Effect of the age of the embryogenic callus culture on A. tumefaciens -mediated transformation efficiency. Transformation efficiency was assayed 6 weeks after the end of co-cultivation by GFP epifluorescence. The transformation efficiency was estimated by the proportion (p) of transformed calli (p = x/n), where x is the number of transformed calli and n the number of co-cultivated calli. A 3 δ confidence limit for binomial distribution was calculated using the formula with a level of confidence of 99%. Confidence interval is indicated by vertical bars. All the transformation experiments were conducted independently in five to six replicates each comprising 40 calli for each culture age (200 to 240 calli/culture age).