A pressure chamber and a root pressure probe technique have been used to measure hydraulic conductivities of rice roots (root Lp(r) per m(2) of root surface area). Young plants of two rice (Oryza sativa L.) varieties (an upland variety, cv. Azucena and a lowland variety, cv. IR64) were grown for 31-40 d in 12 h days with 500 micromol m(-2) s(-1) PAR and day/night temperatures of 27 degrees C and 22 degrees C. Root Lp(r) was measured under conditions of steady-state and transient water flow. Different growth conditions (hydroponic and aeroponic culture) did not cause visible differences in root anatomy in either variety. Values of root Lp(r) obtained from hydraulic (hydrostatic) and osmotic water flow were of the order of 10(-8) m s(-1) MPa(-1) and were similar when using the different techniques. In comparison with other herbaceous species, rice roots tended to have a higher hydraulic resistance of the roots per unit root surface area. The data suggest that the low overall hydraulic conductivity of rice roots is caused by the existence of apoplastic barriers in the outer root parts (exodermis and sclerenchymatous (fibre) tissue) and by a strongly developed endodermis rather than by the existence of aerenchyma. According to the composite transport model of the root, the ability to adapt to higher transpirational demands from the shoot should be limited for rice because there were minimal changes in root Lp(r) depending on whether hydrostatic or osmotic forces were acting. It is concluded that this may be one of the reasons why rice suffers from water shortage in the shoot even in flooded fields. (+info)
QTL analysis of photosynthesis and water status traits in sunflower (Helianthus annuus L.) under greenhouse conditions.
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The identification of QTL for several physiological traits in sunflower is described. Traits related to photosynthesis (leaf chlorophyll concentration, net photosynthesis and internal CO(2) concentration) and water status (stomatal conductance, transpiration, predawn leaf water potential, and relative water content) were evaluated in a population of recombinant inbred lines under greenhouse conditions. Narrow-sense heritabilities were low to average. Using an AFLP linkage map, 19 QTL were detected explaining 8.8-62.9% of the phenotypic variance for each trait. Among these, two major QTL for net photosynthesis were identified on linkage group IX. One QTL co-location was found on linkage group VIII for stomatal movements and water status. Coincident locations for QTL regulating photosynthesis, transpiration and leaf water potential were described on linkage group XIV. These results lead to the first description of the organization of genomic regions related to yield in sunflower. (+info)
Direct measurement of sodium and potassium in the transpiration stream of salt-excluding and non-excluding varieties of wheat.
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The xylem-feeding insect Philaenus spumarius was used to analyse sodium and potassium fluxes in the xylem of intact, transpiring wheat plants. Two cultivars were compared: the salt-excluding (Chinese Spring) and the non-excluding (Langdon). Chinese Spring accumulated much less sodium in its leaves than the salt-sensitive Langdon. After 7 d in 150 mol m(-3) NaCl, the sodium concentration in the leaf sap of Langdon reached over 600 mol m(-3). This was some three-fold greater than that in Chinese Spring. Similar findings have previously been reported from these cultivars. The reduced ion accumulation was specific to sodium; accumulation of K(+) was unaffected by NaCl in Chinese Spring, such that it developed a much lower leaf Na(+)/K(+) ratio than Langdon. The spittlebug, P. spumarius was used to sample xylem sap from both cultivars. This approach showed that the leaf xylem sap of Chinese Spring had much lower levels of sodium than that of Langdon. In the 150 mol m(-3) NaCl treatment, sodium levels in the leaf xylem reached only 2-3 mol m(-3) in Chinese Spring, compared with 8-10 mol m(-3) in Langdon. Transpiration rates were found to be similar in the two varieties. The lower leaf xylem content alone was thus sufficient to account for the reduced accumulation of sodium in leaves of Chinese Spring. The mechanisms by which xylem sodium might be lowered are discussed and it is concluded that sodium is probably excluded from the xylem in the root of Chinese Spring. (+info)
Effect of temperature on cuticular transpiration of isolated cuticular membranes and leaf discs.
