Stress enhances the gene expression and enzyme activity of phenylalanine ammonia-lyase and the endogenous content of salicylic acid to induce flowering in pharbitis

The involvement of salicylic acid (SA) in the regulation of stress-induced flowering in the short-day plant pharbitis (also called Japanese morning glory) Ipomoea nil (formerly Pharbitis nil) was studied. Pharbitis cv. Violet was induced to flower when grown in 1/100-strength mineral nutrient solution under non-inductive long-day conditions. All fully expanded true leaves were removed from seedlings, leaving only the cotyledons, and flowering was induced under poor-nutrition stress conditions. This indicates that cotyledons can play a role in the regulation of poor-nutrition stress-induced flowering. The expression of the pharbitis homolog of PHENYLALANINE AMMONIA-LYASE, the enzyme activity of phenylalanine ammonia-lyase (PAL; E.C. 4.3.1.5) and the content of SA in the cotyledons were all up-regulated by the stress treatment. The Violet was also induced to flower by low-temperature stress, DNA demethylation and short-day treatment. Low-temperature stress enhanced PAL activity, whereas non-stress factors such as DNA demethylation and short-day treatment decreased the activity. The PAL enzyme activity was also examined in another cultivar, Tendan, obtaining similar results to Violet. The exogenously applied SA did not induce flowering under non-stress conditions but did promote flowering under weak stress conditions in both cultivars. These results suggest that stress-induced flowering in pharbitis is induced, at least partly, by SA, and the synthesis of SA is promoted by PAL.     

Photoreceptors and signals in the photoperiodic control of development

Many plant species are sensitive to changes in the seasons, especially with regard to their reproductive behaviour. Sexual or vegetative reproductive structures are often only formed at times of the year when days are sufficiently long, or short. Plants perceive daylength in the leaves, but reproductive changes occur in shoot apices in response to the movement of signals throughout the plant. There is good evidence that phytochrome A is an essential component of the daylength-sensing mechanism in long-day plants, and preliminary evidence suggests a potential interaction between phytochrome C and daylength sensitivity in short-day plants. The sensitivity of reproductive processes to photoperiodic control is directly altered by photoreceptor action, particularly in the case of phytochrome B, which has a major influence on whether flowering or tuberization occurs under non-inductive conditions in both long- and short-day plants, but is not involved in daylength measurement. The signals which move from leaves to the sites of reproductive development are not known, but there is good evidence that gibberellins may be important and some preliminary indication that brassinosteroids might also be involved in photoperiodic signalling.     

Seed dormancy in alpine species

In alpine species the classification of the various mechanisms underlying seed dormancy has been rather questionable and controversial. Thus, we investigated 28 alpine species to evaluate the prevailing types of dormancy. Embryo type and water impermeability of seed coats gave an indication of the potential seed dormancy class. To ascertain the actual dormancy class and level, we performed germination experiments comparing the behavior of seeds without storage, after cold-dry storage, after cold-wet storage, and scarification. We also tested the light requirement for germination in some species. Germination behavior was characterized using the final germination percentage and the mean germination time. Considering the effects of the pretreatments, a refined classification of the prevailing dormancy types was constructed based on the results of our pretreatments. Only two out of the 28 species that we evaluated had predominantly non-dormant seeds. Physiological dormancy was prevalent in 20 species, with deep physiological dormancy being the most abundant, followed by non-deep and intermediate physiological dormancy. Seeds of four species with underdeveloped embryos were assigned to the morphophysiologial dormancy class. An impermeable seed coat was identified in two species, with no additional physiological germination block. We defined these species as having physical dormancy. Light promoted the germination of seeds without storage in all but one species with physiological dormancy. In species with physical dormancy, light responses were of minor importance. We discuss our new classification in the context of former germination studies and draw implications for the timing of germination in the field.     

中红侧沟茧蜂滞育临界光周期和敏感光照虫态的测定

室内对中红侧沟茧蜂Microplitis mediator滞育诱导的光周期反应及敏感光照虫态进行了研究。结果表明：在16℃和18℃温度下,当光周期为10L∶14D时,中红侧沟茧蜂的滞育率最高为98%和87.8%;其次是光周期为8L∶16D时,滞育率为78.7%和73%;光周期为12L∶12D时,滞育率为55.8%和50.8%;而在全光照LL, 光周期22L∶2D,20L∶4D,18L∶6D,16L∶8D,14L∶10D ,2L∶22D和全黑暗DD时的滞育率均为0。中红侧沟茧蜂属于典型的长日照发育型,在16℃时诱导滞育的临界光周期是7.03L∶16.97D和12.21L∶11.79D;在18℃时是6.75L∶17.25D和12.03L∶11.97D。16℃时中红侧沟茧蜂对光照反应最敏感时期为2龄幼虫第5～8天,2龄幼虫第1～4,9～12和13～16天及3龄期次之,卵期和1龄幼虫不敏感。     

Effect of Darkness on Growth and Flowering of Chenopodium rubrum and C. murale Plants in vitro

Chenopodium rubrum, a short-day plant, and C. murale, a long-day plant, were grown in vitro in continuous darkness. Control C. rubrum plants exposed to continuous darkness for 15 d at cotyledonary phase, did not flower, while 80 % of plants flowered on the medium with 5 % glucose and 10 mg dm⁻³ GA₃. Control C. murale plants exposed to continuous darkness for 10 d at the age of 4^th pair of leaves, did not flower, while GA₃ (1 – 5 mg dm⁻³) stimulated flowering up to 65 %. This revised version was published online in July 2006 with corrections to the Cover Date.     

Phytochrome and photoperiodic induction

The photoreceptor phytochrome has been extensively characterized at the chromophore, protein and gene level. It consists of a family of red/far-red reversible molecules and the genes for three members have been sequenced. Phytochromes are chromoproteins, which probably exist as dimers in vivo. Photoperiodism in higher plants involves the interaction of phytochrome with an endogenous timekeeping system. The interaction is complex, and several distinct actions of light can be distinguished. The possible involvement of different phytochromes in different actions of light in both long-day plants and short-day plants is discussed. Potential roles for different members of the phytochrome family and homo-and hetero-dimers of phytochrome are proposed.