**4. Conclusion**

**Figure 6.** PEG effect on growth traits and free proline concentration of plants cultivated in tissue culture [28].

) chlorophyll fluorescence and their ratio (F<sup>v</sup>

according to their field-assessed drought tolerance (ctrl—control; drought; DT—drought tolerance).

/Fm) in sugar beet genotypes grouped

**Figure 5.** Maximal (Fm) and variable (Fv

82 Plant, Abiotic Stress and Responses to Climate Change

Tolerance to drought is very complex. Experiments in three different environments (tissue culture, greenhouse, and field) with 11 genotypes, where many different parameters were followed, revealed that it is not easy to find single criteria for classification with respect to drought tolerance. However, the results suggest that free proline accumulation may be used as a reliable parameter. The classification based on changes in concentration of free proline in plants exposed to drought in greenhouse and tissue culture corresponded to classification made on the bases on field observations. Therefore, similar fast tests, conducted with young plants and possibly aided by the use of molecular markers, can be useful for estimation of breeding material with respect to tolerance to water deficiency, which will significantly enhance sugar beet breeding for expected future changes in climate.
