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Precipitates of insoluble inorganic salts were used to clog apoplastic pores in cell walls of the outer part of rice roots (OPR) in two rice cultivars (lowland cv. IR64 and upland cv. Azucena). Aerenchyma of two different root zones (20–50 and 50–100 mm from the apex) was perfused with 1 mmpotassium ferrocyanide (K4[Fe(CN)6]) while the whole root segments were bathed in 0.5 mmcopper sulphate (CuSO4) medium. In another experiment, salts were applied on opposite sides of the OPR. The copper-ferrocyanide precipitation technique resembles the famous osmotic experiments of the German botanist Wilhelm Pfeffer, in which he used them with clay diaphragms. Precipitates were observed on the side where ferrocyanide was applied, suggesting that Cu2+ and SO42– were passing the barrier including the Casparian bands of the exodermis much faster than ferrocyanide. There was a patchiness in the formation of precipitates, correlated with the maturation of the exodermis. The intensity of copper ferrocyanide staining decreased along developing rice roots. No precipitates were observed in mature parts beyond 70–80 mm from the root apex, except for sites around the emergence of secondary roots, which were fairly leaky to both water and ions. Blockage of the apoplastic pores with precipitates caused a three- to four-fold reduction of hydraulic conductivity of the OPR (LpOPR). The reflection coefficient of the OPR (σsOPR) increased in response to the blockage with precipitates. The osmotic versus diffusive water permeability ratios of the OPR (PfOPR/ PdOPR) were around 600 for immature and 1200 for mature root segments. Treatment significantly affected the bulk rather than the diffusive water flow and caused a three- to five-fold reduction of thePfOPR/ PdOPR ratios. Results indicated that despite the existence of an exodermis with Casparian bands, most of the water moved around cells rather than using the cell-to-cell passage. [ABSTRACT FROM AUTHOR]

Wild species of Oryza may serve as sources of superior drought tolerance alleles for cultivated rice. In a series of three screenhouse experiments, we compared traits associated with leaf water status, stomatal conductance, membrane stability, and root development in several wild Oryza accessions and O. sativa cultivars, when grown under well-watered or water-deficient conditions. One accession of O. longistaminata had greater stomatal conductance under stress and maintained leaf elongation better under stress than most other genotypes. Several upland-rice cultivars from the japonica subspecies also maintained greater conductance and leaf expansion than an indica cultivar or O. rufipogon accessions. Leaves of an accession of O. latifolia were particularly sensitive to water deficit. Oryza longistaminata and several O. rufipogon accessions had greater membrane stability, measured as electrolyte leakage from leaf disks after heat treatment, than cultivated rice. Drought treatments tended to reduce the levels of electrolyte leakage observed. Wild species had levels of osmotic adjustment under stress that were similar to O. sativa. Entries differed in the growth of roots with water deficit: O. longistaminata accessions and some japonica cultivars showed either an increase in total root mass or an increase in the proportion of root mass in deeper soil levels when water deficit was imposed, whereas indica cultivars and O. rufipogon accessions did not. Changes in root mass and distribution were correlated with values of stomatal conductance and leaf elongation; this suggests that leaf measurements made at key times in the drying cycle may reveal possible genetic differences in rooting behavior, which are otherwise difficult to measure. These studies indicate that O. longistaminata and O. rufipogon may serve as sources of novel alleles for improved leaf traits under drought stress–specifically, for maintenance of leaf elongation, stomatal conductance, and membrane stability. Alleles for improved root growth and distribution under water deficit exist in some japonica rice cultivars. [ABSTRACT FROM AUTHOR]

Osmotic adjustment is one of several characters putatively associated with drought tolerance in rice. Indica cultivars are known to have a greater capacity for osmotic adjustment than japonica cultivars. We developed an advanced back-cross population using an indica donor, IR62266-42-6-2, to introgress osmotic adjustment into an elite japonica cultivar, IR60080-46A. One hundred and fifty BC3F3 families were genotyped using microsatellites and RFLP markers, and a few candidate genes. We evaluated osmotic adjustment in these lines under greenhouse conditions using the re-hydration technique. Using the composite interval mapping technique, we detected 14 QTLs located on chromosomes 1, 2, 3, 4, 5, 7, 8 and 10 that together explained 58% of the phenotypic variability. Most, but not all, of the alleles with positive effects came from the donor parent. On chromosome 8, two QTLs were associated in repulsion. The QTL locations were in good agreement with previous studies on this trait on rice and in other cereals. Some BC3F3 lines carried the favorable alleles at the two markers flanking up to four QTLs. Intercrossing these lines followed by marker-aided selection in their progenies will be necessary to recover lines with levels of osmotic adjustment equal to the donor parent. The advanced back-cross strategy appeared to be an appropriate method to accelerate the process of introgressing interesting traits into elite material. [ABSTRACT FROM AUTHOR]

Data from field trials using cultivars, a mapping population, and a set of BC3 introgression lines were used to examine the relationship between plant water status (measured as leaf relative water content, RWC) and yield and spikelet sterility (%SS) for aerobically grown rice exposed to water deficit during the reproductive stage. Control plots were irrigated several times each week to maintain soil moisture near field capacity, while stress plots experienced periods of soil drying. In an experiment comparing 35 lines and cultivars, control grain yield in the dry season was correlated with RWC measured during a period of water exclusion (r=0.72,P<0.01). This correlation was strongly influenced by genetic differences in yield potential and maturity. In a mapping population comprising approximately 80 lines, leaf RWC in the irrigated control was significantly correlated with %SS (r=−0.43,P<0.01) and yield (r=0.45,P<0.01). No correlation was observed between leaf RWC at the end of a 14-day stress period and %SS or yield in stress plots. In a set of BC3 introgression lines, RWC was not associated with yield or %SS in either stress or control treatments. Leaf water status appears to be related to yield and %SS in some cases, but the relatively weak correlations indicate that other characteristics of the lines are also important in determining the integrated response to reproductive stage drought stress. When the range of contrasting characteristics was reduced through the use of introgression lines, the association between yield and RWC disappeared, even though lines differed significantly in their yield response to water deficit. [Copyright &y& Elsevier]