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The effect of dehydration stress on membrane competence among and within Fragaria species was evaluated using index of injury, I _d , and tissue ionic conductance, g _Ti. Single accessions of F. chiloensis ssp. lucida Duch., F. virginiana ssp. glauca (S. Watson) Staudt, F. virginiana ssp. virginiana Duch., F. ×ananassa Duch., and F. vesca L. were used to study interspecific variation. Leaf thickness and total electrolyte content were greatest for the F. chiloensis ssp. lucida accession and least for the F. virginiana ssp. glauca accession, but foliar electrolyte concentration did not vary across accessions. The g _Ti values were >5-fold higher from 0 to 2 hours than for other intervals, declining over time. Significant differences in g _Ti and I _d values were only evident at 2 and 4 hours within stress levels, and increased as stress level increased. While the F. chiloensis ssp. lucida accession exhibited low g _Ti values at 70% relative water content (RWC), it showed greater relative membrane injury than the other species expressed as g _Ti , I _d , or the ratio of stress g _Ti to control g _Ti as dehydration level increased. Although the F. virginiana ssp. glauca accession had the highest g _Ti values, even at 100% RWC, its relative injury as stress level increased was not as great as that of the F. chiloensis accession. In a second experiment, intraspecific variation was examined using four accessions each of F. chiloensis and of F. virginiana which were dehydrated to 50% RWC. The species mean g _Ti , I _d , and g _Ti ratio values at 2 and 4 hours for the F. chiloensis accessions were lower than those for the F. virginiana accessions, but significant intraspecific variation was also observed. In spite of the differences between species means, the evidence of intraspecific variation indicates that not all accessions of a species exhibit similar drought responses (i.e., membrane competence). Due to the consistent conclusions derived from using either g _Ti or I _d after 2 or 4 hours of incubation for characterization of membrane competence, g _Ti and I _d were comparable techniques for identification of potential drought tolerance in Fragaria.

The expression g_ti, or tissue ionic conductance, was proposed to describe the efflux of ions from leaf disks (Whitlow et al., 1992, Plant Physiology, 98:198-205). The objective of this study was to determine the effectiveness of the g_ti method to screen germplasm for heat and desiccation tolerance using representative selections of 5 Fragaria species. Leaf disks were exposed to 4 levels of heat, 25, 35, 45, and 55 C, and 4 levels of desiccation. 60, 70, 80 and 100% relative water content (RWC). F. virginiana glauca was consistently ranked as the leakiest in all treatments including controls, with g_ti values 70 to 100% higher than in the other species. Temperatures of 25 to 45 C did not influence g_ti over time. A temperature of 55 C was lethal to the tissue and, thus, the ion flux was initially very high but soon diminished. At 70% RWC F. virginiana glaucu tissue was the leakiest, and F. virginiana and F. vesca tissues were somewhat leakier than those of F. × ananassa and F. chiloensis. Differences among species diminished with time in bathing solutions.