Zeolite and selenium application and their effects on production and physiological attributes of canola cultivars under water stress

Aplicación de zeolita y selenio y sus efectos en la producción y atributos fisiológicos de los cultivares de canola bajo estrés hídrico

Hossein Zahedi¹*, A. Hossein Shirani-Rad², H. Reza Tohidi-Moghadam³

¹ Department of Agronomy and Plant Breeding, Eslamshahr Branch, Islamic Azad University, Eslamshahr, Iran. * Author for correspondence. (hzahedi2006@gmail.com).

³ Department of Agronomy, Varamin-Pishva Branch, Islamic Azad University, Varamin, Iran.

Received: August, 2011.
Approved: May, 2012.

Abstract

To study the effects of zeolite and selenium (Se) application on production and physiological attributes of three canola (Brassica napus L.) cultivars under water stress, an experiment was conducted in two growing seasons (2006 and 2007), at Karaj, Iran. The experimental design was randomized complete blocks in factorial split plot with three replications, and three factors: 1) irrigation: complete and restricted at stem elongation stage; 2) zeolite: 0 and 10 t ha^-1; 3) Se (as sodium selenate): 0, 15 and 30 g L^-1. These treatments were randomized into Zarfam, Okapi and Sarigol canola cultivars. According to the results, Zarfam cultivar showed the lowest electrolyte leakage and the highest biological yield due to zeolite application and 30 g L^-1 Se. The highest chlorophyll content and seed yield was related to Okapi cultivars under the same treatments, while Sarigol was next. Zeolite and Se applications increased seed yield, but under water stress they did not change this variable. Furthermore, the biological yield increased due to Se and zeolite application, especially under water stress. Thus, application of 15 g L^-1 Se and 10 t ha^{- 1} zeolite may increase canola seed yield under water stress.

Key words: biological yield, Brassica napus L., water stress, cellular electrolyte leakage, selenium, zeolite.

Para estudiar los efectos de la aplicación de zeolita y selenio (Se) en la producción y atributos fisiológicos de tres cultivares de canola (Brassica napus L.) bajo estrés hídrico, se realizó un experimento en dos épocas de cultivo (2006 y 2007) en Karaj, Irán. El diseño experimental fue de bloques completamente al azar en parcelas divididas en un arreglo factorial, con tres repeticiones, y tres factores: 1) riego: completo y restringido en la etapa de elongación de los tallos, 2) zeolita: 0 y 10 t ha^-1, 3) Se (como sodio selenita): 0, 15 y 30 g L^-1. Estos tratamientos fueron distribuidos al azar en los cultivares de canola Zarfam, Okapi y Sarigol. Según los resultados, el cultivar Zarfam mostró la menor pérdida de electrolito y el mayor rendimiento biológico debido a la aplicación de zeolita y 30 g Se L^{- 1}. El contenido de clorofila y rendimiento de semilla más alto se relacionó con el cultivar Okapi con los mismos tratamientos, seguido de Sarigol. Las aplicaciones de zeolita y Se aumentaron la producción de semillas, pero bajo estrés hídrico no cambiaron esta variable. Además el rendimiento biológico aumentó debido a la aplicación de Se y zeolita, especialmente bajo estrés hídrico. Así, la aplicación de 15 g L^-1 Se y 10 t ha^{- 1} de zeolita pueden aumentar el rendimiento de semilla de canola bajo estrés hídrico.

Palabras clave: rendimiento biológico, Brassica napus L., estrés hídrico, pérdida de electrolitos celulares, selenio, zeolita.

INTRODUCTION

In the arid and semi-arid environment of Iran, rainfall and soil moisture are the most important factors affecting crop production. Water stress causes some undesirable biochemical and physiological changes in plants (Pattangual and Madore, 1999), significantly decreases biological yield and seed yield especially at flowering stages (Deepak and Wattal, 1995), and decreases yield in canola (Brassica napus L.) (Triboi-Blondel and Renard, 1999). The most important effect of environmental stress is cell membrane degradation which decreases membrane selective permeability and increases cellular electrolyte leakage. The amount of electrolyte leakage is measured as an index for determining stress intensity. Chlorophyll content has close and negative correlation with water stress, and chlorophyll measurement can be a useful index to explain stress intensity (Shen et al., 2008).

Membrane lipid peroxidation is due to the reactive oxygen species caused by water stress. Lipid peroxidation decreases cell membrane elective permeability (Basaga, 1989). Chloroplasts, mitochondria, and peroxisomes are intracellular generators of activated oxygen species, such as H₂O₂, superoxide and hydroxyl radicals in the plant cells (Salin, 1991). According to Yu et al. (1998) and Shen et al. (2008), research is focusing on the effects of calcium and potassium on the cell membrane stability and the increase in resistance to environmental stress. There are evidences based on the beneficial effects of selenium (Se) on plants, which is an important trace element for animals and plants. Selenium plays an important role in activity of enzymes such as glutathione peroxidase (Gladyshev et al., 1998), improves plant growth and increases antioxidant capacity (Seppanen et al., 2003).

