Water Uptake Rates of Citrus Root Orders.

Physiological understanding of the function of root orders is limited. In a study on four-year old Citrus volkameriana trees, a method to monitor the water flux among root orders was developed using miniaturized chambers. Different root orders of were analyzed with respect to frequency, root morphology and water flux rates. The eight root orders examined showed an overlap in diameter but differences in tissue densities and specific root area were clearly distinguishable. Although first order branch roots biomass was about 30% of the total root system, 50% of surface area (SA) were possessed by this group while the fifth root order accounted for 5% of SA (20% biomass). A significantly higher rate of water uptake was measured by the first order roots followed by the second and third-root orders. In contrast, the fourth-/fifth-root orders showed water excess. The water excess suggested the occurrence of hydraulic redistribution as a result of differences in osmotic potentials. We suggest that plants may utilize hydraulic redistribution to prevent coarse root desiccation and/or to increase nutrient acquisition. Our study identified the root order as determinant of water flux and showed that the novel ‘miniature depletion chamber’ method enabled direct measurement of water fluxes per root order. The method is suggested to be a major tool for future studies on root order traits.