Respostas fisiológicas e morfológicas de plantas de mandioca submetidas a um e dois ciclos de secamento do solo

Abstract

The occurrence of short droughts, during the growing season of cassava in Rio Grande do Sul, may cause several soil drying cycles which induce physiological disorders in plants, affecting cassava productivity in the state. The objectives of this study were to verify the physiological mechanisms used by plants during the first and second soil drying cycle, the recovery of leaf area in plants under water stress after rehydration, the response of plants under one drying cycle with two distinct physiological ages and the possibility of acclimation of plants under two drying cycles, in sequence. Two experiments were conducted with cassava plants, Fepagro RS 13 cultivar, inside a plastic house at Federal University of Santa Maria, RS, Brazil. Planting dates were on 29/09/2011 for EXP 1 and 24/11/2011 for EXP 2. Water regimes for the two experiments were RH1, without water deficit during the two periods (control treatment); RH2, with water deficit during the two periods (two cycling cycles); RH3, with one drying cycle on P1, matching with the first drying cycle on RH2 and RH4, with one drying cicle on P2, matching with the second drying cycle on RH2. FTSW is the portion of available water in the soil which plants use in the transpiration. The physiological mechanisms that stood out on P1 of RH2 and RH3 and P2 of RH4 were the reduction of total leaf area and stomatal control of cassava plants. On P2 of RH2, it was the reduction of total leaf area (without leaf senescence). RH3 plants showed fast recovering after rehydration on P2, with high NFE and CFA, regarding RH1. The drying cycles with low (RH3) and high (RH4) physiological age presented no difference on FTSW threshold for TR and CFR. The high CT of RH2, on P2 of EXP1, indicates that plants had lower activation of stomatal control during high DPV periods. Thus, plants of RH2 acclimated to the second drying cycle with FTSW threshold of 0,09 to CFR and 0,13 to TR, presenting late CFR and TR declines in comparison with drying cycles of RH3 and RH4.