Soil Science and Plant Nutrition
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Effects of silicon application on drought resistance of cucumber plants Cheng Cang Ma , Qing Fang Li , Yu Bao Gao & Tian Rong Xin To cite this article: Cheng Cang Ma , Qing Fang Li , Yu Bao Gao & Tian Rong Xin (2004) Effects of silicon application on drought resistance of cucumber plants, Soil Science and Plant Nutrition, 50:5, 623-632, DOI: 10.1080/00380768.2004.10408520 To link to this article: https://doi.org/10.1080/00380768.2004.10408520
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Soil Sci. Plant Nutr., 50 (5), 623 - 632, 2004
Effects of Silicon Application on Drought Resistance of Cucumber Plants Cheng Cang Ma*'**, Qing Fang Li**, Yu Baa Gaa*,l, and Tian Rang Xin* *Department of Biology, Nankai University, Tianjin, 300071 PR China; and **Department of Biology, Huaibei Coal Normal College, Huaibei City, An/lUi Province, 235000 PR China Received September 12,2003; accepted in revised form March 22, 2004
A study on the effects of silicon supply on the resistance to drought in cucumber plants was conducted in pot experiments. The results suggested that in the absence of stress, silicon slightly enhanced the net photosynthetic rate, but significantly decreased the transpiration rate and stomatal conductance in cucumber plants. Silicon enhanced the net photosynthetic rate of cucumber plants under drought stress. Since silicon decreased the stomatal conductance, enhanced the capacity of holding water, and kept the transpiration rate at a relatively steady rate during drought stress, the photosynthesis of the cucumber plants was sustained. And under drought stress, silicon increased the biomass and water content of leaves in cucumber plants. Silicon decreased the decomposition of chlorophyll in cucumber plants under drought stress, limited the increase of the plasma membrane permeability and malondialdehyde (MDA) content in leaves, alleviated the physiological response of peroxidase (POD) to drought stress, maintained the superoxide dismutase (SOD) normal adaptation, and increased the activity of catalase (CAT). Under severe stress, these physiological biochemical reactions showed positive correlations with the amount of silicon supply. These findings demonstrated that silicon enhanced the resistance of the cucumber plants to drought. Statistical analysis indicated that under drought stress the cumulative value of biomass showed a highly significant correlation with the cumulative value of diurnal photosynthesis (r = 0.9812, p < 0.01), and was significantly correlated with the water content of leaves (r = 0.8650, p < 0.05). These results demonstrated that under drought stress the first factor responsible for the effects of silicon application on the cumulative value of biomass was the increase of photosynthesis, and the second factor was the enhancement of the water holding capacity. Based on these facts, it was concluded that silicon enhanced the resistance to drought mainly by taking part in the metabolism of plants.
Key Words:
cucumber plant, drought resistance, photosynthesis, silicon, transpiration.
In 1926, Sommer from California University in America, first pointed out that silicon was the essential element which made rice plants grow well (Xia et al. 2001), thus drawing the attention to the beneficial effects of silicon application on crops. Research in the field dealt mainly with three aspects. The first aspect was the effect of silicon supply on plant growth, revealing that silicon was a beneficial element to many higher plants. Silicon improved the crop metabolism (Ma et al. 2002), promoted crop growth and development, and increased crop production (Epstein 1994; Inanaga et al. 2002). Silicon induced stem cell wall strengthening, enhanced the resistance to lodging (Savant et al. 1999), --------~
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To whom correspondence should be addressed. E-mail:
[email protected] I
and improved the leaf orientation of rice plants (Savant et al. 1999; Ando et al. 2002). Silicon also increased the cell wall extensibility (Hossain et al. 2002), inhibited transpiration and improved water use efficiency (Savant et al. 1999). It enhanced the photosynthetic rate of crops, increased the efficiency of Nand P fertilization (Ma and Takahashi 1989; Savant et al. 1999), and affected the contents of N, P, K, Ca, and Mg in plants. The second aspect was the association of silicon with abiotic stresses. It suggested that silicon alleviated salt injuries (Liang et al. 1996) and toxicities of Fe, Mn, Cd, Cr, AI, and Cu (Liang et al. 1993; Savant et al. 1999; Neumann and Nieden 200 I; Morikawa and Saigusa 2002), and enhanced the resistance to cold (Savant et al. 1999) and ultra-violet rays (Goto et al. 2003). The third aspect was the association of silicon with biotic stresses. It demon-
c.c. MA et al.
