Changes in plant growth, photosynthetic gas exchange, chlorophyll fluorescence and stem diameter of soybean [Glycine max (L.) Merr.] plants under drought stress were studied. Total plant dry mass was reduced by 30 % compared to wellwatered control plants. Leaf water potential was slightly decreased by water stress. Water stress induced daytime shrinkage and reduced night-time expansion of stem. Photosynthetic rate, stomatal conductance and transpiration rate were significantly declined by water stress, while the intercellular CO 2 concentration was changed only slightly at the initiation of stress treatment. The maximum photochemical efficiency of photosystem 2 and apparent photosynthetic electron transport rate were not changed by water stress.Additional key words: Glycine max, growth, net photosynthetic rate, photochemical efficiency of photosystem 2, stomatal conductance, transpiration rate, water stress. ⎯⎯⎯⎯Soybean is one of the major and widespread crops in the world and is rather sensitive to water stress. Plants regulate their diurnal water status at a favourable level by the control of stomatal aperture (Farquhar and Sharkey 1982). Stomatal closure contributes to maintain high leaf water content and high leaf water potential, but it leads to a decrease in leaf photosynthesis. Stomatal closure reduces intercellar CO 2 concentration in leaves which imposes limitations CO 2 assimilation, and it causes an imbalance between photochemical activity at photosystem 2 (PS 2) and electron requirement for photosynthesis, and leads to increased susceptibility to photo-damage (He et al. 1995, Flagella et al. 1998. Several investigators have shown that the stem diameter of woody plants fluctuate diurnally (e.g., Imai et al. 1990 in grape, Simonneau et al. 1993 in peach, Ito et al. 1999 in pear). Simmoneau et al. (1993) used the micromorphometric techniques to observe rapid changes in the stem diameter in peach tree, which was closely related to the water status throughout the day. Water status of plants has been estimated directly by measuring the changes in stem diameter, however, the effect of water stress on growth in soybean plants has not been examined by this method.The present study was undertaken to evaluate the responses of photosynthesis to water stress in soybean plants, both in terms of CO 2 assimilation, as measured by leaf gas exchanges, and of the functionality of the photosynthetic apparatus, as assessed by chlorophyll fluorescence measurements. We also analysed the effect of water stress on stem diameter of intact soybean plants grown under drought.Seeds of soybean (Glycine max (L.) Merr.), latematuring cultivar New Tanbakuro were sown in mixture of granite regosol soil, Perlite and peat moss (2:1:1 (v/v/v)), and were irrigated daily. At 24 d after ⎯⎯⎯⎯ Abbreviations: c i -intercellular CO 2 concentration; DAT -days after the stress treatment; E -transpiration rate; ETR -apparent photosynthetic electron transport rate; F v /F m -variable to maximum chlorophyll fluorescence ratio (maximum photochemica...

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This study investigated the factor of the physiological characteristics causing the reduction of yield of soybean plants (Glycine max (L.) Merr.) by drought stress, by monitoring changes in stem diameter and pod thickness, and photosynthetic activity, partitioning of 13 C-labeled photosynthate. Drought stress reduced the whole plant dry weight due to the decrease in leaf and pod dry matter accumulation; however, this stress did not have a significant effect on stem growth. Leaf photosynthesis was also severely decreased by drought stress in the early stage of stress treatment as leaf water potential decreased. Imposition of stress decreased pod thickness, but stem diameter increased. The adverse effect of drought stress on pod thickness was more evident at night than during the day. The stem diameter also shrank during the day and expanded at night, but the nocturnal increase in stem diameter during drought stress treatment was greater for stressed plants compared with well-watered controls. Drought stress significantly promoted 13 C partitioning from the fed leaf to other parts of the plant; the stem was the largest beneficiary. Soluble carbohydrates accumulated in various plant parts under the influence of the stress, but starch concentration declined in all organs except the stem. These results indicated that stem growth was promoted by drought stress compared to pod growth at the early grain-filling stage.

a b s t r a c tOptical, electrochemical, and electron-transporting properties of disilane-and siloxane-bridged biphenyl and bithiophene derivatives were investigated, in comparison with those of the monosilane-bridged analogues (siloles). The UV spectra and cyclic voltammograms indicated that elongation of the silicon bridge suppresses the p-conjugation, in accordance with the results of DFT calculations. The DFT calculations indicated also that the disilane-bridged biphenyl and siloxane-bridged bithiophene should have the low-lying HOMOs and LUMOs. The electron-transporting properties were evaluated by the performance of triple-layered OLEDs having vapor-deposited films of the Si-bridged compound, Alq 3 , and TPD, as the electron-transport, emitter, and hole-transport, respectively. Of these, the device with a disilane-bridged biphenyl exhibited the high performance with the maximum current density of 590 mA/cm 2 at the applied electric field of 12 Â 10 7 V/m (applied bias voltage = 13 V) and the maximum luminance of 22 000 cd/m 2 at 13 Â 10 7 V/m.

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