Impact of ascorbic acid on seed yield and its components of sweet corn (Zea mays L.) under drought stress

Introduction

Drought stress is regarded as the main growth-restricting factor for plants, especially in arid and semiarid areas where plants are often exposed to periods of water deficit stress which is one of the main reasons for the crop loss in the world (Anjum et al. 2017). Drought responses in plants are complex, and it is well documented that drought stress damages multiple physiological and metabolic processes (Witt et al. 2012). Drought stress reduces grain yield in the corn plant. However, the yield reduction depends on the stress intensity, duration, and incidence at the crop stage (Talaat et al. 2015). Drought occurring two weeks before and during silking phase decreases seed setting and kernel size, which causes yield losses of about 20-50% (Moharramnejad et al. 2019; Zarabi et al. 2011).

To alleviate deleterious effects of drought and to improve the growth of stressed plants in relation to varying physiological and biochemical characteristics, different strategies have been adopted. The exogenous use of chemicals as foliar spray or pre-sowing seed treatment is regarded as the most appreciable strategy. These compounds are absorbed by plants when applied exogenously (Noman et al. 2015). One approach for inducing oxidative stress tolerance would be to increase the cellular level of enzyme substrates such as ascorbic acid (AsA). AsA is a small, water-soluble antioxidant molecule which acts as a primary substrate in the cyclic pathway of enzymatic detoxification of hydrogen peroxide. AsA is the first substance in detoxification and neutralizing of superoxide radicals (Noctor and Foyer 1998). Application of ascorbic acid as an antioxidant can decrease the harmful effects of abiotic and biotic stresses (Shalata and Neumann 2001; Pignocchi and Foyer 2003; Noman et al. 2015).

The goal of current study was to investigate the effect of application of foliar ascorbic acid in increasing the tolerance of sweet corn to drought stress and improving the grain yield under this stress.

Materials and Methods

The sweet corn seed for this experiment (Hybrid Chase) was purchased from the Seminis Company. The experiment was conducted as a split-plot experiment based on randomized complete block design with three replicates under field condition in the Agricultural Research Station of Ardabil during 2016 growing season. Before the commencement of the experiment, soil samples were taken to determine the physical and chemical properties. A composite soil sample was collected at a depth of 0-30 cm. It was air dried, crushed, and tested for physical and chemical properties. The experimental area had a silty loam soil. The main plots were four levels of drought stress as irrigation after 70 (as control), 100, 130 and 160 mm evaporation from the class A pan, and the sub-plots were four levels of foliar ascorbic acid (AsA); (0 as the control, 150, 200 and 250 ppm). Ascorbic acid was sprayed on the sweet corn plants at the 4 to 6-leaf stage after general irrigation of the field, while the drought stress levels were applied until harvest.The experimental plots were four rows of 3-m long with 0.75 m spacing and within row spacing of 0.25 m between hills. Three seeds were planted in a hill and thinned to two plants per hill three weeks after sowing to attain a population density of 60,000 plants per hectare. The measured traits included ear length, number ­­of kernels per ear, number of kernel rows per ear, number of kernels per row, dry weights of husks, 300–kernel weight and grain yield.

Statistical analysis

Statistical analysis of data included the test of assumptions of normality of errors and homoscedasticity, analysis of variance and comparison of means using SPSS software. Means were compared by the Tukey^’s test at p≤ 0.05.

Results and Discussion

Analysis of variance showed that the interaction of drought stress with AsA was significant for number of kernels per ear, number of kernel rows per ear, number of kernels per row, 300-kernel weight and grain yield in sweet corn. The interaction of drought stress with AsA was not significant for husk weight and ear length, however, the effect of drought stress on these traits was significant (data not shown).

According to Figure 1, drought stress significantly reduced number of kernels per ear, especially at 160 mm evaporation from the class A pan.  The highest number of kernels per ear was observed at the normal conditions. Golbashy et al. (2010) and ELSabagh et al. (2015) reported on the deleterious effects of drought stress on number of kernels per ear. It seems that drought stress reduced pollen number in sweet corn and/or anthesis-silking interval, which has caused the lack of fertilization of uppermost ovaries on the ear. On the other hand, foliar application of AsA increased the number of kernels per ear at all irrigation levels. Although at normal conditions, the foliar application of AsA was effective at all levels (150, 200, and 250 ppm), but at 130 and 160 mm evaporation from the class A pan, AsA it was effective only at 200 and 250 ppm levels as compared to the control (0 ppm).