Zeolites have a porous structure that can accommodate Na+, K+, Ca²+, Mg²+, and others cations. These positive ions are rather loosely held and can readily be exchanged for others in a solution. Zeolite application in the soil increases water retention capacity and acts as a chemical sieve allowing some ions to pass through, while blocking others (Mumpton and Fisherman, 1977). Zeolite, because of its high cation exchange capacity (CEC), decreases nutrient leaching, especially nitrate; thus, zeolite application in clay soils subjected to water stress can improve final yield via increase in soil water holding (Zahedi et al., 2009).

Selective absorption and controlled release of nutrients by zeolite helps plant growth under poor conditions (Ok et al., 2002). High CEC, selective absorption and structure stability, make zeolite a suitable substance as soil amendment to overcome water stress and fertilization optimize (Mumpton and Fisherman, 1977). Thus, the objetive of this study was to evaluate the effect of zeolite soil and Se application on growth, seed production, and some physiological canola attributes under conditions of water stress.

This study was conducted at an experimental field at Karaj, Iran (35° 59' N, 50° 75' E, and altitude of 1313 m) on three canola cultivars, Zarfam, Okapi, and Sarigol in 2006 and 2007 growing seasons. The yearly average precipitation (30 years long-term period) was 244 mm, mostly concentrated during the autumn and winter months (November to February). Before beginning the experiment, soil samples were taken to determine the physical and chemical properties.

A composite soil (clay loam) sample collected from 0-30 and 30-60 cm depth was air dried, crushed, and tested for physical and chemical properties (Table 1). Chemical fertilizers and zeolite were distributed on the soil and incorporated at a depth of 30 cm. The plots were 5 m long and consisted of six rows, 0.3 m apart. Between the blocks and main plots, an alley between 6 and 2.4 m was kept to eliminate all influence of lateral water movement. The canola seeds were disinfected and sown in early October (2006 and 2007). The distance between the plant rows was 30 cm and the plant density was 1 000 000 plants ha^-1 at sowing time. Irrigation was carried out uniformly in all plots until flowering stage. Non-stressed plants were irrigated after reaching 80-mm evaporation from Class A pan evaporation. Weeds were effectively controlled by hand.

Cellular electrolyte leakage assay

For this assay, five fully mature and expanded leaves of each treatment were clipped. The leaf disks were cut and immersed in 20 mL of manitole in test tube (-2 MPa osmotic potential). After 24 h of darkness the electrical conductivity of the samples was measured.

Chlorophyll content assay

Chlorophyll a (Chl a) and chlorophyll b (Chl b) were extracted and estimated according to the method of Lichtenthaler (1987). About 100 mg of each leaf was cut into tiny segments and kept in 10 mL of chilled 80 % acetone in a capped glass tube. After 48 h of extraction in dark at 4 °C, the leaf segments were further extracted for residual pigments. The contents of Chl a and Chl b were measured at 666 and 653 nm. Total chlorophyll concentration (Chl a+b) was Chl a + Chl b.

Experimental design and statistical analysis

The experimental design was randomized complete blocks with a factorial split plot arrangement of treatments in three replications. The treatments were: 1) irrigation (I): complete and restricted (I₂) at the stem elongation stage; 2) zeolite (Z): 0 (Z1), and 10 t ha^-1 (Z2); 3) Se, 0, 15, and 30 g L^-1 sodium selenate (S1, S2, and S3) at the initial silique stage. These treatments were applied on Zarfam, Okapi and Sarigol cultivars. Analyses of variance were carried out using the GLM procedure (SAS Institute, 2002), assuming that the residuals were random, homogenous and with a normal distribution about a mean of zero. Treatment means were compared using LSMEANS (p≤0.05).

Combined analysis of variance for 2006 and 2007 showed that the effect of year was not significant, except for seed and biological yields (Table 2). In addition, in most of the cases, interaction of years (Y) with treatments was not significant. It is worth mentioning that quadripartite interaction had significant effect on cellular electrolyte leakage and chlorophyll content (Table 2).

Analysis of treatments means (Table 3) showed that the highest electrolyte leakage was observed at Zarfam cultivar, and the highest chlorophyll content at Okapi cultivar. Seed yield was similar among the canola cultivars, while the highest biological yield was found at Zarfam cultivar. Under conditions of water stress, the highest and lowest electrolyte leakage occurred in Zarfam and Okapi cultivars, because of their differences in sensitivity to water stress, whereas the lowest chlorophyll content was related to Zarfam cultivar (Table 3).

