MEMBRANE-PERMEABILITY, MORPHOLOGY, AND DESICCATION TOLERANCE OF DAUCUS-CAROTA SOMATIC EMBRYOS AS INFLUENCED BY DRYING RATE

Carrot (Daucus carota) somatic embryos that were treated with abscisic acid during their development have been shown to acquire complete des iccation tolerance when slowly dried, but fail to do so when rapidly d ried. We studied plasma membrane permeability by a spin probe techniqu e. On rehydration, the plasma membranes of rapidly dried somatic embry os became permanently leaky, whereas those of slowly dried embryos ret ained permeability levels as low as for fresh embryos. The leakage was associated with extensive deesterification of the phospholipids. Usin g low-temperature scanning electron microscopy (LTSEM), we examined (r e)hydrated and dry somatic and zygotic embryos. No differences in surf ace morphology were detected between dry tolerant and intolerant somat ic embryos that were obtained by slow and rapid drying, respectively. However, on rehydration, the nonviable somatic embryos had lost turgor and intracellular organization, as revealed by LTSEM images of fractu red embryos, whereas the viable somatic embryos had turgescent cells w ith intact cell structure. In 3-day-imbibed, viable embryos, cell exte nsion was visible. These findings are consistent with the electron par amagnetic resonance data. Somatic embryos always had reduced and abnor mal cotyledons, mostly fused, and the surface was irregular. The surfa ce of dry somatic embryo cells was considerably more wrinkled than tha t of dry zygotic embryo cells. Maturation in polyethylene glycol-conta ining medium increased the percent dry matter in the fresh somatic emb ryos and reduced the extent of wrinkling after drying. Wrinkling of th e dried embryo cells apparently depends on the filling with dry matter in the hydrated state. We suggest that wrinkling per se does not limi t desiccation tolerance of somatic embryos.