Physiological Assessments of Sweet Sorghum Inoculated with Azospirillumbrasilense according to Nitrogen Fertilization and Plant Growth Regulators

Sweet sorghum (Sorghum bicolor Moench) is the name given to the sorghum plant able to produce and accumulate sucrose in the stem, reaching amounts close to those of the sugarcane. 

The sorghum belongs to the family Poaceae. It is believed that its center of origin lies on the region where Sudan and Ethiopia are located in the African continent and that; apparently, it was disseminated worldwide by the native Africans who were smuggled into slavery. Nowadays, it is grown in most tropical and subtropical regions and is the fifth most produced cereal worldwide (MOREIRA, 2011).

 Sorghum is considered a positive energy balance culture and stands out as a promising raw material to generate bioenergy, due to its high fermentable fiber and sugar contents, which may be exploited on a large scale and with great adaptability to different weather and soil conditions (SANTOS et al., 2014).In addition, the short production cycle (six months for high biomass yield) and the low water demand, in comparison to that of the sugarcane and corn, favor the sorghum, which shows better energy efficiency (NAGAIAH et al., 2012; SERNA-SALDÍVAR et al., 2012). 

In addition to the sucrose production by sorghum, it can be said that some factors, such as productivity increase and crop production cost reduction, are still to be assessed as a way to improve the sweet sorghum success prospects. Among such factors, the high cost of fertilizers is a bottleneck that narrows the cost/benefit ratio in the sweet sorghum cultivation and in the sugar and alcohol generation (MOURA et al., 2005).

Although nitrogen is abundant in the atmosphere, the plants metabolize it just in its nitric and ammonia forms. Nitrogen fertilizers are expensive, and the biosphere reserve comes from the atmosphere.It cannot be directly assimilated by plants in its molecular form; however, the biological N fixation processes by prokaryotic organisms make such assimilation possible. The proliferation of soil bacteria adhered to the plant root surface was discovered in the early nineteenth century, along with the discovery of nitrogen fixation (MOREIRA, 2007). 

Thus, the use of nitrogen-fixing bacteria benefited the yield of several crops such as soybean (Glicine max (L.) Merrill). Therefore, whenever nitrogen-fixing bacteria are inoculated in seeds, there is potential release nitrogen of approximately 150 kg N ha-1throughout the crop cycle. Another advantage is the nutrient availability according to the needs of each stage of the cycle, which increases the plant metabolic efficiency and productivity (ARAÚJO; HUNGRIA, 1999). 

Plant growth regulators are substances or a mixture of substances that may be directly applied to the plants in order to enable changes in the vital and structural processes, as well as to allow increasing the sucrose levels, the maturation time and the productivity (CASILLAS et al., 1986).

 It is known that the sweet sorghum crop has been gaining prominence in the sucrose production chain, despite the high consumption of nitrogen fertilizers, and that the Azospirilum brasilense bacteria provides low-cost nitrogen, and even that the use of plant growth regulators is able to improve plant metabolism. Therefore, it is necessary assessing the effects of the inoculation with Azospirillum brasilense and of different doses of nitrogen fertilizer, as well as of the plant growth regulator Stimulate®, on the plant photosynthesis and on the total recoverable sugar production. It is also necessary assessing the relation between the plant growth regulator doses at different nitrogen fertilization levels and the behavior of phenological features such as the leaf area index and the dry mass weight of plants.

 Thus, the hypothesis of the current study is that the physiological action of the plant growth regulator Stimulate®-based product, in interaction with the nitrogen fertilization, affects the photosynthetic metabolism by increasing the total recoverable sugar production in sweet sorghum plants inoculated with Azospirillum brasilense.

 The aim of the current study is to assess the physiological responses of sweet sorghum plants inoculated with Azospirillum brasilense to nitrogen fertilization and to plant growth regulators.