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strated that silicon increased the resistance to diseases and insect pests in rice (Rodrigues et al. 2003), wheat, maize, and cucumber (Liang et al. 2002; Schuerger et al. 2003). Silicon is abundant and widely distributed in soils. In the stratum its content is only lower than that of oxygen (Liang and Sun 1993). Since silicon in soils occurs in the fonn of silicate crystals or sediments and its concentration is commonly low in the soil solution, it is often deficient in soils. If silicon fel1ilizer could be applied to the silicon-deficient soils, crop growth may improve, and the production may increase. Cucumber is an important horticultural plant. Basic laboratory studies have demonstrated that silicon increased the seed germination rate, enhanced seedling growth and metabolism (Miyake and Takahashi 1983; Li and Ma 2002), as well as the resistance to satta disease, powdery mildew, fusarium wilt, and root rot in cucumber plants (Fawe et al. 1998; Matichenkov et aJ. 2000; Itoh et aJ. 2001), and strikingly increased production (Yasuto 1988). However, it remained to be determined whether silicon could enhance the resistance of cucumber to drought. Therefore, we conducted an experiment to observe the effects of silicon application on photosynthesis, water metabolism, biomass, chlorophyll content, membrane lipid peroxidation, penneability of plasma membrane, and protective enzymes of cucumber plants under drought stress, in order to elucidate the functional mechanisms whereby silicon enhances the resistance to drought in cucumber plants. This might enable to determine how silicon increases crop production, and may provide a scientific basis for the application of silicon fertilizer to the fields.
MATERIALS AND METHODS Cucumber seedling culture. Luvic cinnamon soil was used, with the following properties: pH 6.92, organic matter content 7.5 g·kg- I , total nitrogen content 1.51 g·kg- I , available phosphorus 7.76 mg·kg- ' , available potassium 140 mg·kg- I , available silicon 55.1 mg·kg- I (extracted with 1.0 moIL -I acetate buffer at pH 4.0, and determined by silicon molybdenum blue colorimetry, Nanjing Agricultural University 1981) and field capacity 15.29%. The basic fertilizer contained; NH4N0 3 , CaH ZP0 4 and K l S0 4 , and the silicon feltilizer KzSiO,. The amount of potassium contained in K2 Si0 3 was deducted from the application of K2S04 (Liang et aJ. 1999). After fertilizer application, the soil pH reached a value of 6.96, and there was no significant difference among the soil samples. Under open-air conditions, cucumber plants were cultured in pots 30 cm in diameter and each pot contained
2.5 kg soil samples. There were three groups of pots: the first group included 9 pots without silicon addition; the second group included 6 pots which were supplied with Si0 2 at the rate of 120 mg·kg-- I soil; and the third included 6 pots which were supplied with SiO l at the rate of 240 mg·kg- I soil. Even and full cucumber seeds (No.4 Jinchun) were selected and soaked in 1.0 g-L- 1 HgCl z solution for 10 min. After being washed with distilled water, the seeds were gelminated at 2YC for 18 h in a temperature-controlled light incubator, and then were sown (March 25, 2002) at the rate of 15 seeds per pot and the pots were covered with plastic film. The plastic film was taken off on April 2, and the seedlings were thinned out to 5 plants per pot. The seedlings were watered depending on the water content in soils and stress was imposed 60 d later. Drought stress treatment. The drought stress treatment lasted for 10 d, from May 26 to June 4 in 2002. The drought stress intensity was set as follows: control: 80-90% field capacity (FC), mild stress: 5570% FC, and severe stress: 35-50% Fe. From each of the three groups of pots mentioned above, 3 pots were used in the mild stress treatment, 3 pots in the severe stress treatment, and 3 pots left in the first group were used as control. Several days before the drought stress treatment, the water content of the soils was set at the pre-determined level. During the stress, we regulated the amounts of water in order to keep the water content in soils at the designed level of stress. The net photosynthetic rate, transpiration rate and stomatal conductance were measured on May 23, May 30, and June 4, which corresponded to the stage preceding drought stress, the middle stage of drought stress, and the later stage of drought stress, respectively. On May 25, just before the drought stress, and June 5, which corresponded to the later stage of drought stress, we measured the fresh weight, dry weight, and leaf relative water content of the plants, respectively. Chlorophyll and malondialdehyde (MDA) contents, penneability of the leaf plasma membrane and activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) were measured during the period from June 5 to June 6. Determination of plant physiological characteristics. The diurnal changes in the net photosynthetic rate, transpiration rate, and stomatal conductance were detennined using the CI-30l POItable Photosynthesis System (Cm Co., Vancouver, USA). The measurements were taken once every 2 h from 7:00 to 17:00. At the same time, the changes in the photosynthetically active radiation (PAR), air humidity, air temperature, leaf temperature, and CO 2 concentration in the air were recorded automatically (Fig. I). Chlorophyll was extracted out of leaves using a mixture of acetone and ethanol (1 : I). The extract absorbency was mea-
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Silicon Application and Drought Resistance in Cucumber Plants
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sured with a 722-spectrophotometer (Ruili Optical Instrument Co., Beijing, China) at 663 and 645 nm, and the chlorophyll content was calculated by the formula C T = 8.02A 661 + 20.21A MS ' Penneability of the plasma membrane of leaf was monitored by the electrolyte leakage method and expressed by the relative electric conductivity (Xue 1985). MDA content was detennined by the 2-thiobarbitmic acid (TBA) chromotest (Liu 1994). The POD activity was determined by the guaiacol method (Zhang 1990). The SOD activity was determined by the nitrotetrazolium blue chloride (NBT) photochemical reduction reaction (Liu 1994). The CAT activity was determined by the titration method (the amount of residual HP2 after CAT decomposition was titrated with Na 2S 20 3 ). The activity of the enzyme was expressed as the decomposition rate of HP2 (Shandong Agricultural College and Xibei Agricultural 1985). Statistical analysis. Net photosynthetic rate, transpiration rate in some critical periods of diurnal changes were tested using one sample I-test. Other physiological characteristics were tested using Duncan's multiple range test.
