The soluble carbohydrate levels in the taproot and crown of alfalfa (Medicago sativa L.) contribute to freezing tolerance and the regrowth of new shoots. To evaluate the role of carbohydrates, we transformed alfalfa using Agrobacterium tumefaciens with the binary vector pVINV. This vector contained the CaMV35S promoter and the yeast derived suc-2 gene (invertase) with a transit peptide targeting the enzyme to the vacuole. Rooted clonal cuttings of seven independent transformants and a non-transformed control were planted at Elora research station in May 1999. The plants were evaluated physiologically in September 1999 prior to acclimation, and in December 1999 following acclimation. In addition, stand counts and vigour ratings were taken in a separate experiment using the same plant material to assess winter survival. Invertase activities, soluble protein, total soluble carbohydrate, starch concentrations and dry matter production in both root and crown tissues were evaluated.

Invertase, TSC and dry weights varied significantly within the transgenic group. Some transgenics varied significantly from the control in the above factors, while others demonstrated no significant, indicating a possible insertional effect due to transformation.

Mean invertase activities and soluble protein content were not significantly different in pre-acclimated plants compared to post-acclimated plants. However, mean TSC and starch concentrations and dry weights in pre-acclimated plants were significantly different from post-acclimated plants, with TSC and dry weights being higher and starch being lower. Invertase activity was negatively correlated with protein concentration and postively correlated with starch concentrations in the roots of post-acclimated plants. There was no correlation between invertase and TSC or dry weight in either root or crown tissues in either pre or post-acclimated plants.

The transgenic plants were not significantly different from the control plants in terms of winter survival or vigour rating following the 1999/2000-winter season. This may be due to the mild winter conditions experienced or it is possible that the presence of a constituitively expressed invertase gene had little effect on the survivability of these plants under these field conditions.