Root system structure determines the volume of soil accessible to crop plants as well as the pathway for water and solute uptake (Kramer and Boyer, 1995). Increasing the length of small-diameter fibrous roots within a crop root system increases the amount of water and nutrients available to the crop (Tinker and Nye, 2000). The goal of fertilizer application is to place nutrients within the crop root zone to ensure the most efficient nutrient uptake. Maintaining sufficient water and nutrient concentrations within soil occupied by the crop root system is essential for optimal nutrient uptake (Scholberg et al., 2002). Therefore, understanding the spatial distribution of fibrous roots is essential to ensure proper fertilizer placement, improve nutrient uptake efficiency, and reduce leaching below the root zone.
Several Florida studies showed that tree size and yield were related to fibrous root dry weight density or distribution in the deep sandy soils of central Florida (Castle and Krezdorn, 1975; Ford, 1954, 1964, 1968, 1969, 1972). Castle and Krezdorn (1975) described two general types of citrus root systems, one characterized by extensive lateral and vertical development and the other by intensive higher fibrous root density near the soil surface. Trees on rough lemon (Citrus jambhiri Lush), Volkamer lemon (C. volkameriana Pasquale), and Palestine sweet lime (C. limettioides Tan.) rootstocks are typical of citrus trees with extensive root structure in which 50% of the fibrous roots were at soil depths greater than 0.7 m. Large, high-yielding trees with extensive root systems dominated the citrus industry in Florida when trees were irrigated less intensively and planted at much lower densities. Unfortunately, rough lemon has been virtually eliminated as a commercial rootstock as a result of citrus blight disease (unknown etiology) in the 1970s and 1980s (Castle, 1980). Carrizo citrange and Swingle citrumelo are examples of the intensive-type root systems with few fibrous roots below 0.7 m and less lateral development (Castle and Krezdorn, 1975). These rootstocks now dominate the Florida citrus industry and are well suited for high-density, intensively managed plantings (Castle, 1978).
Little root density distribution data exist for citrus relative to increase in tree size. Fibrous root dry weight density ranged between 300 and 1200 g·m−3 depending on rootstock, distance from the tree, and soil depth (Castle, 1978, 1980). Maximum fibrous root length density (FRLD) ranged from 0.53 cm·cm−3 for Swingle citrumelo to 2.02 cm·cm−3 for trifoliate orange (P. trifoliata) (Eissenstat, 1991). Fibrous root dry weight density in the 0- to 0.3-m soil depth increased from 450 to 1000 g·m−3 between trees when the in-row distance decreased from 4.5 to 2.5 m (Elezaby, 1989). He attributed the root density increase to overlapping root systems. Eissenstat et al. (1999) found that water and nutrient uptake of citrus to be a function of FRLD and soil water content. Root water and solutes uptake modeling is commonly based on incorporating spatial root distribution and root length or root length density (Chandra and Rai, 1996; Hayhoe, 1981; Mmolawa and Or, 2000).
Fibrous root weight distribution in the previously mentioned references related to tree age, but relationships between fibrous root dry weight or FRLD to tree size have not been described. Many factors such as rootstock, soil type, irrigation practices, and nutritional status can contribute to trees of the same age differing in tree size. Thus, tree age is probably not a reliable index on which to base root distribution relationships. Therefore, the following hypotheses were tested: 1) root distribution of similar sized trees is significantly affected by rootstock, 2) distributions of FRLD are similar for trees of similar size grown on the same soil type and rootstock, and 3) generic relationships can be developed for well-drained soils that describe citrus root densities at various depths from the soil surface and distances from the tree as a function of tree size. To test these hypotheses, we determined spatial FRLD distribution with respect to lateral distance away from the tree trunk and soil depth for mature ‘Hamlin’ orange trees grown on two commercial citrus rootstocks. We also developed relationships that define FRLD of fibrous roots at various soil positions within the root zone as a function of tree size. A relationship of vertical and horizontal FRLD distribution to tree size could then be used to estimate spatial FRLD distribution for citrus water and nutrient uptake models.
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Castle, W.S. 1980 Fibrous root distribution of ‘Pineapple’ orange trees on rough lemon rootstock at three tree spacings J. Amer. Soc. Hort. Sci. 105 478 480
Castle, W.S. & Krezdorn, A.H. 1975 Effect of citrus rootstocks on root distribution and leaf mineral content of ‘Orlando’ tangelo tree J. Amer. Soc. Hort. Sci. 100 1 4
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