Role of Silicon in Mitigating Effects of Deficit Irrigation on Production of Sorghum (Sorghum Bicolor L.) Under Arid Land Conditions

Growth of sorghum under irrigation water stress i.e. under reduction of irrigation water from 100% to 60% ETc resulted in significant reduction in growth components of sorghum. Plant height, number of tillers and total fresh forage yield decreased with decrease in irrigation water from 100% to 80% and 60% ETc. Folliar application of silicon mitigated and alleviated the harsh conditions created by drought stress on sorghum plant growth. Silicon when applied at concentration of 2 kg/ha with water stress level 2 (80%ETc) gave the maximum plant height (109.0 cm) and the maximum fresh forage yield (44.00 t/ha) compared to control treatment (100% ETc). Root volume was maximum (0.031 m3) under 60% ETc and 2 kg ha-1 Si treatments compared with control (100% ETc). For production of sorghum fodder under the arid land environmental conditions of Saudi Arabia deficit irrigation with (80% and 60% ETc) with application of 2 kg ha-1 silicon (Si) as foliar spray seems a reasonable option than irrigating the crop with 100% ETc due to scarcity of irrigation water in this country. It gives higher yields than 100% irrigation and optimum moister level in the root zoon.

INTRODUCTION Saudi Arabia demands for green forage is ever increasing due to the rapidly expanding livestock industry particular in dairy animals. Sorghum bicolor L forage crop may be one of the solutions to this problem for it is a drought resistant crop and can grow in limited irrigation water, to give very low cost fodder. (Marsalis et al., 2009). It is an animal fodder cereal crop and also human food belonging to Poaceae family and is cultivated in all around the world particularly in arid regions. Plants mostly suffer from drought stress more than from any other factor, and drought stress on crop production is more harmful as compared to other environmental stresses (Dreesen et al., 2012;Rollins et al., 2013). The decrease in rainfall events, continuous water losses, high ambient air temperature and bad irrigation practices during growing season causes drought stress in a specific region (Lobell et al., 2011;Vadez et al., 2011;Wahid et al., 2007;Trenberth, 2011). Leaf area index (LAI) and shoot to root ratio are greatly reduced due to drought stress which ultimately results in reduction of whole plant growth and development (Anjum, et al., 2011b). Due to drought stress, the diffusion of carbon dioxide from the atmosphere decreases in plants which causes in low photosynthetic activity (Keenan, et al., 2010). In plants, Si concentration varies from 0.1 to 10% of dry weight according to genotypes (Hodson et al., 2005). Si regulates the nutrients uptake in many crops particular under environmental stresses (Wallace, 1993;Ma and Takahasi, 2002). When applied, in proper amount, Si mitigates the drought stress and improves production. Sonobe et al., (2017) studied the positive effects of Si on sorghum root response to drought stress, they found that the roots of plants treated with Si absorbed more water compared with control which increased the dry weight and reduced the osmotic potential in the roots. Deposited Si in the plants prevents the compression of xylem during high transpiration rate under drought stress (Mitani and Ma, 2005). In a pot experiment silicon Si (ml/L of K2SiO3 per kilogram of soil) was used in two concentrations (100) and Si (200). Si (200) treatment improved chlorophyll content, leaf area index, root dry weight, leaf dry weight, shoot dry weight, total dry weight and specific leaf weight compared to untreated plants (Ahmad et al., 2011b). The application of Si enhances morphological characters and net yield in sorghum when applied under drought condition (Kaya et al., 2006). Coskun et al., 2016 examined the productive role of Si in many crop plants under drought and salinity stress, the results indicated that silicon mitigated drought and salinity stresses and increased the yield. This study investigates the mitigation role of silicon Si on the harsh environmental drought stresses in this arid land of Saudi Arabia.

