Increasing human population demands more fuel supply causing the release of hazardous greenhouse gases that could be compensated by supplementing with renewable and environment-friendly alternatives such as biofuels. The argument against the use of food crops for biofuel production that it may cause food shortages can be countered by using feedstock outside the human food chain. This opens up the possibility of using halophytes, algae and photosynthetic bacteria as sources of the carbon neutral biofuel which can be produced sustainably without compromising conventional agriculture. In this review we assess the suitability of these non-food resources as bio-fuel alternates.

Alpinia zerumbet (Zingiberaceae) is a unique ornamental and medicinal plant primarily used in food ingredients and traditional medicine. While organic amendments such as biochar (BC) and compost (Co) have been demonstrated to improve plant productivity, no studies have examined their effects on the growth, physiology, and secondary metabolites of A. zerumbet. This study evaluated the impact of the amendment of BC, Co, or a biochar and compost mixture (BC+Co) on modifying and improving the growth, photosynthesis, antioxidant status, and secondary metabolism of A. zerumbet grown on sandy loam soil. The morpho-physiological and biochemical investigation revealed variation in the response of A. zerumbet to organic amendments. The amendment of BC and BC+Co significantly increased net photosynthetic rates of plants by more than 28%, chlorophyll a and b contents by 92 and 78%, respectively, and carboxylation efficiency by 50% compared with those grown in the sandy loam soil without amendment. Furthermore, the amendment significantly decreased plant oxidative stress, measured as leaf free proline and glycine betaine. Enzymatic antioxidant activity, total phenols, and flavonoids also varied in their response to the organic amendments. In conclusion, this study shows that BC and/or Co amendments are an efficient and sustainable method for improving the metabolite contents and reducing oxidative stress in A. zerumbet.

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Plant photosynthesis and biomass production are closely associated traits but critical to unfavorable environmental constraints such as salinity and drought. The relationships among stress tolerance, photosynthetic mechanisms, biomass and ethanol yield were assessed in Phragmites karka. The growth parameters, leaf gas exchange and chlorophyll fluorescence of P. karka were studied when irrigated with the control and 100 and 300 mM NaCl in a nutrient solution and water deficit conditions (drought, at 50% water holding capacity). The plant shoot fresh biomass was increased in the low NaCl concentration; however, it significantly declined in high salinity and drought. Interestingly the addition of low salinity increased the shoot biomass and ethanol yield. The number of tillers was increased at 100 mM NaCl in comparison to the control treatment. High salinity increased the photosynthetic performance, but there were no significant changes in drought-treated plants. The saturated irradiance (Is) for photosynthesis increased significantly in low salinity, but it declined (about 50%) in high salt-stressed and (about 20%) in drought-treated plants compared to the control. The rates of dark respiration (Rd) and compensation irradiance (Ic) were decreased significantly under all treatments of salinity and drought, with the exception of unchanged Rd values in the control and drought treatments. A-Ci curve analyses revealed a significant improvement in the Jmax, Vc, max, and triose-phosphate utilization (TPU) at lower salinity levels but decreased at 300 mM NaCl and drought treatments compared to the control. In the chlorophyll fluorescence parameters (Fv/Fm, maximum photochemical quantum yield of PSII, and Y(NO)), the non-photochemical yields were not affected under the salt and drought treatments, although an effective photochemical quantum yield (YII) and electron transport rate (ETR) were significantly enhanced in water deficit compared to control plants. P. karka regulates an efficient photosynthesis mechanism to grow in saline and arid areas and can therefore be used as a sustainable biofuel crop.