Cold-induced imbibition damage of lettuce embryos: a study using cryo-scanning electron microscopy

Abstract

The impact of rehydration on a multicellular organism was studied in lettuce (Lactuca sativaL.) embryos, using cryo-scanning electron microscopy (cryo-SEM). Naked embryos were sensitive to imbibitional stress, whereas embryos with an intact, thick-walled endosperm were not. Imbibitional injury to naked embryos was mainly confined to the outer 2–3 cell layers of the axis. These cells failed to swell and appeared poorly hydrated. Deeper layers were not affected even after extended periods of cold rehydration. The proportion of damaged cells (6–7% of total) roughly corresponded with the additional K+that gradually leached from the embryos. Damaged embryos were able to survive the loss of their surface layers and form adventitious roots. The swelling of inner tissues caused the dead surface layers to rupture into patches. Plasma membranes in dried embryos showed normal bilayer structure with a homogeneous distribution of intra-membrane particles (IMPs), also after non-injurious rehydration. Imbibitionally damaged plasma membranes showed many irregularities, such as globular insertions, that probably resulted from malfusions in the ruptured membrane, but the IMPs were still randomly distributed.