Benito S. Vergara

Intraspecific responses of 188 rice cultivars to enhanced UVB radiation

Abstract Phytotron studies were conducted to determine the intraspecific variation in sensitivity of rice ( Oryza sativa L.) to enhanced UVB and to test the hypothesis that rice cultivars originating from regions with higher ambient UVB radiation are more tolerant to enhanced UVB. Out of the 188 rice cultivars (from various rice growing regions and ecosystems) tested, 143 had significantly reduced plant height, 52 had smaller leaf area, 61 had lower plant dry weight and 41 had less tiller number under elevated UVB radiation (13.0 kJ m −2 day −1 ) for 3 weeks. Six cultivars showed significant positive growth response to enhanced UVB radiation, although the mechanism is not clear at present. These six cultivars were from the summer rice crop of Bangladesh and from high elevation rice areas where prevailing UVB radiation is most likely to be greater. However, there was no correlation between the dry matter changes under enhanced UVB and the ambient UVB level at the origin of the cultivar across the 188 cultivars tested. Therefore, cultivars originating from regions with higher ambient UVB are not necessarily more tolerant to enhanced UVB radiation.

Component analysis of duration from seeding to heading in rice by the basic vegetative phase and the photoperiod-sensitive phase

SummaryInheritance of the duration from seeding to heading in rice was analyzed by dividing the vegetative growth period into the basic vegetative phase (b.v.p.) and the photoperiod-sensitive phase (p.s.p.). Concurrent determination of the two physiologic phases on pure lines and hybrids was facilitated by testing vegetative tillers of the same plant under two photoperiods.In nine crosses of photoperiod-sensitive by insensitive arieties where the parents differed appreciably in b.v.p. estimates obtained under a 10-hour photoperiod, two to three Ef genes of relatively discrete effect controlled the F2 variation in b.v.p. Short b.v.p. was controlled by dominant genes of cumulative but unequal effect. In three other sensitive × insensitive crosses where the parents differed less in b.v.p., the F2 segregation could be ascribed to two to four pairs of metrical alleles with equal or unequal effect.Concurrent data obtained from sensitive × insensitive crosses grown under a 16-hour photoperiod indicate the action of one or two (duplicate) dominant Se genes in controlling strong sensitivity to a long photoperiod. One insensitive variety appeared to carry a recessive inhibitor, i-Se. The Se gene(s) is epistatic to the Ef genes in the expression of earliness under a short photoperiod. Pooled data also suggest an association between photoperiod sensitivity and a short b.v.p. in a large proportion of F2 plants.Field data on the two principal components of the vegetative growth period obtained under natural daylength generally indicated agreement with duplicate plantings grown under controlled photoperiods. Segregation for the optimum photoperiod and critical photoperiod under a changing photoperiod in the field plantings probably resulted in modified expression of the Se and Ef genes.

Influence of supplemental ultraviolet-B on indoleacetic acid and calmodulin in the leaves of rice (Oryza sativa L.)

IR68 and Dular rice cultivars were grown under ambient, 13.0 (simulating 20% ozone depletion) and 19.1 (simulating 40% ozone depletion) kJ m-2 day-1 of biologically effective ultraviolet-B (UV-BBE) for 4 weeks. Plant height and leaf area were significantly reduced by supplemental UV-BBE radiation. Greater reduction in leaf area than of plant height was observed. A decrease in indole-3-acetic acid (IAA) content and increase in peroxidase and IAA oxidase activities of UV-B treated plants in both cultivars were observed compared with ambient control. Calmodulin content also decreased after plants were treated with high supplemental UV-B for two weeks and medium UV-B treatment for four weeks. The results indicated that peroxidase and IAA oxidase activities in rice leaves were stimulated by supplemental UV-B, resulting in the destruction of IAA which in turn may cause inhibition of rice leaf growth. Although the mechanism is unclear, calmodulin is most likely involved in leaf growth.

