Seed dormancy in rice interrelates to the weedy characteristics shattering, awn, black hull color, and red pericarp color. A cross between the weedy strain SS18-2 and the breeding line EM93-1 was developed to investigate the genetic basis and adaptive significance of these interrelationships. These characteristics or their components differed in average degree of dominance from -0.8 to 1.5, in heritability from 0.5 to 0.96, and in their contribution to phenotypic or genotypic variation in dormancy by up to 25%. Five dormancy, four shattering, and three awn-length quantitative trait loci (QTLs) were detected in the BC1 population replicated in 2 years. Two QTLs for hull color were identified, and the SS18-2-derived and EM93-1-derived alleles increased the intensity of black, and red or yellow pigmentations, respectively. The only QTL for pericarp color co-located with the red pericarp gene Rc, with the SS18-2-derived allele increasing the intensity of black and red pigmentations. Four of the five dormancy QTLs were flanked or bracketed by one to four QTLs for the interrelated characteristics. The QTL organization pattern indicates the central role of seed dormancy in adaptive syndromes for non-domesticated plants, implies that the elimination of dormancy from cultivars could arise from the selections against multiple interrelated characteristics, and challenges the use of dormancy genes at these loci in breeding varieties for resistance to pre-harvest sprouting (PHS). However, another QTL (qSD12) provides candidate gene(s) for PHS resistance because it has a large effect in the population and it is independent of the loci for interrelated characteristics.

Electrophysiological measurements were made on root tip cells in the elongation zone of diclofop-methyl-resistant (SR4/84) and -susceptible (SRS2) biotypes of annual ryegrass (Lolium rigidum Gaud.) from Australia. The phytotoxic action of diclofop-methyl (methyl 2-[4-(2',4'-dichlorophenoxy)phenoxy]propanoate) on susceptible whole plants was completely reversed by a simultaneous application of 2,4-dichlorophenoxyacetic acid (dimethylamine salt). The phytotoxic acid metabolite, diclofop (50 micromolar), depolarized membrane potentials of both biotypes to a steady-state level within 10 to 15 minutes. Repolarization of the membrane potential occurred only in the resistant biotype following removal of diclofop. The resistant biotype has an intrinsic ability to reestablish the electrogenic membrane potential, whereas the susceptible biotype required an exogeneous source of IAA to induce partial repolarization. Both biotypes were susceptible to depolarization by carbonylcyanide-m-chlorophenylhydrazone (CCCP), and their membrane potentials recovered upon removal of CCCP. A 15-minute pretreatment with p-chloromercuribenzenesulphonic acid (PCMBS) blocked the depolarizing action of diclofop in both biotypes. However, PCMBS had no effect on the activity of CCCP. The action of diclofop appears to involve a site-specific interaction at the plasmalemma in both Lolium biotypes to cause the increased influx of protons into sensitive cells. The differential response of membrane depolarization and repolarization to diclofop treatment may be a significant initial reaction in the eventual phytotoxic action of the herbicide.DM' is a postemergence graminicide that selectively controls grasses in broadleaf crops. Unlike other postemergence graminicides, DM selectively controls wild oat (Avena fatua) and other grasses in wheat (Triticum aestivum). Biotypes of annual ryegrass (Lolium rigidum) with resistance to DM have appeared in all major cereal cropping areas of Australia (6, 7). The physiological or biochemical mechanism for resistance to DM has not been determined for these ryegrass biotypes.The 13,18,19,27,33) and (b) inhibition of ACCase, a key enzyme in acyl lipid biosynthesis located in the chloroplasts and plastids of nongreen tissues (2,10,20,21,29 The basis for resistance in annual ryegrass biotypes is not due to differential metabolism and detoxification of DM (9,26) or to altered sensitivity of ACCase to inhibition by diclofop (14). The plasma membrane of several DM-resistant and -susceptible biotypes of annual ryegrass was sensitive to membrane depolarization by diclofop (5, 9). However, only the resistant biotypes appeared to reestablish the electrogenic component of Em upon removal of the herbicide (5, 9). Therefore, the resistance of annual ryegrass biotypes to DM and other herbicide analogs may be related to the differential response of the plasmalemma to membrane depolarization (biophysical mechanism). The mechanism for the repolarization response is unknown. The objective of this research 1415 www.plantphysi...

No abstract

No abstract