Linking Hydraulic Conductivity to Anatomy in Plants that Vary in Specific Root Length

In Citrus L. sp., specific root length of whole root systems has been correlated positively with root hydraulic conductivity, but there is little mechanistic understanding of the causes for this association. The hydraulic conductivity of individual roots in relation to root anatomical characteristics in seedlings of three citrus rootstocks [sour orange (SO) (Citrus aurantium L.), trifoliate orange (TO) (Poncirus trifoliate (L.) Raf.), and Swingle citrumelo (SC) (C. paradisi Macf. x P. trifoliata)] that vary widely in specific root length (SRL) was measured. Among fibrous roots, first-order and secondorder laterals were examined. Relative differences among rootstocks in the overall hydraulic conductivity (LP) and radial conductivity (LR) for individual 1-month-old and 6-month-old secondand first-order roots generally were consistent with hydraulic conductivity determined previously for entire root systems. There were no significant differences in axial conductance per unit pressure (Kh) in either firstor second-order roots among the rootstocks. This was consistent with the similarity in number and diameter of xylem vessels. One-month-old second-order roots had no suberized exodermis but varied in cortical radius. Six-month-old second-order roots of TO, however, had more nonsuberized cells (passage cells) in the exodermis than roots of SC and SO, although the cortical radius of SC and SO roots were not different. Compared to 6-month-old second-order roots, 1-month-old second-order roots had much higher LP and LR but lower Kh. Differences in overall root hydraulic conductivity among the citrus rootstocks were mainly related to structural differences in the radial pathway for water movement, suggesting that radial hydraulic conductivity was the primary determining factor of water uptake in citrus rootstocks. hydraulic conductivity of roots with large SRL may be attributed to a greater radial hydraulic conductivity of individual roots with smaller diameters. Roots of small diameter may have a shorter radial path for water movement from the root surface into the xylem than large-diameter roots. Variation in root hydraulic conductivity among different citrus rootstocks could also be linked to specific root anatomical features such as suberization and thickness of the secondary wall of the exodermis and endodermis (Huang and Nobel, 1994). Rieger and Litvin (1999) have measured root-system hydraulic conductivity of whole, intact plants, and root anatomical features of two woody and three herbaceous species. They found that root-system hydraulic conductivity is inversely related to root cortical thickness and the presence of a suberized exodermis. The objectives of this study were to 1) determine overall hydraulic conductivity, radial and axial hydraulic conductivity of different types and ages of fibrous roots for three citrus rootstocks [sour orange (SO), trifoliate orange (TO) (Poncirus trifoliate (L.) Raf.), and Swingle citrumelo (SC) (C. paradisi Macf. x P. trifoliata)] that vary in SRL and root anatomical features; and 2) determine anatomical features that are correlated with radial and axial hydraulic conductivity of individual roots in the three rootstocks. The rootstocks, SO and SC, have similar SRLs and seedling root-system hydraulic conductivity, whereas TO has a much higher SRL and root system hydraulic conductivity (Eissenstat 1997). Materials and Methods PLANT MATERIALS AND GROWTH CONDITIONS. Rootstocks were grown in sand in 2-L pots for 6 months after seedling emergence in a growth chamber with average day/night temperatures of 30/ 25 C, a photosynthetic photon flux (PPF) of 600 μmol·m·s at Received for publication 25 May 1999. Accepted for publication 29 Nov. 1999. Kansas Agricultural Experiment Station contribution 99-279-J. We thank J.P. Syvertsen and Dian Achor for helpful reviews of this manuscript. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. Corresponding author. Water flow from the soil to the root xylem is determined by hydraulic conductivities of three components of the root–soil pathway: the soil, the root–soil interface, and the root (reviewed by Passioura, 1988). Under soil drying conditions, the soil rather than the root is a major limiting factor for water uptake (Nobel and Cui, 1992). Rieger and Duemmel (1992) reported that hydraulic conductivities of entire root systems were similar in drying soil in several Prunus L. sp., even though they differed in drought resistance. When soil moisture is readily available, however, root hydraulic conductivity can represent two-thirds of the limitations on water movement within the soil–plant system (Nobel and Cui, 1992; Passioura, 1988). Therefore, differences in root hydraulic conductivity in wet soils could result in differences in water transport to shoots that could influence leaf water status, and in turn, plant growth and physiological responses (Passioura, 1988). Hydraulic conductivity of whole root systems has been reported to be correlated positively with shoot growth rate in several citrus rootstocks (Syvertsen, 1981). Relatively vigorous rootstocks [e.g., rough lemon (Citrus jambhiri) and Carrizo citrange] tend to have higher conductivities, stomatal conductance, and transpiration rates than the less vigorous rootstocks [Cleopatra mandarin (Citrus reshni Hort. ex Tan) and sour orange (C. aurantiacum)] (Syvertsen and Graham, 1985). Root hydraulic conductivity for entire root systems also is higher in citrus rootstock species that have higher specific root lengths (SRLs) (Eissenstat, 1992, 1997; Graham and Syvertsen, 1985). High