Caracterização fisiológica e influência da época de plantio no florescimento de cultivares de cana-de-açúcar

Abstract

Brazil is the largest sugarcane producer in the world, and this extremely important crop gives rise to significant social, economical and environmental implications for the country. Brazil is also the largest industrial manufacturer of sugarcane alcohol and thus occupies a strategic position in terms of supply of this alternative and renewable source of energy. Owing to its unrivalled agricultural and industrial productivity, and lower production costs in comparison with other countries, Brazil is a competitive exporter of both sugar and alcohol, and exerts a strong influence on the international price of these products. The aim of the present work was to determine the photosynthetic activity and the levels of nutrients and soluble carbohydrates in four sugar cane cultivars, with different flowering cycles, planted at three different times of the year in Serra do Ouro (State of Alagoas, Brazil). The experimental design constituted a 4 x 3 factorial random arrangement involving four sugarcane cultivars (F150, RB72454, RB842021 and RB92579) and three planting periods (20th July 2005, 20th August 2005 and 20th September 2005 ). For each crop, the chlorophyll content, the gas exchange and the total number of flowering stalks were determined XIV from March 2006 until the end of the flowering period. The nutrients present in the cortex of the stalk were also analysed prior to flowering. In terms of flowering, the smallest response to planting earlier than normal (i.e. in July rather than September) was presented by the F150 cultivar, whilst the RB92579 cultivar exhibited the greatest response. In general, the non-flowering cultivar RB842021 showed lower stomatal conductance, photosynthetic rate, carbon fixation efficiency and transpiration compared with the flowering cultivars. The highest chlorophyll content determined in the F150 cultivar may have contributed to its superior photosynthetic rate compared with the other cultivars and also for its premature flowering. The concentration of most macronutrients decreased according to the time of flowering, i.e., the concentration was higher in the premature cultivars compared with the late cultivars. This was not the case for Ca and S, however, since the concentration of these two elements was reduced in the premature cultivar suggesting participation of Ca in the early onset of flowering. The concentration of the micronutrients Zn, Fe and Mn also decreased with the time of flowering, with the late cultivar presenting the smallest values. After flowering the concentration of soluble carbohydrates in cultivars F150, RB72454 and RB92579 increased by 19.74%. A balanced content of nutrients may favour flowering in sugarcane cultivars. Word key: Gaseous changes, nutritious e carbohydrate.