Sweet sorghum productivity for biofuels under increased soil salinity and reduced irrigation

Abstract

Sweet sorghum (Sorghum bicolor (L.) Moench.) is a drought-tolerant crop with high resistance to saline-alkaline soils, and sweet sorghum may serve as an alternative summer crop for biofuel production in areas where irrigation water is limited. A two-year study was conducted in Northern Greece to assess the productivity (biomass, juice, total sugar and theoretical ethanol yields) of four sweet sorghum cultivars (Sugar graze, M-81E, Urja and Topper-76-6), one grain sorghum cultivar (KN-300) and one grass sorghum cultivar (Susu) grown in intermediate (3.2 dS m⁻¹) or in high (6.9 dS m⁻¹) soil salinity with either low (120 mm) or intermediate (210 mm) irrigation water supply (supplemented with 142–261 mm of rainfall during growth). The soil salinity and irrigation water supply effects on the sorghum chlorophyll content index, photosystem II quantum yield, stomatal conductance and leaf K/Na ratio were also determined. The sorghum emergence averaged 75,083 plants ha⁻¹ and 59,917 plants ha⁻¹ in a soil salinity of 3.2 dS m⁻¹ and 6.9 dS m⁻¹, respectively. The most affected cultivar, as averaged across the two soil salinity levels, was the Susu grass sorghum emerging at 53,250 plants ha⁻¹, followed by the Topper-76-6 sweet sorghum emerging at 61,250 plants ha⁻¹. The leaf K/Na ratio decreased with decreasing irrigation water supply, in most cases, but it was not significantly affected by soil salinity. The dry biomass, juice and total sugar yields of sorghum that received 210 mm of irrigation water was 49–88% greater than the yields of sorghum that received the 120 mm of irrigation water. Sorghum plants grown in a soil salinity of 3.2 dS m⁻¹ produced 42–58% greater dry biomass, juice and total sugar yields than the yields of sorghum plants grown in a soil salinity of 6.9 dS m⁻¹. The greatest theoretical ethanol yield was produced by sweet sorghum plants grown in a soil salinity of 3.2 dS m⁻¹ with 210 mm of irrigation water (6130 L ha⁻¹, as averaged across cultivar), and the Urja and Sugar graze cultivars produced the most ethanol (7620 L ha⁻¹ and 6528 L ha⁻¹, respectively). Conclusively, sweet sorghum provided sufficient juice, total sugar and ethanol yields in fields with a soil salinity of 3.2 dS m⁻¹, even if the plants received 50–75% of the irrigation water typically applied to sorghum.

Introduction

Interest in sweet sorghum in Mediterranean environments is increasing because of the use of biofuels (from juice) and raw materials (from bagasse) for energy production (Barbanti et al., 2006). Sweet sorghum is characterized by high sugar content, mainly sucrose, fructose and glucose, in the juice of the stalks, from which ethanol can be easily produced and used as biofuel. For this reason, sweet sorghum has also become a popular energy plant throughout the world (Mastrorilli et al., 1999). Additionally, sweet sorghum biomass is used for fiber, paper, syrup and animal feed (Steduto et al., 1997).

Water is the principal limiting factor of crop production in many areas of the world. Salinity also causes great losses in agriculture by lowering yields of various crops. Land salinization is acute and widespread in Greece (Koukoulakis et al., 2000). In these areas, irrigation is needed to obtain maximal yield because decreasing the water supply by irrigation causes a significant reduction in seasonal evapotranspiration, aerial sorghum dry matter and grain yield (Berenguer and Faci, 2001).

Sweet sorghum grows in marginal areas because of its high tolerance to saline and drought conditions (Berenguer and Faci, 2001, Almodares and Hadi, 2009). Sweet sorghum has higher water-use efficiency than other summer crops under both well-watered and water-stressed conditions (Steduto et al., 1997). From the agronomic point of view, sweet sorghum is more environmentally friendly than maize because of its relatively low nitrogen needs (Barbanti et al., 2006) and water requirements (Mastrorilli et al., 1999). According to Almodares and Hadi (2009), sweet sorghum used for biofuel production may be an alternative crop to maize in marginal irrigated areas where the irrigation water is limited during crop development. Sweet sorghum has also been suggested as a good source for ethanol production because of its rapid growth rate, early maturity and high total energy value (Smith and Buxton, 1993). Moreover, sweet sorghum production is encouraged by new policies regarding nonfood crops in the European Union (Rexen, 1992).

Despite that the potential of sweet sorghum as an alternative energy crop has been emphasized (Smith and Buxton, 1993, Steduto et al., 1997), the ability of various sweet sorghum cultivars to grow under soil salinity and water deficiency field conditions has not been sufficiently determined. Screening sweet sorghum cultivars with the objective of meliorating saline soils are challenges to breeders, plant physiologists and agronomists. When considering the introduction of sweet sorghum cultivars, the acceptance of farmers and their willingness to integrate ecologically appropriate crops must be guaranteed. The motivation of farmers may increase if sorghum cultivars provide direct benefits, such as acceptable biomass and biofuel production, from land where other crops are unproductive (Almodares and Hadi, 2009).

In semiarid Mediterranean areas, the principal limiting factor of crop production is water applied as irrigation to maximize yields. Therefore, to achieve a better planning of available water resources and to establish irrigation strategies that optimize crop yield, it is necessary to know the crop response to a variable irrigation supply (Berenguer and Faci, 2001), especially in saline soils where water stress is increased (Ould Ahmed et al., 2008). The objective of this research was to assess the productivity of four sweet sorghum cultivars grown in intermediate (3.2 dS m⁻¹) or high (6.9 dS m⁻¹) soil salinity with either low (120 mm) or intermediate (210 mm) irrigation water supply under Mediterranean conditions. Sweet sorghum productivity was also compared with the productivity of one grain cultivar and one grass sorghum cultivar.