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Cuticular transpiration was measured in the temperature range between 10 degrees C and 55 degrees C using tritiated water and five species (Vinca major L., Prunus laurocerasus L., Forsythia intermedia L., Citrus aurantium L., and Hedera helix L.). Cuticular water permeabilities measured with isolated cuticular membranes were not different from cuticular water permeabilities measured with leaf discs. Depending on the species cuticular water permeabilities increased by factors between 12 (V. major) to 264 (H. helix) when temperature was increased from 10 degrees C to 55 degrees C. Arrhenius plots (lnP versus 1/T) of all investigated species were characterized by phase transitions occurring in the temperature range of 30-39 degrees C. Activation energies for water permeability across plant cuticles below and above the midpoint of phase transition were calculated from Arrhenius plots. Depending on the species they varied between 26 (F. intermedia) to 61 kJ mol(-1) (H. helix) below the phase transition and from 67 (V. major) to 122 kJ mol(-1) (F. intermedia) above the phase transition. Since the occurrence of phase transitions always lead to significantly increased rates of cuticular transpiration it is argued that temperatures higher than 35 degrees C caused structural defects to the transport-limiting barrier of the plant cuticles of all species investigated. (+info)
Wall relaxation and the driving forces for cell expansive growth.
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When water uptake by growing cells is prevented, the turgor pressure and the tensile stress in the cell wall are reduced by continued wall loosening. This process, termed in vivo stress relaxation, provides a new way to study the dynamics of wall loosening and to measure the wall yield threshold and the physiological wall extensibility. Stress relaxation experiments indicate that wall stress supplies the mechanical driving force for wall yielding. Cell expansion also requires water absorption. The driving force for water uptake during growth is created by wall relaxation, which lowers the water potential of the expanding cells. New techniques for measuring this driving force show that it is smaller than believed previously; in elongating stems it is only 0.3 to 0.5 bar. This means that the hydraulic resistance of the water transport pathway is small and that rate of cell expansion is controlled primarily by wall loosening and yielding. (+info)
Influence of seismic stress on photosynthetic productivity, gas exchange, and leaf diffusive resistance of Glycine max (L.) Merrill cv Wells II.
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Relative growth rate (RGR), leaf water potential (psi w), transpiration rate (Tr), photosynthetic rate (Pn), and stomatal and mesophyll resistances to CO2 exchange were measured or calculated to determine how periodic seismic (shaking) stress decreased dry weight accumulation by soybean (Glycine max [L.] Merrill cv Wells II). Seismic stress was applied with a gyratory shaker at 240 to 280 revolutions per minute for 5 minutes three times daily at 0930, 1430, and 1930 hours. Fifteen days of treatment decreased stem length 21%, leaf area 17%, and plant dry weight 18% relative to undisturbed plants. Seismic stress also decreased RGR 4%, which was due entirely to decreased net assimilation rate. Transpiration decreased 17% and leaf psi w increased 39% 30 minutes after treatment. A reduction in Pn began within seconds after the onset of treatment and had declined 16% after 20 minutes, at which time gradual recovery began. Assimilation rate recovered fully before the next seismic treatment 5 hours later. Resistance analysis and calculation of leaf internal CO2 levels indicated that the transitory decrease in Pn caused by periodic seismic stress was due to increased stomatal resistance on the abaxial leaf surface. (+info)
Effects of CO2 on stomatal conductance: do stomata open at very high CO2 concentrations?
