The influence of cell size and chromosome dosage on cold-hardiness expression in the Triticeae

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Abstract
The influence of cell size and chromosome dosage on cold-hardiness expression was investigated in members of the tribe Triticeae. Mean leaf guard-cell lengths for ploidy levels of 2x, 4x, 6x, and 8x were found to increase by approximately 10 μm with each addition of two basic (x = 7) genomes, indicating that larger cell size was associated with higher ploidy level. Poor expression of cold hardiness in amphiploids was associated with large cell size. However, comparisons among and within species indicated that ploidy level was not the only factor determining cell size. Significant differences in guard-cell length were observed among common wheat (Triticum aestivum L. em. Thell.) cultivars. Cell size differences among cultivars were found in both hardened and nonhardened common wheat plants and these differences were associated with cultivar cold hardiness (r = 0.95, P ≤ 0.01). The evidence indicated that smaller cell size influenced cold tolerance by amplifying the expression of cold-hardiness genes in cold-acclimated plants, probably by reducing the degree of cell contraction from freeze dehydration. A chromosome (gene) dosage effect that favored the expression of genes from the parent species contributing the higher chromosome number was also shown to play an important role in the expression of cold hardiness in interspecific hybrids and amphiploids. Comparison of related species with similar cell size and chromosome number suggested differences in the effectiveness of cold hardiness conferring genes. Observations made on species from the Triticeae indicate that when cold-hardiness potential is limited at the diploid level, a plant group may expand its cold-hardiness range by "loading up" on existing cold-hardiness genes by means of polyploidy. An increased genetic potential may then be further enhanced by selection for smaller cell size within the polyploid nucleotype. This process appears to have been responsible for the superior cold hardiness of hexaploids within the Triticum genus.Key words: cell size, cold hardiness, gene dosage, Triticeae, evolution, interspecific hybrid, Agropyron.

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