Resistência do algodoeiro à ramulose: avaliação de linhagens, indutores químicos, enzimas envolvidas na resposta de defesa e custo fisiológico da indução

Abstract

Cotton is the higher economically important plant fiber producer, however yield can be seriously reduced by ramulosis, caused by the fungus Colletotrichum gossypiivar. cephalosporioides, for that the main control management practice is the use of resistant cultivars. In the present work, it was assessed resistance to ramulose of cotton lines, the induction of resistance by chemical inducers, the activity of enzymes involved in the defense response and the physiological cost of induced resistance in cotton. The resistance evaluation to ramulosis of 34 cotton lines was performed on the basis of epidemiological components, initial disease index (IDI), final disease index (FDI), disease progress rate (DPR) and area under the disease progress curve (AUDPC). It was verified, also, the effect of disease resistance inducers acibenzolar-S-methyl (ASM), Agro-Mós® (AM) and sodium silicate (Si) in the cotton cultivars CNPA GO 2000 – 1256 (intermediate resistant) and CNPA GO 2002 – 7997 (susceptible). The inducers ASM and AM were sprayed 5 and 10 days before inoculation of the pathogen, by foliarspray, while Si was mixed to the susbstract. At 24 days after inoculation, plants were assessed for disease index (DI), AUDPC and percent control (PC). ASM and AM were also tested using three doses of each inducer and determined the activity of -1,3- glucanase, peroxidase and phenilalanine ammonia lyase (PAL), at five and ten days after inoculation of the pathogen. The physiological cost of induced resistance was assessed on the basis of plant response to the inducers ASM, AM and jasmonic acid (JA) in substracts fertilized with two levels of nitrogen. Plants were assessed for plant height (PH), internode length (IL), shoot fresh biomass (SFB), root fresh biomass (RFB), shoot dry biomass (SDB) and root dry biomass (RDB). It was also determined the activity of the enzymes PAL and peroxidase. The line CNPA GO 2002 – 7997 showed high susceptibility to disease, with higher FDI. It was obtained significative(P=0,05) correlations between the variables IDI, FDI and AUDPC. The analysis of Euclidian distance by UPGMA allowed the separation of lines in two groups, one of them, composed only by line CNPA GO 2002 – 7997, and all the others grouped in asecond one. It was not observed influence of the time of inducers treatment. Line CNPA GO 2000 – 1256 displayed lower DI for all disease resistance inducers compared to the control. The variable AUDPC did not present interaction between period, cultivar and inducer, although all inducers were significantly different from control. In general, no significantly difference could be detected among doses and control. It was verified the activity of -1,3-glucanase and peroxidase at 10 DAE. CNPA GO 2002 – 7997 showed considerable increase in -1,3-glucanase, while peroxidase activities only ASM- D1 to CNPA GO 2002 – 7997 and ASM- D3 to CNPA GO 2000 – 1656 showed significantly difference from control and increase enzymatic activity. No increase in PAL activity could be detected. ASM produced control of ramulose in cotton, increasing the levels of enzymes involved in the plant defense response, however showed high physiological cost, with accentuated reduction in PH, SFB and SDBfollowed by a higher activity of peroxidase.