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 °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 °C. The thermal time model, but not the hydrotime model, failed to describe adequately final germination percentages at temperatures other than 25 °C. The hydrotime constant, θ(H), was reduced when the incubation temperature was increased up to 30 °C, the base water potential for 50 % germination,Ψ(b(50)), was less negative at 20 and 30 °C than at 25 °C, indicating either expression or induction of dormancy. At 20 °C this less negative Ψ(b(50)) explained satisfactorily the germination curves obtained at all water potentials, while at 30 °C it had to be corrected towards even less negative values to match observed curves at water potentials below 0. Hence, Ψ(b(50)) appeared to have been further displaced to less negative values as exposure to 30 °C was prolonged by osmoticum. These results suggest expression of dormancy at 20 °C and induction of secondary dormancy above 25 °C. This was confirmed by an experiment showing that inhibition of germination imposed by temperatures higher than 30 °C, but not that imposed at 20 °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.

Phenological responses to temperature of an annual and a perennial Lesquerella species.

BACKGROUND AND AIMS: The annual Lesquerella fendleri, native to the south-western desert of United States and Mexico, and the perennial L. mendocina, native to Argentina, may have potential as new crops for cold-arid environments. The introduction of a new crop requires an understanding of environmental influences on growth and development, particularly temperature, which has been recognized as the main factor affecting the rate of development in crops. The objective of this study was to examine differences in the phenology of L. fendleri and L. mendocina and in the response to temperature in both vegetative and reproductive phases. METHODS: Plants of each species were grown at a range of constant temperatures under controlled conditions and developmental responses were analysed and quantified. KEY RESULTS: The rate of development of L. fendleri increased linearly with temperature in the phase from emergence (EM) to floral bud appearance (FBA) over the range 9-20 degrees C, and for the phase from FBA to first flower open (FL) over the range 9-24 degrees C. In contrast, the rate of development of L. mendocina was insensitive to temperature during the phase EM to FBA. In the phase FBA to FL, L. mendocina had a lower sensitivity to temperature than L. fendleri. In addition, L. fendleri exhibited a quantitative response to supra-optimal temperatures (reducing rate of development with further increases in temperature) whereas L. mendocina showed a qualitative response, with development ceasing to progress at temperatures above the optimum. CONCLUSIONS: This differential behaviour at high temperatures could explain the biennial habit found for L. mendocina sown during late spring under field conditions, whereas it behaves as an annual when sown in autumn-winter. The possibility is discussed of using this information for establishing the coincidence of critical stages with environmental conditions that can limit potential and actual yield through agronomic practices.