Respostas morfológicas, fisiológicas e bioquímicas de acessos de pinhão manso (Jatropha curcas L.) submetidos a estresse salino-sódico

Abstract

Jatropha curcas L. (Physic nut) is a woody oil-seed bearing species of the Euphorbiaceae family that has emerged as a potential source of biodiesel. This species is drought tolerant, with potential for cultivation in semi-arid environments on salt- and sodium-affected soils. However, to reach this objective, it is necessary a better understanding of the morphological, physiological and biochemical responses of physic nut to these adverse climate conditions. Aiming to contribute in this direction, a series of short experiments, under controlled environmental conditions, were conducted at the Soil Department of the Federal Rural University of Rio de Janeiro, Seropédica, RJ. Inter-accession evaluations were conducted in hydroponics, using a Hoagland and Arnon solution at full ionic strength, as determined in a preliminary test. Experimental conditions were as follow: photoperiod was 12 h of 450 µmol m-2s-1 (400-700) photon flux density, with air temperatures of 28/24 ºC for the day/night period. After a pre-treatment period in a sand bed and in greenhouse conditions, physic nut seedlings were transferred to plastic pots in the growth chamber and salinity treatments begun by adding NaCl to the basal nutrient solution. In the first assay, the accession 305 was treated with three levels of NaCl: 0, 50 and 150 mmol l-1 while in the second and third assays, the accessions 356 and 345 were treated with five levels: 0, 50, 75, 100 and 150 mmol l-1, respectively. A fully randomized experimental design with 4 repetitions (assays 1 and 3) or 5 repetitions (assay 2) was used. Plants were randomly distributed with positions changed daily. All the plants were harvested 15 days after beginning the treatment. At harvesting plants were separated into leaf blade + petiole, stems and roots, and the total leaf area was measured using image analysis. In the fresh leaf samples the relative water content (RWC), total chlorophyll, soluble sugars, free-amino acids and proline contents were determined. Ionic composition, evaluated in terms of Na+, K+, Ca 2+and Mg2+ contents were determined in the leaf, stems and roots from dry samples. The accessions exhibited different response patterns to increasing levels of NaCl in the nutrient solution. Using the relative yield of total dry matter as the first criterion of evaluation it was concluded that accession 356 was the most sensitive and 305, the most tolerant. The second criterion in importance was the leaf Na+ content and its associated Na/K ratio. Regarding the accumulation of organic compatible solutes it was concluded that increased sensitivity was associated with increased accumulation of free amino acids and proline. Finally, regarding the relative water content and total chlorophyll content of the leaf blade, these traits may be used only as complementary indicators, due to their low sensitivity to increases in NaCl concentration in the nutrient solution.