THE EFFECT OF MOISTURE-CONTENT ON THE LOW-TEMPERATURE RESPONSES OF ARAUCARIA-HUNSTEINII SEED AND EMBRYOS

The effect of moisture content on the low temperature responses of 're calcitrant' Araucaria hunsteinii seed and excised embryos was investig ated in relation to germination and storage. Ar 6 degrees C seed longe vity was decreased as mean moisture content was reduced from 45 to 30% (fresh weight basis); the time predicted for the loss of one probit o f germination, sigma, fell from about 2000 to 20 d. The estimated base temperature for germination rate, T-h, was near 6 degrees C. Germinat ion occurred during storage at 6 degrees C unless the initial seed moi sture content was below about 45%. Embryo freezing point decreased fro m -6 to -19 degrees C as moisture content was decreased from 44 to 27% . The unfrozen water content of the embryo was estimated by thermocoup le and differential scanning calorimetry methods to be around 25%. Emb ryos exhibited reduced growth in vitro following drying below the mois ture content range 34-41%, equivalent to water potentials below about -4 MPa. Moisture was unevenly distributed among component tissues of t he embryo; in undried material, the radicle had a higher moisture cont ent than the hypocotyl and the hypocotyl was in turn moister than the cotyledons. Individual embryo tissues completely lost the ability for organized growth when dried below 25% moisture content, although some cells survived, as shown by callus production from the radicle. This t ype of survival on drying was also seen in embryos after their subsequ ent cryopreservation in liquid nitrogen and after 4 years storage at - 20 degrees C within 'non-germinable' seeds. Practical storage conditio ns for seeds and embryos are considered in relation to these findings. (C) 1995 Annals of Botany Company