Aerenchyma and an inducible barrier to radial oxygen loss facilitate root aeration in upland, paddy and deep-water rice (Oryza sativa L.)

Abstract

The present study evaluated waterlogging tolerance, root porosity and radial O(2) loss (ROL) from the adventitious roots, of seven upland, three paddy, and two deep-water genotypes of rice (Oryza sativa L.). Upland types, with the exception of one genotype, were as tolerant of 30 d soil waterlogging as the paddy and deep-water types. In all but one of the 12 genotypes, the number of adventitious roots per stem increased for plants grown in waterlogged, compared with drained, soil. When grown in stagnant deoxygenated nutrient solution, genotypic variation was evident for root porosity and rates of ROL, but there was no overall difference between plants from the three cultural types. Adventitious root porosity increased from 20-26 % for plants grown in aerated solution to 29-41 % for plants grown in stagnant solution. Growth in stagnant solution also induced a 'tight' barrier to ROL in the basal regions of adventitious roots of five of the seven upland types, all three paddy types, and the two deep-water types. The enhanced porosity provided a low resistance pathway for O(2) movement to the root tip, and the barrier to ROL in basal zones would have further enhanced longitudinal O(2) diffusion towards the apex, by diminishing losses to the rhizosphere. The plasticity in root physiology, as described above, presumably contributes to the ability of rice to grow in diverse environments that differ markedly in soil waterlogging, such as drained upland soils as well as waterlogged paddy fields.

Fig. 1. Rates of radial O₂ loss (ROL) along intact adventitious roots of 12 rice genotypes when in an O₂‐free root medium with shoots in air. Plants were raised in either aerated (open symbols) or stagnant deoxygenated nutrient solution (closed symbols) prior to the measurements taken along one 100–131 mm adventitious root of each 28–38‐d‐old plant, with the treatments imposed on day 10. Measurements were taken at 25 °C and the O₂‐free root medium contained some basal electrolytes as well as 0·1 % agar to prevent convection. Data given are means of three replicates ± standard errors.

Fig. 1. Rates of radial O₂ loss (ROL) along intact adventitious roots of 12 rice genotypes when in an O₂‐free root medium with shoots in air. Plants were raised in either aerated (open symbols) or stagnant deoxygenated nutrient solution (closed symbols) prior to the measurements taken along one 100–131 mm adventitious root of each 28–38‐d‐old plant, with the treatments imposed on day 10. Measurements were taken at 25 °C and the O₂‐free root medium contained some basal electrolytes as well as 0·1 % agar to prevent convection. Data given are means of three replicates ± standard errors.