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Negative effects of enhanced UV-B radiation have been demonstrated in plants, but impacts under realistic field conditions remain uncertain. Adverse impacts to major crops, such as rice (Oryza sativa L.), that are grown in areas with currently high ambient levels of UV-B, could have consequences for food security. To address the response of rice to UV-B, we conducted an intensive and extensive series of field experiments from 1992 to 1995 documenting the effects of supplemental UV-B (simulating approximately 20% ozone depletion for the Philippines), using irrigated rice cultivars under tropical conditions. This multiseason study indicated that supplemental UV-B had no significant effects on rice grain yield (including the yield components spikelet filling percentage, and 1000-grain weight) or growth parameters (plant height or panicles per square meter). The absence of UV-B effects was consistent across seasonal environment (four dry and three wet seasons), cultivar, and type of exposure system. Thus, rice yields are not likely to be affected by increases in UV-B under realistic field conditions.

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.

The protective effect of a synthetic terpenoid analog of abscisic acid (ABA), coded LAB 173711, on transplant shock in rice was investigated under field conditions. Rice seedlings (Fujisaka 5 and IR 20) were taken from a seed bed and their roots were immersed in aqueous solutions of the growth regulators for 24 h. Afterwards they were drought-stressed for 3 or 6 h by exposing them to direct sunlight before they were transplanted to the field. A pretreatment with either LAB 173711 alone or in combination with the growth retardant tetcyclacis reduced the loss of water during the exposure to sunlight, increased the percentage of survival of the seedlings in the field, and increased the percentage of grain filling of the plants which had survived. The combined application of growth regulators was most effective and may be useful for minimizing transplant shock in rice and other crops.

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.

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.

Plants exhibited differential response for ratoon growth and development under different temperature regimes before and after main crop harvest. Low temperature (23°/20 °C) before cutting induced ratooning whereas similar temperature after cutting delayed and reduced ratoon tillering. Ratoon grain yield and its attributes were significantly affected under various temperature regimes. Ratoon grain yield under low temperature regime before cutting did not differ from that of the control. Ratoon growth duration was maximum under low temperature regime after cutting. High temperature (37°/27 °C) after main crop cutting resulted in high spikelet sterility in the ratoon crop.

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

Genetic 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.

Variability 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.

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.

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.

Inheritance 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.

Rice varieties differ greatly in the time required from sowing until harvest. These periode often are strongly influenced by planting date and maturation in five months or more after seeding is common in the tropics. Obviously, a short duration crop would have several advantages over a long duration crop, even with equal total grain yields. A short duration crop would require less water per crop; would be less exposed to hazards such as insects, pathogenic organisms, drouths and typhoons; and would increase the time the land would be available for subsequent plantings of rice or other crops.

High grain yields in rice have been correlated with high dry matter (DM) production after flowering (1). This high DM production is the result of the balance between the environment and plant growth before flowering.

The growth pattern of two types of dwarfness in rice was studied. The difference in height of the tall and short Peta plants was apparent even at the seedling stage. In the dwarf and in the normal Century Patna 231 the difference in height was apparent only after panicle initiation. The four lines had the same number of elongated internodes, with the taller lines having longer internodes. The application of gibberellin resulted in increased plant height, longer internodes, blades and kaf sheaths, and decreased tiller number. Century Patna23 1-dwarf showed the least, and short Peta the greatest, response to gibberellin application.

ALTHOUGH Oryza sativa L. is generally considered to be a short-day plant, a delay in flowering by short-day treatment has been reported1–7. Varieties with such a response would have a potential use in studies of physiology and breeding, and would be most interesting ecologically. The data reported on such varieties do not present a clear picture of their reaction, however, and are insufficient to warrant their classification as intermediate, much less long-day, types. To clarify this point, we have carried out detailed tests on several varieties of rice previously reported to be long-day or intermediate. ‘BPI-76’, a variety known to be sensitive to photoperiod, was included as a control.