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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Weed research 22 (1982), S. 0 
    ISSN: 1365-3180
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Experiments were carried out in Indonesia to see whether weeds of lowland rice fields could be managed by manipulation of the water table and what factors affected germination of their seeds. It was found that seeds of two Scirpus spp. germinated when buried in anaerobic soil but not when on the surface, whereas seeds of two Echinochloa spp. germinated when on the surface of both aerobic and anaerobic soils but when buried germinated only in aerobic soil.The germination of weed seeds was affected to differing extents by submergence in water at depths down to 20 cm. Germination of Fimbristylis littoralis seeds was greatly reduced and of Ludwigia hyssopifolia was complétély inhibited by submergence. Conversely, germination of Monochoria vaginalis seeds was virtually unaffected. It is suggested that the effect of submergence on germination of some species may be due to their reaction to the amplitude of temperature fluctuation. The germination and emergence of seedlings under water were greatly reduced by a cover of Salvinia molesta. The practical importance of these findings is discussed.
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Plant, cell & environment 7 (1984), S. 0 
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Abstract. It is demonstrated that after the feelling of ash coppice Cirsium palustre becomes established from seed that was produced during the previous coppice cycle and has lain underground for several years. These seeds are light-requiring and it is probable that their germination is triggered by exposure to light during coppicing in winter.It is assumed that the seeds become light-requiring because phytochrome in the far-red absorbing form high. This facilitates their incorporation in the seed bank in summer. However, the disappearance of Pfr is inhibited at low temperatures; this means that the seeds can store the light stimulus in winter and will not germinate until they experience higher temperatures in spring. The significance of the inhibition of seed germination under a leaf canopy is that it enhances the seeds chance of being incorporated in the soil seed bank.
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  • 3
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: The photosynthetic induction state under conditions of different lightfleck frequencies or durations, or different shade periods was studied in soybean leaves in order to examine how it might limit utilization of sunflecks in leaf canopies. Induction following an increase in photon flux density (PFD) from strongly limiting to saturating PFDs exhibited two phases; a fast-inducing one, requiring about 1 min and a slow one, requiring up to 60 min for completion. Transfer of fully induced leaves to low light resulted in a rapid decrease in the fast-inducing component, a slower decrease in the slow-inducing component and an even slower decrease in stomatal conductance. Therefore, the decreases in extent of induction appeared to be due to biochemical factors and not to stomatal closure. Under flashing light regimes consisting of 1-s lightflecks given at different frequencies for long periods, a constant induction state was achieved, the measure of induction state increased with the frequency of the lightflecks. This constant induction state also depended on the growth conditions, with shade leaves having a higher value than those grown at high light at any particular lightfleck frequency. The measure of induction state was mostly lower in flashing light as compared to constant light of the same mean PFD, particularly in leaves with a low light saturation point and in short lightflecks. Initial activities of ribulose-1,5-bisphosphate carboxylase (rubisco) were also higher in continuous light and were highly correlated with the measure of induction state. The rapid decrease in extent of induction of soybean leaves during shade periods is an important limitation to the ability of the leaves to respond to light increases similar to those occurring with sunflecks. At least part of the limitation on carbon assimilation during sunflecks due to photosynthetic induction is based on regulation of rubisco activity.
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Plant, cell & environment 15 (1992), S. 0 
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Leaves of soybean plants grown in contrasting light and nutrient availability conditions were exposed to constant and to flashing light regimes with lightflecks of different frequencies, durations and photon flux density (PFD). The lightfleck characteristics were selected to be representative of the range of variation found for sunflecks in a soybean canopy. CO2 fixation rates were measured using a fast-response gas-exchange apparatus. The net CO2 fixation due to 1-s-duration lightflecks was 1·3 times higher than predicted from steady-state measurements in constant light at the lightfleck and background PFD. This lightfleck utilization efficiency (LUE) was somewhat higher at a high than at a low frequency of one second lightflecks. LUE in flashing light with very short lightflecks (0·2s) and single 1 s lightflecks was as high as 2, but decreased sharply with increasing duration of lightflecks. This decrease occurred because CO2 fixation rates during lightflecks were constrained by carbon metabolism and induction limitations, and because the contribution of post-illumination CO2 fixation to total CO2 fixation decreased with increased duration of lightflecks. LUE increased with increased PFD during the lightflecks, particularly in leaves from plants grown in high-light, high-nutrient conditions. Saturation PFDs were much higher in flashing light than in constant light. Only small differences in LUE were apparent between leaves from the three growth conditions.
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    Plant, cell & environment 25 (2002), S. 0 
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: The possible interference when measuring gas exchange with respiratory CO2 produced under the gasket of commercially available clamp-on leaf chambers was investigated. Two of these chambers were compared with a leaf chamber that accommodated an entire leaf without clamping it under a gasket. An overestimation of dark respiration rate (RD) by 55% was found with Plantago major leaves, a species with homobaric leaves that have high resistance for lateral gaseous transport. The percentage was similar in the heterobaric Ficus benjamina, but was 32% in the highly porous homobaric Nicotiana tabacum. Net photosynthetic rate at low photon flux density was underestimated by 35% in the clamp-on chamber. However, the gasket effect was not detectable at light saturation because the error was small in comparison with the high photosynthetic rates. Estimation of respiration in the light (RL) in Nicotiana as derived from CO2 exchange at low CO2 concentrations was complicated by three factors. The inward diffusion of respiratory CO2 from under the gasket was added to a diffusion of CO2 from outside through the gasket material and through the leaf, which produced an even larger error in RL in comparison with RD at ambient CO2. These errors are significant for estimations of carbon gain at whole plant and canopy level and also at the leaf level when photosynthetic rates are low. Possible improvements in gasket design and corrections of CO2 exchange measurements for the gasket effect are discussed.
