Gibberellins and the photoperiodic control of leaf growth in Poa pratensis

The role of gibberellins (GAs) in photoperiodic control of leaf elongation in Poa pratensis was studied by both application of exogenous GAs and analysis of endogenous GAs. Leaf elongation was strongly increased under long day (LD, 24 h) conditions at both 9 and 21°C, leaf length at 9°C LD being similar to that in plants grown in short days (SD, 8 h) at 21°C. However, even at 21°C leaf elongation was enhanced by LD. Exogenous GA₁ could completely compensate for LD at both 9 and 21°C. Gibberellins A₂₀, A₁₉ and A₄₄ could also partly replace LD, but they were significantly less active than GA₁, GA₅₃ was inactive when applied to plants grown at 9°C in SD and exhibited only marginal activity at 9°C LD and 21°C SD. The total level of GAs of the early 13-hydroxylation pathway (A₅₃, A₄₄, A₁₉, A₂₀ and A₁) increased rapidly when plants were transferred from SD to LD at 9°C. After transfer from 9 to 21°C, there was an increase in GA levels at both LD and SD, followed by a decrease under LD conditions. In all cases, GA₁₉ was the predominant GA, accounting for 60 to 80% of the analysed GAs. Levels of the bioactive GA₁ were low and increased transiently by LD four days after transfer from SD to LD. At both temperatures, the ratio GA₁₉ to GA₂₀ and GA₂₀ to GA₁ at 9°C was enhanced by LD compared with SD. Taken together, these results support the hypothesis that photoperiodic regulation of leaf elongation in Poa pratensis is GA-mediated, and they indicate a photoperiodic control of oxidation of GA₅₃ to GA₄₄ and GA₁₉ to GA₂₀, and perhaps also of 3β-hydroxylation of GA₂₀ to GA₁.     

Frost tolerance and biochemical changes during hardening and dehardening in contrasting white clover populations

Successful winter survival of perennial plants, like white clover, is dependent on proper timing of both hardening and dehardening. The purpose of this study was to investigate the regulation of these processes in two cultivars (AberCrest and AberHerald) and two Norwegian ecotypes (Særheim collected at 58°46′N lat. and Bodø at 67°20′N lat.) of white clover (Trifolium repens L.). For hardening and dehardening, plants were exposed to controlled temperature conditions and frost hardiness of stolons was tested by programmed freezing at the rate of 3°C per hour. In addition, stolons were analysed for starch, soluble sugars and soluble amino acids. Cultivars AberCrest and AberHerald, selected for growth at low temperature and winter hardiness in the United Kingdom, were significantly less hardy than the Norwegian populations. After six weeks of hardening (2 weeks at 6°C and 4 weeks at 0.5°C), estimated LT₅₀ values were ?13.8, ?13.0, ?17.8 and ?20.3°C for AberCrest, AberHerald, Saerheim and Bodø, respectively. The rate of dehardening increased with increasing temperature. At low temperature (6°C), the northern ecotype from Bodø was more resistant to dehardening than AberHerald. However, at 18°C the absolute rate of dehardening (°C day^?1) was twice as high in Bodø as in AberHerald plants. Stolon elongation during dehardening was initiated at lower temperatures in AberHerald than in plants of the Bodø ecotype. The content of total soluble sugars, sucrose and the amino acids proline and arginine were significantly higher in hardy plants of Bodø than in those of AberHerald. Sucrose levels decreased during dehardening and correlations between sucrose content and LT₅₀ during this process were statistically highly significant for both Bodø and AberHerald. The least hardy populations of white clover were characterized by thick stolons, long internodes and large leaves.     

Interaction of gibberellic acid and photoperiod on leaf sheath elongation in normal and gibberellin-insensitive genotype of wheat

Effect of gibberellic acid (GA₃) on leaf sheath elongation in a normal (cv. Møystad) and a gibberellin(GA)-insensitive (cv. Siete Cerros) genotype of wheat ( Triticum aestivum L.) were studied at 18 and 12°C under short (SD, 12 h) or long (LD, 24 h) photoperiod. Leaf sheath length in cv. Møystad was signficantly increased by exogenous GA₃ both under SD and LD. LD alone stimulated leaf sheath elongation and the combined effect of LD and GA₃ was additive, and there was no statistically signficant interaction between photoperiod and GA₃ concentrations. Leaf sheath length in cv. Siete Cerros was not significantly affected by GA₃ under any conditions. However, there was a highly significant stimulation of leaf sheath elongation by LD in cv. Siete Cerros as well. These results indicate that stimulation of elongation growth in wheat leaves by LD is not mediated by gibberellin.     

