Assesing the sustainability of Sorghum bicolor as a biofuel crop grown in a low desert environment: Constraints on productivity and water use efficiency

Background/Question/Methods           

Purpose-grown biofuels have potential to reduce societal reliance on fossil fuels, but demonstration of sustainability must consider productivity, potential environmental impact, and efficient use of limiting resources. Here, we examine the sustainability of forage Sorghum (S. bicolor) in the Imperial Valley, low desert of CA, a high-heat, high-irradiance, and high productivity environment. We combine single leaf-level (LI 6400; net assimilation, A_n) and canopy-level (eddy covariance; net ecosystem exchange, NEE) measurements of carbon exchange along with pressure vessel determinations of leaf water potential, to investigate constraints on carbon assimilation in this extreme environment (T> 40 C and PAR > 2000 µmol m^-2s^-1). Responses of A_nto light and temperature curves were performed on upper and lower leaves, to examine photosynthetic capacity, potential environmental constraints, and distribution of activity throughout the dense canopy.  

Results/Conclusions

Sorghum exploited this high irradiance environment with substantial yields of about 50 tons ha^-1 with a high water use efficiency (40 kg ha^-1mm^-1) and high radiation use efficiency (0.03). Sorghum achieved high rates of A_n , exceeding 50 µmols CO₂ m^-2s^-1,  and NEE, exceeding 45 µmols CO₂ m^-2s^-1. A_n and NEE were significantly correlated with each other (r² = .75) and with photosynthetically active radiation, PAR (r²= .99) and stomatal conductance, g_s (r²= .95). Leaf to air vapor pressure deficit increased to 4.8 kPa and transpiring leaf water potential declined to -1.5 MPa, even under well irrigated conditions. Nevertheless, g_s was closely related to PAR (r²= .97) and to A_n, suggesting no hydraulic limitation on stomatal conductance, even at midday. A_n and g_s were not inhibited by temperatures above 40 C, although responses on the hottest days will be subject to more thorough investigation. Photosynthetic activity in Sorghum in hot, arid environments appears to be light limited. Light availability in this growing environment is high and often saturating until noon. Light availability generally decreases before peak midday temperatures are reached (at roughly 1400 hours), possibly alleviating potential heat stress by reducing photosynthetic activity, and thus g_s, prior to exposure to the most extreme daily temperatures. Sorghum  was not strongly inhibited by moderate water stress occurring under well irrigated conditions. Maintaining high productivity is a key determinant of sustainability in this extreme production environment and Sorghum exhibits high resource use efficiency by avoiding photosynthetic inhibition during peak periods of high summer temperatures and evaporative demand. These observations are positive indicators for a candidate biofuel feedstock production system.