The greenhouse trial was carried out during 1990 to 1992. Tubers with the size of 55 +/- 5 mm were planted into the vegetative pots of 101 content. Pots were filled with arable land. Four varieties were used in the trials: Resy, Karin, Desiree, Kamyk. The trial included other eight treatments (Tab. I), except for control, irrigated during the whole vegetation on 75 % relative soil humidity after Novak (1954). Relative soil humidity as controlled in pots in two- or three-day intervals. Tubers were analyzed, after physiological crop maturity, on starch, dry matter content and the texture in these tubers was evaluated. In half of the treatments (2, 3, 6, 7) there was decreased starch content in agreement with data of Iritani (1981). On the contrary, in the second part of treatments, especially where drought stress was acted in the end of vegetation (8, 9), there was increasing starch content as contrast to the control in treatment 9, from 1.58 % in 1992 to 2.53 % in 1991 in agreement with the results of Necas (1962). On average of three years, positive effect of drought, effective from crop emergence to bud formation, came to light in lesser extent. In some years varieties had the exceptionally highest starch content just in these treatments (4, 5). Variety reactions to drought in this index of tuber quality were very different. Results of dry matter content in individual treatments were enough in agreement with starch content. However, there were found some differences. The highest values were found out in treatments, in which drought stress was effective from full flowering to the end of vegetation (8, 9). Treatments, in which drought was effective from emergence to bud formation, had lower values (4, 5). Tubers from controls (treatment 1) had the fifth highest values of dry matter content. On average of three years, four other treatments placed under the control level. Drought was effective in the period from planting to plant emergence and from buds to full flower (2, 3, 6, 7) in these treatments. High differences enough in reaction to drought stress in various vegetation periods appeared between varieties. Decreasing dry matter accumulation in crops exposed to short-term drought stress is caused by lower interception of photosynthetically efficient radiation by smaller leaf surface owing to substantial leaf reduction and branching suppression (Zrust et al., 1991). Jefferies, Mac Kerron (1989) came to the same conclusion earlier. The finding, that drought in the end of vegetation increases dry matter content in tubers, can be the results of decreasing leaf role in this season and gradual transfer of assimilates from leaves to the tubers, which is accelerated by drought. Tuber texture is significantly correlated to dry matter content. According to Mica et al. (1984), the tuber consistence increases with the increasing dry matter content. This knowledge was not confirmed in the case of crop influencing by drought. Especially drought effective in the end of vegetation and increasing dry matter content in tubers provided no significant results in tuber texture. On the contrary, tubers from these treatments (8,9) were the least consistent. The significant role had greenhouse conditions there, and in these high temperatures in crop maturity period. Even in this qualitative index, various variety reactions to drought stress, effective in individual vegetation periods, appeared.
