Membrane permeability, morphology, and desiccation tolerance ofDaucus carota somatic embryos as influenced by drying rate

Summary

Carrot (Daucus carota) somatic embryos that were treated with abscisic acid during their development have been shown to acquire complete desiccation tolerance when slowly dried, but fail to do so when rapidly dried. We studied plasma membrane permeability by a spin probe technique. On rehydration, the plasma membranes of rapidly dried somatic embryos became permanently leaky, whereas those of slowly dried embryos retained permeability levels as low as for fresh embryos. The leakage was associated with extensive deesterification of the phospholipids. Using low-temperature scanning electron microscopy (LTSEM), we examined (re)hydrated and dry somatic and zygotic embryos. No differences in surface morphology were detected between dry tolerant and intolerant somatic 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 fractured embryos, whereas the viable somatic embryos had turgescent cells with intact cell structure. In 3-day-imbibed, viable embryos, cell extension was visible. These findings are consistent with the electron paramagnetic resonance data. Somatic embryos always had reduced and abnormal cotyledons, mostly fused, and the surface was irregular. The surface of dry somatic embryo cells was considerably more wrinkled than that of dry zygotic embryo cells. Maturation in polyethylene glycol-containing medium increased the percent dry matter in the fresh somatic embryos and reduced the extent of wrinkling after drying. Wrinkling of the dried embryo cells apparently depends on the filling with dry matter in the hydrated state. We suggest that wrinkling per se does not limit desiccation tolerance of somatic embryos.