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Changes in photosynthetic gas exchange, chlorophyll fluorescence, and stem diameter of soybean plants under drought stress condition.

Yoshiyuki Ohashi, Nobuhiro Nakayama, Hirofumi Saneoka and Kounosuke Fujita

Graduate school of Biosphere Science, Hiroshima University, Higashi-Hiroshima, 739-8528, Japan. Email: YO:, NN:, HS:, KF:


The responses of photosynthetic gas exchange, chlorophyll fluorescence, and stem diameter were studied in two cultivars of soybean during water stress and recovery. Photosynthetic rate (Pn), stomatal conductance (gs) and transpiration rate (E) were significantly reduced by water stress, while the intercellular CO2 concentration (Ci) was slightly changed. The maximum photochemical efficiency of PSII (Fv/Fm) and apparent photosynthetic electron transport rate (ETR) were also not altered by water stress in Tanbakuro. Pn, gs, E in cultivar Tanbakurio recovered nearly to the levels of the control plants after rewatering, while cultivar Tamanishiki did not recover during the 24 hours after rewatering. Water stress induced daytime shrinkage and reduced night-time expansion of stem. The plant stem diameter of plants reduced with the initiation of photoperiod and expanded during the night.

Key words

Chlorophyll fluorescence, photosynthesis, soybean, stem diameter, water stress


Plants regulate their diurnal water status at a favorable level by the control of stomatal aperture. Stomatal closure helps to maintain a high leaf water content and thereby a higher leaf water potential, which leads to a reduction in photosynthetic activity. Stomatal closure reduces CO2 entry into leaves which reduces the intercellular CO2 concentration and lowers C fixation. This causes an imbalance between photochemical activity at photosystem II (PSII) 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 with changes in the light and soil water conditions (Imai et al. 1990, Ito et al. 1999, Fujita et al., 2003). 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. The present study measured the responses of photosynthesis to water stress in soybean plants, both in terms of CO2 assimilation and of the functionality of the photosynthetic apparatus. We also analysed the effect of water stress on stem diameter of intact soybean plants grown under drought conditions.


Two soybean cultivars (Tanbakuro and Tamanisiki) were sown in flats containing a mixture of granite regosol soil and perlite 1:1 (v/v) and irrigated daily. At 24 days after germination, plants of uniform size were selected and were transplanted into 3L pots filled with the above-mentioned soil mixture. Water stress treatment was imposed by maintaining the soil water content at 50% of field capacity at 48 days (at flowering stage) after germination. Duration of the stress treatment was 20 days, and the plants subjected to water stress were assessed for recovery 24 h after rewatering to the field capacity. Gas exchange and chlorophyll fluorescence were measured simultaneously on the attached upper most fully expanded leaves with a Li-6400 portable open gas-exchange system and a Li-6400-40 leaf chamber fluorometer (Li-Cor, Lincoln, NE, USA). Changes in stem diameter were recorded at 5 min intervals after the initiation of the stress treatment with a micro-displacement detector for 10 days as decribed by Ito et al. (1999).

Results and Discussion

When the soil moisture level was maintained at 50 % of the field capacity the midday leaf water declined from –0.43 MPa in the control to –0.77 MPa in cultivar Tamanishiki, and from –0.52 MPa to –0.77 MPa in cultivar Tanbakuro. Plant growth was severely reduced by water stress in both cultivars. The decrease in shoot dry weight 7 days after stress treatment was 32 % in cultivar Tanbakuro and 42 % in cultivar Tamanishiki. Soybean plants are sensitive to drought relevant to other crop plants as seen in this study and the previous studies (Korte et al., 1983, Ohashi et al., 1999).

Table 1. Photosynthetic gas exchange and Chlorophyll fluorescence parameters in the water-stressed plant and control at 7 days after water stress treatment (DAT) and 1 days after re-watering (DAR). Pn: photosynthetic rate, gs: stomatal conductance, Ci: intercellular CO2 concentration, E: Transpiration rate, Fv/Fm: maximum photochemical efficiency, ETR: apparent photosynthetic electron transport

Water stress caused a substantial reduction of photosynthesis rate in both Tanbakuro and Tamanishiki (Table 1). There was a similar reduction in Pn and gs in both cultivars suggesting that the reduction in Pn is due to the reduction in gs. Upon rewatering, there was large difference between both cultivars in the recovery of Pn and gs. The Pn and gs in Tanbakuro recovered completely within 24 hours after rewatering, however Tamanishiki did not recover.

Fig. 1. Diurnal changes in stem diameter in cultivar Tanbakuro during the first 3 days of the water stress treatment.

Fig. 2. Day time changes in stem diameter in cultivar Tanbakuro in the water stress and control plants during 10 days of water stress treatment.

In the present study maximum photochemical efficiency (Fv/Fm) in water-stressed plants and control plants were similar (Table 1). Apparent photosynthetic electron transport rate (ETR) in Tanbakuro was not significantly affected by stress and was higher in the stressed plants 24 h. after rewatering (Table 1). These result shows that the leaf photochemistry was rather resistant to water stress and the electron transport chain was maintained under water stress conditions, although Pn and gs were strongly inhibited in Tanbakuro. In contrast the cultivar Tamanishiki showed no effect of stress on Fv/Fm, but a large reduction in ETR.

Stem diameter decreased after sunrise and increased after sunset (Fig. 1). The changes of stem diameter were smaller in water-stressed plants compared with the control (Fig. 2). This observation suggested that change in stem diameter is controlled by plant water status, and shrinkage begins as the plant is expose to sun light and stomatal water loss exceeds the supply of water by roots. Stem diameter responses quickly within one or two days after the water stress treatment with change of Pn, gs and E, suggesting that stem diameter could be used as effective parameter to estimate the water status in soybean under various environmental stress condition.


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