Annual Root Growth Pattern of Young Citrus Trees in Relation to Shoot Growth, Soil Temperature, and Soil Water Content

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Abstract
The annual pattern of root growth of ‘Valencia’ orange [Citrus sinensis (L.) Osb.] trees on rough lemon (C. jambhiri Lush.) and Carrizo citrange [Poncirus trifoliata (L.) Raf. × C. sinensis] rootstocks was studied in relation to shoot growth, soil temperature, and water stress in Plexiglas-walled root observation chambers. The chambers were filled with a reconstituted profile of a fine sand soil and were installed below-ground at a field site in central Florida. The chambers periodically were raised above-ground to record root growth. Under nonlimiting soil water conditions, continuous root extension growth was evident from February to November. The overall seasonal trend in root growth was significantly correlated with soil temperature. The most intense root growth occurred when soil temperatures were above 27°C, and was limited at soil temperatures below 22°. Root growth was cyclic. During periods of shoot elongation, the number of growing roots and the rate of root elongation declined. Shoot growth was a major factor controlling the intensity of root growth when soil temperature and soil water content were nonlimiting. When the soil water content was decreased intentionally, root growth was checked at a soil matric potential of −0.05 MPa. After rewatering, there was a lag period of 2 days before root growth increased. Both number of growing roots and rate of root elongation were influenced by shoot growth, soil temperature, and soil water content; however, the growth response of the root system to these factors was mediated largely through the number of roots initiating growth. No rootstock differences were apparent in the pattern of root growth.

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