Number of kernel rows was not significantly affected by the drought stress, however, the lowest value belonged to the 160 mm evaporation from the class A pan (Figure 2). Sheikhi et al. (2013) indicated the decrease in number of kernel rows due to drought stress. The response of number of kernel rows to foliar application of AsA was erratic. At 70 mm evaporation from the class A pan, foliar application of 200 ppm AsA significantly increased number of kernel rows, however, at 70 mm evaporation from the class A pan it was effective at the dose of 150 ppm.

The combined effect of drought stress and foliar application of AsA on number of kernels per row is shown in Figure 3. Although drought stress decreased number of kernels per row at all stress levels, but it's effect was only significant at 160 mm evaporation from the class A pan. Sabagh et al. (2015) reported that number of kernels per row decreased under drought stress. Some levels of AsA also increased number of kernels per row at 70, 100 and 160 mm evaporation from the class A pan (Figure 3).

According to Figure 4, drought stress significantly reduced 300-kernel weight at all stress levels as compared to the control (70 mm evaporation from the class A pan). The seed weight depends on the photosynthetic capacity of the plant and remobilization of assimilates from the stems. Also, the rate and length of grain filling determines the weight of seeds. In the experiments conducted by Campos et al. (2004) and Echard et al. (2006) on maize, it was shown that drought stress limited the storage of assimilates in the stems and eventually decreased seed weight. Banziger et al. (2002) and Recap (2004) also reported the reduction of seed weight under drought stress conditions. All levels of AsA significantly improved 300-kernel weight at 100 and 160 mm evaporation from the class A pan but at 130 mm evaporation, only the foliar application of 200 ppm AsA significantly increased the 300-kernel weight (Figure 4).

All drought stress levels (100, 130 and 160 mm evaporation from the class A pan) substantially reduced husk weight as compared to the control (Figure 5). Emam and Ranjbar (2001) reported the reduction of ear dry weight under drought stress.

Drought stress significantly affected the ear length  at  all   stress   levels   compared   with  the

Figure 1. Combined effect of drought stress and foliar ascorbic acid (AsA) on number of kernels per ear in sweet corn; b1: 0 ppm, b2: 150 ppm, b3: 200 ppm and b4: 250 ppm AsA.

Figure 2. Combined effect of drought stress and foliar ascorbic acid (AsA) on number of kernel rows per ear in sweet corn; b1: 0 ppm, b2: 150 ppm, b3: 200 ppm and b4: 250 ppm AsA.

Figure 3. Combined effect of drought stress and foliar ascorbic acid (AsA) on number of kernels per row in sweet corn; b1: 0 ppm, b2: 150 ppm, b3: 200 ppm and b4: 250 ppm AsA.

Figure 4. Combined effect of drought stress and foliar ascorbic acid (AsA) on 300-kernel weight of sweet corn; b1: 0 ppm, b2: 150 ppm, b3: 200 ppm and b4: 250 ppm AsA.

Figure 5. Effect of drought stress on dry weight of husks in sweet corn.

control (Figure 6). Thus the highest ear length belonged to 70 mm evaporation from the class A pan as the control. According to the results obtained from Talaat et al. (2015), the cell growth declined under stress conditions.

Combined effect of drought stress and foliar application of AsA on grain yield of sweet corn are presented in Figure 7. All water-stress levels significantly reduced sweet corn yield. Sabagh et al. (2015) also reported the reduction of maize yield and its components under drought stress as compared with the normal irrigation conditions. Similar studies have reported the adverse effects of drought stress on maize (Islam et al. 2011; Koksal 2011, Barutcular et al. 2016). Obviously, drought stress has reduced the final grain yield by limiting the yield components.

Figure 6. Effect of drought-stress levels on ear length of sweet corn.

Figure 7. Combined effect of drought stress and foliar ascorbic acid on grain yield of maize; b1: 0 ppm, b2: 150 ppm, b3: 200 ppm and b4: 250 ppm AsA.