Cell shape and localisation of ice in leaves of overwintering wheat during frost stress in the field.

Wheat leaf pieces were excised and freeze-fixed in the field, preparatory to low-temperature scanning electron microscopy to study distribution of ice within leaf blades, and associated cell shapes, during natural frosts. Pieces of leaf blades from wheat plants (Triticum aestivum L. 7942H1-20-8) overwintering in Indiana, USA (January, 1991), were excised and immediately freeze-fixed by manually plunging in melting freon. Cells in controls were turgid and extracellular ice was absent. The leaves of the frost-stressed plants froze at about - 2.4° C, and at that temperature extracellular ice was mainly located sub-epidermally, including in the substomatal cavity, and occupied about 14% of the fracture faces. The frequency of ice particles per unit leaf area in two specimens was 14 and 210 · mm(-2) (about 140 and 2100 · g(-1) leaf fresh-weight basis). At -9.0° C, ice filled the extracellular spaces, occupying 61% of the fracture faces. Cells were somewhat collapsed at -2.4° C and were much more collapsed at -9.0° C. The epidermal cells were more collapsed than the mesophyll cells. Tissue structure (connections with adjacent cells), wall flexibility, and ice growth may all have influenced the shapes of the collapsing cells. The experiments demonstrate the feasibility of freeze-fixation in the field. The sub-epidermal location of most ice indicates that in the field either (i) ice is nucleated sub-epidermally (implying both the presence of nucleators and the presence of liquid water in the sub-epidermal spaces) or (ii) ice is nucleated on the leaf surface, then propagates into the leaf probably through stomata.