II.
MATERIALS AND METHODS The research experiment was conducted in the Research Station of the Faculty of Meteorology, Environment and Arid Land Agriculture at Hada Al-Sham. The design used in this research was split plot design with main plots and subplots with three replications. The experimental area was divided into three main plots and three subplots. The irrigation was applied as main plots and Si was applied as subplots. The irrigation treatment levels are percentage of crop evapotranspiration's (ETc), and silicon treatments as (0 kg Si ha-1 1 kg Si ha-1 , 2 kg Si ha-1 ). Surrounding the experimental plots, a buffer of two meter (2m) wide was also cultivated with the same sorghum crop to make a fetch to reduce uneven weather and wind effects on the experimental plots. The total number of plots were twenty seven (3*3*3=27). The dimension of each experimental plot was (4m x 2m) in size with one meter clearance between each plot to prevent inter-plot water flow. The experimental area was irrigated then ploughed twice, and levelled.

Plots irrigation system
Four plastic tanks (6000 litters capacity of each) were installed, and all the tanks were interconnected and connected with irrigation pump. Bubbler irrigation system was used. Each plot was provided with eight bubblers of Rain Bird pressure compensating full circle trickle pattern 0.5 GPM discharge. The bubblers within each plot were mounted on two Poly Ethylene (PE) laterals pipelines of one inch size at one meter spacing. The bubblers were mounted using saddle of 1 inch*1/2 inch size (female threads) and six inch long PE risers. Each bubbler had built-in strainer to filter course sand and debris. Each PE lateral line supplied irrigation to three sub-plots running from east to west by applying same irrigation amount. The main and sub-main lines used were two inch PE pipe running from north to south outside the plots (flowmeter). Electrical pressure control valve was used at delivery of pump to deliver even pressure and prevent damage to the pump or lines while keeping bubblers' discharge constant. Flow rate was measured with seven bubblers in each plot and larger capacity pump and found to be very close to the standard discharge with small variation. Four pressure gauges were installed to check the operating pressure at pump, in the main line, in the sub main lines and in the laterals lines.
Method used in irrigation: The water balance irrigation scheduling method involves estimation of the soil water balance by measuring the amount of rainfall, irrigation and then estimating the soil water depletion by calculating water consumed by the crop. It is a flexible method and can forecast irrigation needs in future depending upon weather forecast. The procedure of water balance irrigation scheduling is as follows: AWSC = FC -PWP Where AWSC: is the available water storage capacity of the soil (mm/m).
PWP: is the permanent wilting point (volumetric moister content (%) (Lower limit). Evapotranspiration (ET) is the combination of two separate processes (Evaporation and Transpiration) whereby water is lost from the soil surface by evaporation and from the crops by transpiration.
Planting of sorghum: Sorghum fodder local variety Jizani was sown. After installation of irrigation system and levelling Sorghum crop was sown manually in all plots. The spacing between rows kept at 30cm. Seeds were placed at 5cm depth and then covered manually. Six rows were sown in each plot. Six rows were also sown as buffer area at the same time. Seed rate of 60 kg/ha was used.  (Table 1) indicated highly significant differences in results of the studied growth parameters of sorghum ( number of tillers per square meter, plant height, fresh forage yield, and root volume) between irrigation treatments (100%, 80% and 60% of ETc) and Si foliar treatments (0, 1 and 2 kg/ha) at (P≤ 0.01) during all cuts.

Effect of irrigation and Si levels on number of tillers/m2 of Sorghum crop:
Analysis of variance: Table (1) illustrates that during first cut no significant differences were observed by irrigation treatments on number of tillers per m2 in sorghum, but there were significant differences by irrigation treatments and Si treatments on number of tillers during second and third cuts. Means comparisons of number of tillers/m2 of sorghum crop.
Number of tillers per square meter was not affected very much by Si during the first cut. While during 2nd and 3rd cut the decrease in irrigation amount from 100% to 60% of ETc reduced the number of tillers per square meter by 11.2 % but tiller number increased by 10.11% by application of Si from 0-2 kg/ha, ( Table 2). The interaction of irrigation and Si treatments shows no significant effect on number of tillers per square meter during all cuts.  16%) while increased the level of Si from 0-2 kg/ha increased them from 97.00 cm to 112.33 cm (13.64%), the same trend was followed during 2nd cut, the decrease was 15.86% and increase was 13.56%, during 3rd cut the decrease was 15.43% and increase was 13.93% (Table 3).