Increase in electrolyte leakage represents cell membrane degradation as well as chloroplast destruction, which leads to chlorophyll content reduction (Kumar and Paul, 1997). Okapi cultivar showed the highest seed and biological yields and under all conditions zeolite application increased seed yield in the three cultivars and improved biological yield in Okapi (Table 3). Under full irrigation, Se application (15 g L^-1) and no zeolite, Zarfam cultivar showed the highest electrolyte leakage and the lowestchlorophyll content, while inverse results were observed in Okapi (Table 3).

Under the same treatment conditions, an increase in Se concentration increased chrophyll b content in Zarfam and Sarigol cultivars. In addition, zeolite application under full irrigation plus Se (30 g L^-1) increased the variables (Table 3). Under water stress, the highest electrolyte leakage and chlorophyll content was found in Zarfam cultivar, which suggest high resistance to water stress (Table 3).

Cultivars treated with 15 g L^-1 Se, full irrigation and zeolite did not change seed and biological yields. But 30 g L^-1 Se and full irrigation, increased biological yield of Zarfam. In contrast, 30 g L^-1 Se, full irrigation and zeolite increased seed yield of Okapi and Zarfam and caused the highest biological yield in Zarfam.

Under water stress the Sarigol cultivar produced the highest seed yield and the highest biological yield was observed in Zarfam cultivar; and water stress plus zeolite increased seed yield in Zarfam and Okapi and biological yield in Okapi. The highest harvest index was obtained from Okapi and Sarigol treated with water stress and 15 g L^-1 Se. The analysis of these results show that 15 g L^-1 Se plus water stress and zeolite had no significant effect on seed and biological yields. Water stress plus 30 g L^-1 Se caused the highest biological yield in Zarfam and the highest seed yield in Okapi (Table 3). The highest and lowest seed yields were obtained from Zarfam and Sarigol under water stress, zeolite, and Se application (Table 3).

Stem elongation, flowering, pollination, and seed filling are the most sensitive stages to water stress in canola (Thomas et al., 2004) and at these stages yield is decreased (Wright et al., 1995). It seems that zeolite application improves growth and seed yield by holding water into the soil. Positive effect of zeolite on plant height, number of branches, yield, and yield components can be due to a decrease in nitrogen leaching and increase in nitrogen availability (Polat et al., 2004). Nonetheless, plant response to water stress is variable and depends on stress intensity, and duration, and plant growth stage (Chaves et al., 2003).

Water stress decreases relative water content, chlorophyll, and cell membrane stability over the growing period (Chandrasekar et al., 2000). Increase in cellular electrolyte leakage on account of water stress is due to cell membrane degradation. Kumar et al. (1993) show that electrical conductivity in canola leaves is dependent on relative humidity and turgor potential, while this parameter is dependent on relative humidity in mustard (Brassica campestri), and under conditions of mild stress chlorophyllconcentration increases due to leaf area reduction. Mild water stress increases protoplasm concentration and decreases leaf extension, while severe stress inhibits chlorophyll synthesis completely (Kumar et al., 1993). Loss of cell water content increases chlorophyll concentration in leaves; the effect of water stress on chlorophyll content is erratic and it depends on environmental conditions and genetics of the plant (Ward et al., 1992). Increase in stress intensity leads to chlorophyll degradation (Kumar and Paul, 1997), hasty senescence, chloroplast breakdown and chlorophyll degradation (Lawlor and Leach, 1985). Kumar and Paul (1997) report that water stress at flowering and seed filling stage significantly decreases Chl a and Chl b. Furthermore, when soil water potential reaches to -1.5 MPa, the chlorophyll content decreases 82 % due to pigment degradation (Chandrasekar et al. , 2000).

Zeolite with high CEC acts as a sink for nutrient, such as ammonium, and thus improves plant growth, especially in sandy soils (Polat et al., 2004). Water can penetrate easily into the zeolite structure, and zeolite application increases soil water retention capacity (Rehakova et al., 2004). In addition, Se application increases relative water content and improves water uptake from roots (Kuznestsov et al., 2003).

CONCLUSIONS

Under all conditions of this study, Zarfam cultivar showed the highest electrolyte leakage and lowest chlorophyll content. Okapi cultivar had the highest chlorophyll content under full and limited irrigation. Seed yield decreased significantly as a result of water stress. According to these results, zeolite and selenium applications increased seed yields, but not under water stress. Biological yield increased due to selenium and zeolite application, especially under water stress.

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