ANALYSIS OF EMBRYO GROWTH IN MATURE FRUITS OF CELERY (APIUM-GRAVEOLENS)

A celery 'seed' is a schizocarpic fruit and consists largely of endosperm surrounded by a thin testa and a thick pericarp. Embryo growth in fruits of celery (Apium graveolens L. var. rapaceum [Miller] D.C.) cv. Monarch with low and high densities was studied during incubation in water and in polyethylene glycol (PEG). Embryo growth in water was characterized by both cell division and an increase in cell size, while embryo growth in PEG solutions solely occurred through cell division. Low density fruits differed from high density fruits in a higher cell division rate during incubation both in water and PEG; in water also the cell size of the embryo was larger. As a consequence, the low density fruits reached the embryo length of 1.5 mm, which was critical for radicle protrusion, already after 6.1 days in water and 1.7 days before the high density fruits. In -1.2 MPa PEG the embryos in both fractions reached their maximum length of 0.9 and 0.7 mm in low and high density fruits, respectively, after 13 days of incubation. When incubation for 17 days in PEG was followed by incubation in water, the embryo growth in both fractions was faster than without a pre-incubation in PEG and also the cell size increased. The differences between the fractions persisted, however. It was concluded that embryo cell wall extensibility increased during PEG-incubation. During incubation in water, low density fruits had a higher water content than high density fruits. It was concluded that this difference was caused by a lower pericarp pressure potential (psi(p, pericarp)). When the pericarp was removed, the differences in water content disappeared, the cell size in both fractions became similar, and the difference in mean germination time was reduced. The difference in embryo growth rate of the two fruits during incubation both in water and in PEG could be eliminated by lowering the osmotic potential (psi(pi)) of the incubation medium of low density fruits. It was concluded that the pericarp of celery fruits is an important factor in regulation of embryo growth.