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Potato and wheat plants were grown for 50 d at 400, 1000 and 10000 micromoles mol-1 carbon dioxide (CO2). and sweetpotato and soybean were grown at 1000 micromoles mol-1 CO2 in controlled environment chambers to study stomatal conductance and plant water use. Lighting was provided with fluorescent lamps as a 12 h photoperiod with 300 micromoles m-2 s-1 PAR. Mid-day stomatal conductances for potato were greatest at 400 and 10000 micromoles mol-1 and least at 1000 micromoles mol-1 CO2. Mid-day conductances for wheat were greatest at 400 micromoles mol-1 and least at 1000 and 10000 micromoles mol-1 CO2. Mid-dark period conductances for potato were significantly greater at 10000 micromoles mol-1 than at 400 or 1000 micromoles mol-1, whereas dark conductance for wheat was similar in all CO2 treatments. Temporarily changing the CO2 concentration from the native 1000 micromoles mol-1 to 400 micromoles mol-1 increased mid-day conductance for all species, while temporarily changing from 1000 to 10000 micromoles mol-1 also increased conductance for potato and sweetpotato. Temporarily changing the dark period CO2 from 1000 to 10000 micromoles mol-1 increased conductance for potato, soybean and sweetpotato. In all cases, the stomatal responses were reversible, i.e. conductances returned to original rates following temporary changes in CO2 concentration. Canopy water use for potato was greatest at 10000, intermediate at 400, and least at 1000 micromoles mol-1 CO2, whereas canopy water use for wheat was greatest at 400 and similar at 1000 and 10000 micromoles mol-1 CO2. Elevated CO2 treatments (i.e. 1000 and 10000 micromoles mol-1) resulted in increased plant biomass for both wheat and potato relative to 400 micromoles mol-1, and no injurious effects were apparent from the 10000 micromoles mol-1 treatment. Results indicate that super-elevated CO2 (i.e. 10000 micromoles mol-1) can increase stomatal conductance in some species, particularly during the dark period, resulting in increased water use and decreased water use efficiency. (+info)
Trace gases generated in closed plant cultivation systems and their effects on plant growth.
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Interactions between plants and trace gases, especially ethylene, were investigated from two different viewpoints; ethylene is toxic for plant growth, whereas the ethylene release rate of plants can be utilized as a plant growth indicator. When lettuce plants and shiitake mushroom mycelium were cultivated in closed chambers, ethylene concentration increased with time. Ethylene was released both from lettuce plant and from shiitake mushroom mycelium. Dioctyl phthalate (DOP) and Dibutyl phthalate (DBP) were detected, and these concentrations reached 3.7 ngL-1 for DOP and 2.4 ngL-1 for DBP 4 days after closing. Organic solvents such as xylene and toluene and organic siloxane were detected with GCMS. Visible injury was observed in lettuce plants cultivated in the chambers and it seemed to result from trace contaminants such as DOP, DBP, organic solvents, dimethylsiloxane polymer, and ethylene. In order to obtain basic data of ethylene evolution from plants, ethylene concentration in a closed chamber in which the plants were cultivated under a controlled environment (25 degrees C air temperature, 60-70% relative humidity, 250-300 micromoles m-2 s-1 photosynthetic photon flux density (PPFD)) was measured. Lettuce (Lactuca sativa L. cv. Okayama) released ethylene more than Brassica rapa var. pervidis, Brassica campestris var. communis, and Brassica campestris var. narinosa. Ethylene release rate of intact lettuce plant was highly correlated with plant growth parameters such as dry weight, leaf area and photosynthetic rate. Ethylene release rates of intact lettuce plant were affected by cultivation conditions such as ambient CO2 concentration, light intensity and light/dark period. Increase in ambient ethylene level influenced lettuce growth even at the concentration of 0.1 microliter L-1. The level of ethylene inhibited leaf expansion and slightly accelerated chlorophyll degradation. It did not affect photosynthesis and transpiration, and also little affected dry matter accumulation. Thus, ethylene release characteristics were clarified and an effect of ethylene on lettuce growth was revealed. These findings are useful for determination of a threshold level of ethylene and a capacity of ethylene removal system in CELSS. On the other hand, a possibility of plant growth diagnosis by measuring ethylene concentrations was evaluated. As a result, it became clear that the measurement of ethylene concentration in CELSS is one of the useful non-destructive measurement methods for plant growth diagnosis. Further research is needed to investigate the applicability of the method to environmental stresses other than Ni and Co in nutrient solution. (+info)