Symbiotic growth of indigenons white clover (Trifolium repens) with local Rhizobium leguminosarum biovar trifolii

To study a possible adaptation of the symbiosis between white clover (Trifolium repens L.) and Rhizobium leguminosarum biovar trifolii with regard to light and temperature at northern latitudes, local seed populations of white clover and isolates of R. leguminosarum biovar trifolii from 3 different latitudes in Norway, 58°48'N, 67°20'N and 69°22'N, were used. The commercial cultivar Undrom was used as a reference plant. The experiments were done at 18 and 9°C under controlled conditions in a phytotron during the natural growing season at 69° 39'N. Growth of the plants was evaluated by number and size of leaves, dry matter production and total N-content. At 18°C the white clover plants were harvested twice while at 9°C there was only one growth period. The results from first harvest at 18°C and total growth at 9°C, showed that white clover populations from northern Norway had a lower growth potential than the population from the south and cv. Undrom. This difference was not apparent in the second growth period at 18°C. Growth of the plants from seeds to first harvest was enhanced by mineral nitrogen compared to plants dependent on Rhizobium only. However, after a second growth period dry weight and total nitrogen content of the plants with nitrogen fixation were comparable to the plants receiving mineral nitrogen. Statistical analysis showed that the most important factor for the variation in dry matter production was the plant population. Within the populations at 9°C and at first harvest at 18°C, there were no significant differences in dry matter production with different Rhizobium inoculum. In the second growth period at 18°C, different inoculum gave significantly different amount of dry matter within a population. The results showed a significant interaction between plant population and Rhizobium inoculum, and the results indicated that plants from the north gave higher yield when nodulated by Rhizobium from the north than from the south.     

Interaction of growth retardants,daylength, and gibberellins A19, A20, and A1 on shoot elongation in birch and alder

Gibberellins A₁₉, A₂₀, and A₁ were applied to seedlings of birch (Betula pubescens Ehrh.) and alder (Alnus glutinosa (L.) Gaertn.) in order to test their ability to counteract growth inhibition induced by growth retardants (ancymidol and BX-112) or short day (SD, 12 h) photoperiod. Ancymidol inhibits early and BX-112 inhibits late steps in gibberellin biosynthesis. BX-112 inhibited stem elongation in both species while ancymidol, applied as a soil drench, was effective in alder only. Growth retardants affected stem elongation mainly by inhibiting elongation of internodes. All three gibberellins were equally active when applied to seedlings treated with ancymidol; however, only GA₁ was able to counteract the growth inhibition induced by BX-112. SD-induced cessation of elongation growth in birch was counteracted by GA₁, and to some degree, by GA₂₀, while GA₁₉ was inactive. SD treatment did not induce cessation of apical growth in alder. These results are consistent with the hypothesis that of gibberellins belonging to the early C-13 hydroxylation pathway, GA₁ is the only active gibberellin for stem elongation.     

Ectopic expression of oat phytochrome A in hybrid aspen changes critical daylength for growth and prevents cold acclimatization

Survival of temperate-zone tree species under the normal summer-winter cycle is dependent on proper timing of apical growth cessation and cold acclimatization. This timing is primarily based on the perception of daylength, and through evolution many tree species have developed photoperiodic ecotypes which are closely adapted to the local light conditions. The longest photoperiod inducing growth cessation, the critical photoperiod, is inherited as a quantitative character. The phytochrome pigment family is the probable receptor of daylength, but the exact role of phytochrome and the physiological basis for the different responses between photoperiodic ecotypes are not known. This report shows for the first time that over-expression of the oat phytochrome A gene ( PHYA ) in a tree significantly changes the critical daylength and effectively prevents cold acclimatization. While the critical daylength for elongation growth in the wild-type of hybrid aspen ( Populus tremula × tremuloides ) was approximately 15 h, transgenic lines with a strong expression of the oat PHYA gene did not stop growing even under a photoperiod of 6 h. Quantitative analysis of gibberellins (GA) as well as indole-3-acetic acid (IAA) revealed that levels of these were not down-regulated under short days in the transgenic plants expressing high levels of oat PHYA , as in the wild-type. These results indicate that photoperiodic responses in trees might be regulated by the amount of PHYA gene expressed in the plants, and that the amount of phytochrome A (phyA) affects the metabolism of GAs and IAA.     

Defining the cost of the Egyptian lymphatic filariasis elimination programme

BACKGROUND: Ultrasonography (USG) is known to be a suitable tool for diagnosis in lymphatic filariasis as the adult filarial nematode Wuchereria bancrofti in scrotal lymphatic vessels of infected men can be detected by the characteristic pattern of movement, the Filaria Dance Sign. In onchocerciasis, moving adult worms have not yet been demonstrated by USG. In addition the verification of drug effects on living adult Onchocerca volvulus filariae in trials is hampered by the lack of tools for longitudinal observation of alterations induced by potentially macrofilaricidal drugs in vivo. The present study was carried out to determine the frequency of detection of moving adult filariae of O. volvulus by USG. METHODS: In an endemic region for onchocerciasis in Ghana, 61 patients infected with onchocerciasis were recruited by palpation and onchocercomas examined by USG using an ultrasound system equipped with a 7.5 - 10 MHz linear transducer. Onchocercomas were recorded on videotape and evaluated with regard to location, number and size, as well as to movements of adult filariae. RESULTS: In the 61 patients 303 onchocercomas were found by palpation and 401 onchocercomas were detected by USG. In 18 out of 61 patients (29.5%), altogether 22 nodules with moving adult O. volvulus filariae were detected and are presented in animated ultrasound images as mp-4 videos. CONCLUSION: Ultrasonographical examinations of onchocercomas where living adult filariae can be displayed may serve as a new tool for the longitudinal observation in vivo of patients with onchocerciasis undergoing treatment and as an adjunct to histological evaluation.