Germination responses to temperature and water potential in Jatropha curcas seeds: a hydrotime model explains the difference between dormancy expression and dormancy induction at different incubation temperatures

Background and Aims Jatropha curcas is a drought-resistant tree whose seeds are a good source of oil that can be used for producing biodiesel. A successful crop establishment depends on a rapid and uniform germination of the seed. In this work we aimed to characterize the responses of J. curcas seeds to temperature and water availability, using thermal time and hydrotime analysis, Methods Thermal and hydrotime analysis was performed on germination data obtained from the incubation of seeds at different temperatures and at different water potentials. Key Results Base and optimum temperatures were 14.4 and 30 degrees C, respectively. Approximately 20% of the seed population displayed absolute dormancy and part of it displayed relative dormancy which was progressively expressed in further fractions when incubation temperatures departed from 25 degrees C. The thermal time model, but not the hydrotime model, failed to describe adequately final germination percentages at temperatures other than 25 degrees C. The hydrotime constant, theta(H), was reduced when the incubation temperature was increased up to 30 degrees C, the base water potential for 50% germination, Psi(b(50)), was less negative at 20 and 30 degrees C than at 25 degrees C, indicating either expression or induction of dormancy. At 20 degrees C this less negative Psi(b(50)) explained satisfactorily the germination curves obtained at all water potentials, while at 30 degrees C it had to be corrected towards even less negative values to match observed curves at water potentials below 0. Hence, Psi(b(50)) appeared to have been further displaced to less negative values as exposure to 30 degrees C was prolonged by osmoticum. These results suggest expression of dormancy at 20 degrees C and induction of secondary dormancy above 25 degrees C. This was confirmed by an experiment showing that inhibition of germination imposed by temperatures higher than 30 degrees C, but not that imposed at 20 degrees C, is a permanent effect. Conclusions This study revealed (a) the extremely narrow thermal range within which dormancy problems (either through expression or induction of dormancy) may not be encountered; and (b) the high sensitivity displayed by these seeds to water shortage. In addition, this work is the first one in which temperature effects on dormancy expression could be discriminated from those on dormancy induction using a hydrotime analysis.