Mecanismos fisiológicos associados ao papel da enxertia na resistência de mudas de cajueiro expostas a salinidade

Abstract

Soil salinity can affect plant metabolism due to its primary effects, such as water deficit, ionic toxicity and nutritional imbalance, as well as secondary disturbances associate to oxidative damages. Currently the techniques for correction of soil salinity are temporary solutions and require high investments. Thus, an effective alternative for agricultural exploitation in areas prone to salinization is the cultivation of species and/or genotypes that are more resistant to salinity. For perennial crops, such as cashew, one possibility for increase salt resistance is the use of grafts and/or rootstocks that are more resistant to salt stress. The present study aim evaluate the role of genotypes from scion and rootstock of dwarf cashew tree on physiological mechanisms involved with the K+/Na+ ion balance, photosynthetic efficiency and oxidative protection under salinity conditions. The experiment was carried out in a 4 x 3 factorial arrangement, with four scion/rootstock combinations (CCP 76/CCP 76, CCP 76/CCP 09, CCP 09/CCP 09 and CCP 76/CCP 09) and three NaCl levels (0, 50 and 100 mM) applied by the nutrient solution. Were evaluated plant growth, Na+ and K+ contents, gas exchange and photochemical activity, as well as indicators of damages and protection oxidative. Results show that of evaluated genotypes the scion CCP 09 presented higher sensibility to salt stress, as indicated by a strong leaf-drying associate to low K+ content. Rootstocks CCP 09 and CCP 76 presented a K+ content reduced in roots tissue under salinity only when were grafted with scion CCP 09, suggesting that these rootstock can be not used in this combination for growth in salinized areas. Photosynthetic efficiency was more dependent on the graft/rootstock combination than on the cashew-isolated genotypes, as shown by the higher PN, gS and PN/Ci ratio observed only in the CCP 76/CCP 09 combination. On the other hand, all combinations showed a low photochemical activity associated with an apparent oxidative protection under salinity. This response was corroborate by relative stability in H2O2 and TBARS content, as well as activity of the SOD, APX e POX enzymes, but a slight increase in content of the ASC and GSH non-enzymatic antioxidants. This photo-protection observed for all combinations evaluated, indicated that this characteristic is present in specie and did not depend from scion or rootstock. Together, the data show characteristics associated both salt-sensitivity and salt-resistance in the grafted cashew seedlings. Some these features should be attributed to scion genotype, others to rootstock as well as the scion/rootstock interaction, besides of those not dependent of the genotypes and apparently specie specific. These results display part of the complexity of the physiological interaction between scion and rootstock in the grafting process, mechanisms still little known, mainly under salinity conditions.