Rice Plant Growth and Development

The life cycle of the rice plant is generally 100 to 210 days; the mode falls between 110 and 150 days. In temperate climates, the average duration from sowing to harvest is about 130 to 150 days. Cultivars with growth duration of 150 to 210 days are usually photoperiod sensitive and planted in the deepwater areas. Temperature and day length are the two environmental factors affecting the development of the rice plant, which can be divided into three main phases (Vergara 1970): vegetative phase—from seed germination to panicle initiation reproductive phase—from panicle initiation to anthesis ripening phase—from anthesis to full maturity

Improvement of chilling resistance in rice by application of an abscisic acid analog in combination with the growth retardant tetcyclacis

The protective effect of a synthetic terpenoid Analog of the plant hormone abscisic acid (ABA) coded LAB 173711 applied alone or in combination with the growth retardant tetcyclacis on chilling injury of rice was studied under growth chamber, greenhouse, and field conditions. The compounds were applied as a foliar spray before and after the onset of chilling treatment, as a substrate via the root system, and as a medium for seed soaking. The ABA analog increased chilling resistance in a manner similar to ABA. Combination of the analog with tetcyclacis revealed additive effects. Increased chilling resistance involved several processes: stomatal closure which reduced water loss during chilling, stabilization of the membranes, stabilization of the chlorophyll level, and stabilization of the root system. Possibilities for practical use of the compounds in rice production are discussed.

Feasibility of selection for traits associated with cold tolerance in rice under rapid generation advance method

SummaryVariability of the hybrid population can be preserved through generations during the process of rapid generation advance (RGA). Effective selection can be done in an early generation for those traits having high heritability value like growth duration and plant height. But selection is not advantageous for traits having low heritability. Thus, selection for panicle exsertion, cold tolerance at seedling stage and panicle length would not be wise during the early generations. However, in the low temperature areas growth duration and plant height are two important factors which determine varietal suitability. As selection of these two characters are possible through RGA, it should be a standard parctice during RGA to eliminate the plants with long growth duration and select plants with optimum plant height.

Induced variability for salt tolerance in rice (Oryza sativa L.) after N-methyl-N-nitrosourea treatment of fertilized egg cells

Abstract The response to salt stress of 245 M1 plant progenies derived from N-methyl-N-nitrosourea treatment of fertilized egg cells, was studied. On the average, 29% of the progenies showed variation for increased salt tolerance. Of a population of 15,000 M2 seedlings, 1.3% were scored tolerant. The behavior of salt-tolerant variants was studied in the M3 generation; two variants were categorized as tolerant and seven as intermediate in reaction. Root and shoot growth of variants showed differential inhibition under salt stress. Decrease in shoot and root growth was notably less in variants scored as tolerant than in the parent variety, Taichung 65. Mutagenesis of fertilized egg cells of rice seems to be an effective approach for inducing variability for salt tolerance.

Induced mutations for aluminum tolerance after N-methyl-N-nitrosourea treatment of fertilized egg cells in rice (Oryza sativa L.)

Abstract MNU applied to fertilized egg cells induced an increased range of variation for Al tolerance in M 2 . For the M 3 the induced variability in M 2 was heritable; 50 variants expressed different degrees of tolerance for Al toxicity, some of which did not show any growth inhibition even at 30 ppm Al. Tolerance for Al toxicity and longer root development were interrelated. Mutagenesis of fertilized egg cells appears to be useful for increasing genetic variability for Al tolerance in rice.

Genetic analysis of ratooning ability of rice (Oryza sativa L.)

SummaryGenetic analysis of generation means of F1, F2, F3, F4, and the parental populations of the cross IR10154-23-3-3A/IR15795-232-3-3-2 for ratooning ability was conducted. Good ratooning ability is a recessive trait. The parents differed by at least two pairs of major genes for ratooning ability. A large proportion of transgressive segregants in F2 suggested that the expression of ratooning ability is influenced not only by major genes but also by modifiers. Broad-sense heritability estimates computed by different methods ranged from 0.66 to 0.88. Narrow-sense heritability by the F4–F3 regression method was 0.39. The heritability estimates were 0.42 and 0.33 by the variance component method. The prevalence of additive x additive type of gene effects along with prominent additive effects imply some scope for selection in the segregating generations. However, non-additive type of gene action also affects the expression of ratooning ability.

Ultraviolet-B Irradiance Measurements under Field Conditions

Most of the studies reported on the effects of UV-B on plants were conducted using supplemental UV-B coming from UV-emitting lamps. The output of UV-B irrandance from the lamps, however, could be affected by different factors, a critical consideration in the study of UV-B effects on crops. An experiment was conducted at the IRRI field to determine UV-B irradiance at different voltages (160–220V), solar irradiance, and film photo-oxidation. Supplemental UV-B was provided by pre-aged (100h) UV-emitting fluorescent lamps that were enclosed by filters of either cellulose acetate (UV-B treatment) or mylar polyester (control treatment). UV-B irradiance was measured using an Optronic 752 Spectroradiometer.