresponding values for lateral roots were 20 and 24%, and 3 and 2%, for Swingle and Volk respectively. Water and N supply had little effect on root size distribution. Roots of Swingle were thinner than those of Volk and, as the soil became drier, roots became thicker. This may be related to increased lignification of the roots. Application of N resulted in an increase of fine root weight and root length. Maximum root length and fine root weight for Swingle occurred at a N rate of 10 mg. Fine root weight for Volk, on the other hand, continued to increase which was related to a thickening of fine roots at higher N rates. Although root length decreased at lower soil moisture levels, application of N to dry compartments partially overcame this decrease in root length. Citrus seedlings can modify the partitioning of growth to specific parts of the root system in order to maximize the uptake of both water and N. However, an even distribution of N to a moist, but not excessively irrigated soil may promote both root and seedling growth.


citrus rootstocks; soil moisture; nitrogen placement; split-root systems; specific root length; root distribution; shoot-root ratio

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Proc. Fla. State Hort. Soc.     ISSN 0886-7283