A cana-de-açúcar no Rio Grande do Sul e suas interações com as baixas temperaturas

Abstract

The sugarcane, a thermophilic plant, has its growth, development and ecophysiology affected due to the rigorous winter of Rio Grande do Sul State (RS). Among the main response processes of the plants to this condition are the acclimatization to the cold and the lethal temperature of the plant. However, studies describing the ecophysiological response that sugarcane presents under such thermal conditions are rare. Therefore, the present thesis aims to determine the thermal conditions in frost nights and the response of sugarcane plants to cold in subtropical conditions, in Santa Maria, at Central region RS, Brazil. The work was divided in three thematic chapters. In the first chapter a bibliographical review was carried out for the culture in RS highlighting historical information, current situation and agroclimatic zoning. In the second chapter, the lower lethal temperature of the sugarcane cultivated in the central region of RS was determined for cultivars RB867515 and RB855156, besides performing an analysis of the thermal availability before and after nights with frost occurrence. In the third chapter was characterized the vertical profile of the minimum temperatures in nights of frost in sugarcane canopy. For the second and third chapter an experiment was carried out, with 2,200 m² cultivated with sugarcane in Santa Maria, during the period of 2013 to 2016. The apical meristems temperature of the sugarcane plants was monitored during the experimental period. In addition, the air temperature data of the automatic and conventional meteorological stations were used. It was verified the presence of sugarcane crop in RS since 1637 and according to studies of climatic zoning, the thermal availability, especially the high risk of frost, has always been one of the most limiting factors to sugarcane cultivation in this state. However, the inclusion of variables such as land use and land declivity in the last agroecological crop zoning, significantly interfered in the classification of the areas. In relation to lower lethal temperature, under field conditions, the cultivars were exposed to the air minimum temperature of the -0.2 °C, measured in the conventional station meteorological shelter; -0.6 °C, measured at 1.5 m height inside the canopy of sugarcane plants; and -1,5 °C, in the apical meristem, Measured in the palm heart region near the meristem; even though no damage to the apical meristem was observed in the three years of experiments. It is inferred that this cold tolerance is related to a possible acclimatization of the sugarcane plants to cold, due to the low thermal availability occurred in the days before and after the nights with frost formation, together with the lowest photoperiod of the year for the region. In the vertical profile of the minimum hourly temperatures in the nights favorable to frost formation, it is observed that absolute minimum occurs predominantly shortly before dawn and is observed in the upper third of the canopy of sugarcane plants.