الفهرس 
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Abstract
This study was carried out at Horticulture Dept., Fac Agric El Minia University, El Minia Governorate, Egypt .The study was conducted under in vitro and in vivo conditions to develop the practical and effective method for screening potato varieties for drought tolerance, by adding sorbitol in the culture medium. In this study a sequential procedure, from a single isolated shoot tip of potato (Solanum tuberosum) were cultured on Murashige and Skoog (MS) Medium. Four concentrations of sorbitol (0.0, 0.1, 0.2 and 0.3 mole / liter) were used in MS medium for inducing the water deficiency stress on four potato varieties (Diamant; Burren; Maritiema and Lady Balfour) with three replications in completely randomized design.
Data revealed that the studied potato cultivars were significant differed for in plantlet heights, number of leaves per plantlet, number of nodes per plantlet, number of roots per plantlet and plantlet weight in the medium without sorbitol. Also, the result show different responses of potato cultivars to various levels of sorbitol concentrations. The higher drought stress by the increase in the concentration of sorbitol lead to significant reduction in previous characteristics than those not treated with sorbitol. However, the decrease in previous characteristics was more pronounced in Maritiema and Lady Balfour varieties which are considered as susceptible varieties for drought stress. The results indicated that the Diamant variety is a drought-tolerant more than the other studied potato varieties in our experiments.
After the emergence of roots from the plantlets, they were removed and planted in polyethylene plastic pots filled with soil mixture until the production of mini-tubers. The variance analysis results showed that increasing the osmotic pressure levels lead to decrease in the mini-tuber average size, mini-tubers weight, and number of stems.
These results indicated that the simulation of drought stress under in vitro conditions during the regeneration process constitutes a convenient way to study the effects of drought stress on potato responses. Sorbitol can be used to simulate drought stress in potato plants as a non-penetrating osmotic agent lowering the water potential in a way similar to soil water stress conditions. In-vitro this technique was shown to be useful in identifying relatively drought-tolerant cultivars at the early stages of development and it can be a very useful tool for screening a large number of varieties or breeding lines of cultivars within a short time.