A pressure chamber and a root pressure probe technique have been used to measure hydraulic conductivities of rice roots (root Lp(r) per m(2) of root surface area). Young plants of two rice (Oryza sativa L.) varieties (an upland variety, cv. Azucena and a lowland variety, cv. IR64) were grown for 31-40 d in 12 h days with 500 micromol m(-2) s(-1) PAR and day/night temperatures of 27 degrees C and 22 degrees C. Root Lp(r) was measured under conditions of steady-state and transient water flow. Different growth conditions (hydroponic and aeroponic culture) did not cause visible differences in root anatomy in either variety. Values of root Lp(r) obtained from hydraulic (hydrostatic) and osmotic water flow were of the order of 10(-8) m s(-1) MPa(-1) and were similar when using the different techniques. In comparison with other herbaceous species, rice roots tended to have a higher hydraulic resistance of the roots per unit root surface area. The data suggest that the low overall hydraulic conductivity of rice roots is caused by the existence of apoplastic barriers in the outer root parts (exodermis and sclerenchymatous (fibre) tissue) and by a strongly developed endodermis rather than by the existence of aerenchyma. According to the composite transport model of the root, the ability to adapt to higher transpirational demands from the shoot should be limited for rice because there were minimal changes in root Lp(r) depending on whether hydrostatic or osmotic forces were acting. It is concluded that this may be one of the reasons why rice suffers from water shortage in the shoot even in flooded fields.

Genetic analysis of heat tolerance will help breeders produce rice (Oryza sativa L.) varieties adapted to future climates. An F6 population of 181 recombinant inbred lines of Bala (tolerant) × Azucena (susceptible) was screened for heat tolerance at anthesis by measuring spikelet fertility at 30°C (control) and 38°C (high temperature) in experiments conducted in the Philippines and the United Kingdom. The parents varied significantly for absolute spikelet fertility under control (79–87%) and at high temperature (2.9–47.1%), and for relative spikelet fertility (high temperature/control) at high temperature (3.7–54.9%). There was no correlation between spikelet fertility in control and high‐temperature conditions and no common quantitative trait loci (QTLs) were identified. Two QTLs for spikelet fertility under control conditions were identified on chromosomes 2 and 4. Eight QTLs for spikelet fertility under high‐temperature conditions were identified on chromosomes 1, 2, 3, 8, 10, and 11. The most significant heat‐responsive QTL, contributed by Bala and explaining up to 18% of the phenotypic variation, was identified on chromosome 1 (38.35 mega base pairs on the rice physical genome map). This QTL was also found to influence plant height, explaining 36.6% of the phenotypic variation. A comparison with other studies of abiotic (drought, cold, salinity) stresses showed QTLs at similar positions on chromosomes 1, 3, 8, and 10, suggesting common underlying stress‐responsive regions of the genome.

A new pressure-perfusion technique was used to measure hydraulic and osmotic properties of the outer part of roots (OPR) of 30-day-old rice plants (lowland cultivar: IR64, and upland cultivar: Azucena). The OPR comprised rhizodermis, exodermis, sclerenchyma and one cortical cell layer. The technique involved perfusion of aerenchyma of segments from two different root zones (20-50 mm and 50-100 mm from the tip) at precise rates using aerated nutrient solution. The hydraulic conductivity of the OPR (Lp OPR =1.2·10 )6 m s )1 MPa )1 ) was larger by a factor of 30 than the overall hydraulic conductivity (Lp r =4·10 )8 m s )1 MPa )1 ) as measured by pressure chamber and root pressure probe. Low reflection coefficients were obtained for mannitol and NaCl for the OPR (r sOPR =0.14 and 0.09, respectively). The diffusional water permeability (P dOPR ) estimated from isobaric flow of heavy water was smaller by three orders of magnitude than the hydraulic conductivity (Lp OPR /P fOPR ). Although detailed root anatomy showed well-defined Casparian bands and suberin lamellae in the exodermis, the findings strongly indicate a predominantly apoplastic water flow in the OPR. The Lp OPR of heat-killed root segments increased by a factor of only 2, which is in line with the conclusion of a dominating apoplastic water flow. The hydraulic resistance of the OPR was not limiting the passage of water across the root cylinder. Estimations of the hydraulic properties of aerenchyma suggested that the endodermis was rate-limiting the water flow, although the aerenchyma may contribute to the overall resis-tance. The resistance of the aerenchyma was relatively low, because mono-layered cortical septa crossing the aerenchyma ('spokes') short-circuited the air space between the stele and the OPR. Spokes form hydraulic bridges that act like wicks. Low diffusional water permeabilities of the OPR suggest that radial oxygen losses from aerenchyma to medium are also low. It is concluded that in rice roots, water uptake and oxygen retention are optimized in such a way that hydraulic water flow can be kept high in the presence of a low efflux of oxygen which is diffusional in nature.Abbreviations A r : surface area of root AE C i : osmotic concentration of xylem sap AE C o : osmotic concentration of medium AE d: unstirred layer AE Dp: osmotic pressure difference AE DP r : change of root pressure AE DV s : change of volume AE D s : diffusion coefficient of solutes AE J Vr : water flow AE k rw : rate constant for water AE k sr : rate constant for solutes AE Lp: hydraulic conductivity AE Lp AER : hydraulic conductivity of aerenchyma AE Lp OPR : hydraulic conductivity of outer part of the root AE Lp r : hydraulic conductivity of the root AE Lp SPK : hydraulic conductivity of spokes AE OPR: outer part of roots AE P: steady-state pressure AE P d : diffusional water permeability AE P dOPR : diffusional water permeability of outer part of the root AE P fOPR : hydraulic water permeability of outer part of the root AE P gas : pneumatic pressure AE P ro : origin...

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