**2.3 Assessment of drought tolerance in castor bean by pollen**

The climatic conditions of the south of Ukraine often have an adverse effect on the growth and development of castor bean plants, which are relatively unstable to drought and high temperatures. Therefore, the search for drought-resistant genotypes is the key aspect of plant breeding programs, which aim to obtain high-yielding varieties of this crop.

Express methods based on the analysis of pollen can be classified as promising in evaluating sporophytes for resistance to abiotic environmental factors, which is due not only to the experimentally revealed expression of a large number of genes at both stages of the plant life cycle but also to the ease of manipulation with such substance as pollen.

The correlation coefficient we established between the drought resistance of the sporophyte and the resistance of the male gametophyte to the action of high temperatures in the range of 0.72–0.89 opens the way to distinguish drought-resistant castor

bean genotypes at the pollen level. An indicator of pollen resistance to elevated temperatures is the degree to which the percentage of pollen germination or the length of the pollen tube decreases after a stress factor is applied to mature pollen grains. When treating pollen of drought-resistant genotypes at the temperature of 40°C for 1 h, the degree of decrease in pollen germination percentage, as a rule, does not exceed 25%, while the length of a pollen tube may practically be of the same size.

As an example, data on the heat resistance of pollen can be provided for two genotypes of castor beans contrasting in resistance to drought (Khortytskaya 1 variety—tolerant to drought, K161 line—not tolerant to drought). Drought resistance of genotypes at the sporophyte stage was evaluated according to the method based on determining the osmotic pressure of cell sap by a refractometric method (−1.95 for Khortytskaya 1 variety, −1.0 for K161 line). In this case, the highest negative values of the water potential indicate the maximum ability of tissues to maintain it better, that is, to prevent the adverse effects of drought.

To estimate the tolerance of male gametophyte to high temperatures, pollen was subjected to temperature treatment at 40°C for 1 h, and then germinated *in vitro*. The tolerance of male gametophyte was assessed by the degree of decrease in pollen germination and pollen tube length in comparison with the control (pollen not subjected to heat processing).

As it turned out, in those genotypes whose sporophytes are characterized by high drought resistance, the degree of decrease in the germination of pollen after temperature treatment is minimal, while not drought-resistant lines show more significant inhibition of pollen germination when exposed to high temperatures (in Khortytskaya 1–18.8%, in K161 line—63.7%). The thermal treatment used also revealed significant differences between the accessions according to their ability to develop a long pollen tube. After heating pollen, in genotypes that were assessed as drought-resistant at the sporophyte stage, such stress caused a less significant decrease in the length of the pollen tubes than in not drought-resistant genotypes. In view of the above, a method of pollen evaluation can be proposed for screening drought-resistant castor bean samples.