Effect of interaction between irrigation and Si levels on fresh forage yield of Sorghum crop:
The interaction of irrigation and Si (Irr.*Si) levels on fresh forage yield was also significant (P≤ 0.05) during 1st cut while it was non-significant (P≤ 0.05) during 2nd and 3rd cut. Interaction between irrigation and Si treatments means ( Table 6) showed within same irrigation levels, the increase in Si dose increased the fresh forage yield (t/ha) and the decrease in irrigation levels (increase drought stress) decreased the fresh forage yield according to LSD, compared with control during all cuts. For the 1st cut the maximum fresh forage yield was 52.33 (t/ha) while it was 47.10 (t/ha) for the 2nd cut and 51.33 (t/ha) for the 3rd one. In the 1st cut, compared with controls, the plants treated with Si treatment two (2kg Si ha-1) and irrigation treatment one (80%ETc)

Effect of irrigation and Si levels on root volume of sorghum crop:
Means comparisons of root volume of Sorghum crop: Results of table (7) shows that decrease in irrigation level from 100% to 60% of ETc increased the root volume, and also increase in Si dose from 0 to 2 kg/ha also increased the root volume (Table 7).

Interaction effect of Irrigation and Si levels on the root volume of Sorghum crop.
The interaction of irrigation and Si (Irri*Si) levels and its effect on root volume of sorghum forage plant is significant (P≤ 0.05). According to the values of the interaction between irrigation and Si treatments means (Table 8), it is clear that within each irrigation level, the increase in Si dose resulted in an increased in the root volume, and also the decrease in irrigation level (increase drought stress) within each Si treatment level increased the root volume according to LSD compared with control. The maximum root volume (0.030 m3) was observed in the highest deficit irrigation (60% ETc) and 2 kg ha-1 Si dose. With passage of time, more Si was accumulated in the plants causing lateral growth in the roots. The root depth was almost same in all plots which increased gradually. The increase in root volume increased the water uptake and consequently the studied crop parameters were improved. The increase in root volume also reduced moisture level in the soil near harvesting due to more water uptake efficiency. . The foliar application of silicon increased number of tillers per square meter (almost 1.5%), plant height (14%), fresh yield (25%) and root volume as compared to control. During 2nd cut number of tillers per square meter increased as compared with 1st cut and the trend started decreasing during 3rd cut. The reason of more branching in Si treated plots is probably due to more water uptake efficiency of Si, nature of ratoon crops and increase root depth of sorghum. These results agree with previous studies on tillering effects by application of Si on crops (Tabassam et  VI. CONCLUSION Growth of sorghum under irrigation water stress i.e. under reduction of irrigation water from 100% to 60% ETc resulted in significant reduction in growth components of sorghum. Plant height, number of tillers and total fresh forage yield decreased with decrease in irrigation water from 100% to 8% and 60% ETc. The foliar application of silicon on sorghum leaves increased number of tillers per square meter (almost 1.5%), plant height (14%), fresh yield (25%) and root volume as compared to control in all cuttings. Foliar application of silicon mitigated and alleviated the harsh conditions created by drought stress on sorghum plant growth. Silicon when applied at concentration of 2 kg/ha with the water stress level 2 (80%ETc) gave the maximum plant height (109.0 cm) and the maximum fresh forage yield (44.00 t/ha) compared with control treatment (100% ETc). Root volume was maximum (0.031 m3) under 60% ETc and 2 kg ha-1 Si treatments compared with control (100% ETc). For production of sorghum fodder under the arid land environmental conditions of Saudi Arabia deficit irrigation with (80% and 60% ETc) with application of 2 kg ha-1 silicon (Si) as foliar spray seems a reasonable option than irrigating the crop with 100% ETc due to scarcity of irrigation water in this country. It gives higher yields than 100% irrigation and optimum moister level in the root zoon.