RESULTS Diurnal changes in net photosynthetic rate, transpiration rate, and stomatal conductance of cucumber plants before drought stress Figure 2A shows that there was no significant difference in the diurnal changes in the net photosynthetic rate between the silicon-supplied plants and control. However, in the silicon-supplied cucumber plants, the net photosynthetic rate was slightly higher than that of the control in the morning. Based on the cumulative value of diurnal photosynthesis (Table 1), the effects of silicon on the net photosynthetic rate were not significant without drought stress.
Since silicon reduced the stomatal conductance of leaves (Fig. 2C), so the transpiration rate of the siliconsupplied cucumber plants was lower than that of the control (Fig. 2B). The stomatal conductance and transpiration rate were not significantly affected by the amount of silicon supply. Based on the cumulative value of diurnal transpiration (Table 1), the transpiration rate decreased by 14.3% due to silicon application.
Diurnal changes in net photosynthetic rate, transpiration rate, and stomatal conductance of cucumber plants during drought stress Figure 3A shows that the diurnal changes in the net photosynthetic rate of cucumber plants with a silicondeficient status was represented by a single-humped curve under drought stress, and that the control and the silicon-supplied plants displayed a double-humped curve. The net photosynthetic rate was lower in the severe stress treatment than in the mild stress treatment, and was lower than in the control. The net photosynthetic rate of the silicon-supplied cucumber plants was higher than that of the silicon-deficient plants. Under mild stress, the amount of silicon supplied to the plants did not show a significant effect; under severe stress, however, the net photosynthetic rate increased with increase of the amount of silicon supply. The cumulative value of diurnal photosynthesis (Table 1) was 9.9% higher in the silicon-supplied cucumber plants than in the silicondeficient plants under mild stress, and in the plants with 120 and 240 mg·kg- I of silicon supply, the values were 10.3 and 20.2% higher than those in the silicon-deficient plants under severe stress, respectively. Figure 3B shows that the transpiration rate was lower in the cucumber plants under stress than in the control plants, and that the values were lower under severe stress than under a mild one. From 7:00 to 13:00, the transpiration rate of the silicon-supplied cucumber plants was lower than that of the silicon-deficient plants
c.c. MA et al.
626
(9:00 and 11:00, p < 0.05, t-test), and after 15:00 the values of the former were higher than those of the latter (15:00, p < 0.05, t-test). The data showed that silicon decreased the transpiration rate of cucumber plants. The increase of the transpiration rate after 15 :00 might be due to the low transpiration rate during the 7:00-13:00 period, which resulted in a high leaf water content. And it might account for the appearance of the diurnal changes in the net photosynthetic rate of the silicondeficient cucumber plants as a single-humped curve, while as a double-humped curve for the silicon-supplied plants. There were no significant differences in the transpiration of cucumber plants between the amounts of silicon supply. Based on the cumulative value of diurnal transpiration (Table 1), although the high water content of leaves resulted in a high transpiration rate in the silicon-supplied plants after 15:00, the cumulative values of
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diurnal transpiration were 4.6-8.3% lower than those of the silicon-deficient plants. Under drought stress, the changes in stomatal conductance coincided with those of the transpiration rate in the cucumber plants supplied with different amounts of silicon, indicating that the changes in stomatal conductance led to the changes in the transpiration rate (Fig. 3C). As in the case of the middle stage, the diurnal changes in the net photosynthetic rate of the cucumber plants with a silicon-deficient status were represented by single-humped curves, while the control and the siliconsupplied plants were similar and showed doublehumped curves at the later stage of drought stress (Fig. 4A). At the later stage, the changes in the net photosynthetic rate with the stress intensity and the amount of silicon supply were also similar to those of the middle
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