Intrusive trichome bases in the leaves of silverleaf nightshade (Solanum elaeagnifolium; Solanaceae) do not facilitate fluorescent tracer uptake.

PREMISE OF THE STUDY: Solanum elaeagnifolium (silverleaf nightshade), having originated in the Americas, is now a serious summer-growing, perennial weed in many countries, including Australia. Most surfaces of the plants have a dense covering of trichomes, giving them a silvery-white appearance, hence the common name. We aimed to identify structural and functional properties of its leaves, especially the trichomes, that may affect the uptake of foliar-applied tracer dyes. METHODS: The structure of leaves of Solanum elaeagnifolium was examined by light and scanning electron microscopy. The potential for transport of materials between trichomes and veins was studied with symplastic (carboxyfluorescein diacetate) and apoplastic (lucifer yellow) tracer dyes. KEY RESULTS: Mature leaves had a dense covering of complex, stellate trichomes on both surfaces, particularly the abaxial. The basal cells of Solanum elaeagnifolium trichomes penetrated into the underlying palisade mesophyll layers. The innermost lobes of these basal cells sometimes contacted the bundle sheath of the veins, but were not observed to directly contact the xylem or phloem. We found that neither symplastic nor apoplastic dyes were transferred between the basal cells of the trichomes and the vascular tissues. The trichome layer repelled water-based tracer dyes, while one of four adjuvants tested facilitated entry of both symplastic and apoplastic dyes. CONCLUSIONS: Our results did not support a transport function for the trichomes. The trichomes may protect the mesophytic leaves from invertebrate herbivory, while also probably decreasing radiation absorbed resulting in cooler leaves in this summer-growing species.

Structural changes in acclimated and unacclimated leaves during freezing and thawing.

Freeze-induced damage to leaf tissues was studied at different states of acclimation to low temperatures in snow gum, Eucalyptus pauciflora Sieber ex Sprengel. Intact, attached leaves of plants grown under glasshouse or field conditions were frozen at natural rates (frost-freezing) and thawed under laboratory conditions. Leaves were cryo-fixed unfrozen, during frost-freezing or after thawing for observation in a cryo-scanning electron microscope. Frost-freezing in unacclimated tissues caused irreversible tissue damage consistent with tissue death. Intracellular ice formed in the cambium and phloem, killing the cells and leaving persistent gaps between xylem and phloem. Many other cells were damaged by frost-freeze-induced dehydration and failed to resorb water from thawed extracellular ice, leaving substantial amounts of liquid water in intercellular spaces. In contrast, acclimated leaves showed reversible tissue displacements consistent with leaf survival. In these leaves during freezing, massive extracellular ice formed in specific expansion zones within the midvein. On thawing, water was resorbed by living cells, restoring the original tissue shapes. Possible evolutionary significance of these expansion zones is discussed. Acclimated leaves showed no evidence of intracellular freezing, nor tissue lesions caused by extracellular ice. While the observations accord with current views of freeze-sensitivity and tolerance, cryo-microscopy revealed diverse responses in different tissue types.