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Matriconditioning (MC) of carrot seeds, with Micro-Cel E, improved their germinability by shortening the mean germination time and reducing the spread. Cytological studies indicated that the catabolic phase of germination sensu stricto advanced during the 8 d period of MC, but that the observed changes differed in embryo and endosperm tissues. Complete degradation of storage protein and lipid bodies, followed by starch accumulation were found to occur in the embryonic radicles of carrot seeds. In the endosperm, the catabolic changes were restricted to the micropylar area enclosing the radicle, where extensive breakdown of storage cell walls, partial degradation of protein bodies and no storage lipid hydrolysis were noted. The time course and localization of these changes suggested that weakening of the endosperm represented a secondary effect of priming, which was probably controlled and stimulated by the metabolically advanced embryonic axis. The benefits of seed matriconditioning have been explained on the basis of the fine structure observations. © 1997 Annals of Botany CompanyKey words: Daucus carota L., seed ultrastructure, matriconditioning, embryo/endosperm relation. INTRODUCTIONEnhanced speed and synchrony of germination and increased tolerance to low and high temperatures are the well known beneficial effects of various prehydration seed treatments Khan, 1992a). Like other Umbelliferae, carrot seed germination at harvest time is slow and irregular. Osmoconditioning (OC) of carrot seeds improves their germination performance by shortening the mean germination time and lowering the spread . Recently, conditioning of seeds with solid matrix carriers (matriconditioning, matric priming, MC) has been developed (Eastin, 1990;. Priming with the solid carrier, Micro-Cel E, synchronizes and accelerates germination of seeds (including carrot), more effectively than osmoconditioning. As with OC, the benefits of matriconditioning are retained after drying and re-hydration . In spite of the general acceptance of priming as a technique for improving seed quality, its physiological basis is not clearly understood. It is widely accepted that OC allows seeds to progress through the prolonged lag phase (phase II) of imbibition that prepares the radicle for protrusion, but prevents the start of phase III (radicle protrusion) . There are also reports of seed metabolic activity during priming in osmotic solutions, including synthesis of repair and replicative DNA, rRNA, proteins (Dell' Aquila and Bewley, 1989; and activity of certain enzymes involved in sugar, protein and lipid catabolism {Smith and .There are several indications that the following physiological mechanisms are involved in seed priming: (a) repair of the age-related cellular and subcellular damage that could accumulate during seed development and storage and (b) an advancement of metabolic events during the prolonged lag phase II (germination sensu stricto) of the water uptake .It has also been observed that small, endosperm-containing seeds (celery, car...

SUMMARYThe following sequence of structural changes which are related to the utilization of storage materials have been shown to occur in the apple embryo: degradation of protein bodies, degradation of lipid bodies, appearance and degradation of starch. These processes take place in the embryonic a.xis during stratification and in the cotyledons during the germination of stratified embryos. Important differences were observed in the localization of these processes during the early post-embryonic (up to 12 days) growth of dormant and stratified embryos. It is concluded that during the germination of dormant embryos the embryonic axis infiuences certain hydrolytic processes in cotyledons by delaying and inhibiting their progress, whereas in non-dormant embryos these are stimulated by the embryonic axis.