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  • 6
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Physiologia plantarum 68 (1986), S. 0 
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Seeds of Plantago major L. ssp. major were exposed to continuous light of various red/far-red ratios (R/FR). Germination was strongly inhibited at low R/FR. The quantitative response of the seeds to the R/FR appeared to depend on various environmental factors. A low R/FR was less inhibitory after a stratification pretreatment, at higher temperatures, in the presence of nitrate and at high osmotic potentials. This is discussed in relation to the concept of a variable requirement of Pfr for germination, which depends on state of dormancy as influenced by pretreatment and on conditions during the germination test. The data are also discussed in relation to field emergence. The species is known to form a persistent seed bank, among other things due to its almost absolute light requirement. It is predicted that germination is effectively inhibited under a leaf canopy in summer after dispersal, and perhaps even in spring to some extent when R/FR is not so low but temperatures are low.
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  • 7
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: We examined the effect of growth temperature on the underlying components of growth in a range of inherently fast- and slow-growing plant species. Plants were grown hydroponically at constant 18, 23 and 28 °C. Growth analysis was conducted on 16 contrasting plant species, with whole plant gas exchange being performed on six of the 16 species. Inter-specific variations in specific leaf area (SLA) were important in determining variations in relative growth rate (RGR) amongst the species at 23 and 28 °C but were not related to variations in RGR at 18 °C. When grown at 18 °C, net assimilation rate (NAR) became more important than SLA for explaining variations in RGR. Variations in whole shoot photosynthesis and carbon concentration could not explain the importance of NAR in determining RGR at the lower temperatures. Rather, variations in the degree to which whole plant respiration per unit leaf area acclimated to the different growth temperatures were responsible. Plants grown at 28 °C used a greater proportion of their daily fixed carbon in respiration than did the 18 and 23 °C-grown plants. It is concluded that the relative importance of the underlying components of growth are influenced by growth temperature, and the degree of acclimation of respiration is of central importance to the greater role played by NAR in determining variations in RGR at declining growth temperatures.
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Plant, cell & environment 6 (1983), S. 0 
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Abstract. The effect on seed germination of the red/far-red ratio of light (ζ) as modified by a leaf canopy was studied. The artificial light sources used and daylight filtered through leaves appeared to have identical effects in tests with lettuce seed.The sensitivity of the seeds to ζ varied with the environmental conditions during pretreatment and during the actual germination test. Freshly harvested and dry-stored seeds of Cirsium palustre were strongly inhibited at low ζ; after stratification, the inhibition was much less. The fact that C. palustre emerges mainly after the coppice is felled could not be explained from these results combined with field data on ζ and the response of germination to temperature.The ecological significance of inhibition at low ζ is to be found directly after dispersal. Seed germination will then be delayed until the following spring or later. Geum urbanum showed primary dormancy, which was released by a stratification treatment. No inhibition of germination at low ζ was found in the spring and seeds germinated in light and darkness at low temperatures. These results explain why the species emerges early in spring regardless of the presence of the ash canopy.
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  • 9
    ISSN: 1432-1939
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary A diurnally fluctuating temperature was found to be almost an absolute requirement for the germination of Fimbristylis littoralis and Scirpus juncoides. It is demonstrated that in f. littoralis sensitivity to the amplitude of temperature fluctuation can functuation as a depth sensing mechanism under water. This species did not germinate at all in the absence of oxygen. S. juncoides germinated best in anaerobic conditions and at low oxygen concentrations, but normal atmospheric concentrations inhibited germination. It is shown that this requirement for little or no oxygen together with the requirement for a fluctuating temperature cause the germination of S. juncoides to be restricted to the upper layers of waterlogged soil.
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  • 10
    ISSN: 1432-1939
    Keywords: Canopy structure ; Leaf nitrogen ; Light climate ; Lysimachia vulgaris L. ; Specific leaf weight
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary A hypothesis that a dense stand should develop a less uniform distribution of leaf nitrogen through the canopy than an open stand to increase total canopy photosynthesis was tested with experimentally established stands of Lysimachia vulgaris L. The effect of stand density on spatial variation of photon flux density, leaf nitrogen and specific leaf weight within the canopy was examined. Stand density had little effect on the value of the light extinction coefficient, but strongly affected the distribution of leaf nitrogen per unit area within a canopy. The open stand had more uniform distribution of leaf nitrogen than the dense stand. However, different light climates between stands explained only part of the variation of leaf nitrogen in the canopy. The specific leaf weight in the canopy increased with increasing relative photon flux density and with decreasing nitrogen concentration.
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