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Genetic analysis of heat tolerance will help breeders produce rice (Oryza sativa L.) varieties adapted to future climates. An [F.sub.6] population of 181 recombinant inbred lines of Bala (tolerant) x Azucena (susceptible) was screened for heat tolerance at anthesis by measuring spikelet fertility at 30[degrees]C (control) and 38[degrees]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. doi: 10.2135/cropsci2009.09.0516

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]

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]