Drought represents one of the most critical environmental constraints limiting plant growth and productivity. During early developmental stages, plants exhibit heightened sensitivity to soil water availability. In crop species such as winter wheat (Triticum aestivum L.), overcoming water deficit at the seedling stage is vital for plant establishment and achieving stable yields. Drought tolerance is therefore a primary agronomic and economic trait. Water shortage affects numerous physiological and biochemical processes, particularly photosynthesis, which is essential for plant metabolism and carbon assimilation. Under drought, plants initiate multiple adaptive responses to optimize water use efficiency while maintaining basic metabolic activity. This study aimed to evaluate the impact of drought stress on growth, chlorophyll content, and photosynthetic performance in two winter wheat varieties—Katya (tolerant) and Zora (sensitive). A controlled pot experiment was conducted in which two-week-old plants at the third-leaf stage were subjected to a seven-day drought period followed by rehydration. Chlorophyll a fluorescence, thermoluminescence, and pigment content were measured to assess photosystem II functionality and recovery potential. The results revealed distinct varietal responses: Katya maintained greater photosynthetic stability and recovery, while Zora exhibited reduced chlorophyll content and incomplete restoration. These findings underscore Katya’s superior drought resilience, crucial for sustaining wheat productivity under changing climatic conditions .

Drought stress during early growth stages severely limits wheat productivity globally. Understanding varietal physiological responses to drought stress is critical for breeding climate-resilient cultivars. Two-week-old plants from two winter wheat (Triticum aestivum L.) cultivars—Katya (drought-tolerant) and Zora (drought-sensitive)—were subjected to drought for seven days, followed by rehydration. The experiments were conducted in pots in controlled conditions. The photosystem II (PSII) function was evaluated using chlorophyll a fluorescence (OJIP transients), thermoluminescence emissions and pigment content analysis. Under drought, Katya maintained functional PSII integrity with stable quantum efficiency and increased chlorophyll content, while Zora exhibited chlorophyll degradation. Fresh and dry weight declined in both genotypes but significantly only in Zora; recovery occurred after rehydration. Chlorophyll fluorescence revealed that varietal divergence was localized to the O–J phase of PSII photochemistry, indicating differences in reaction-center behavior confirmed by thermoluminescence. Katya demonstrated preserved PSII reaction-center density, balanced energy partitioning, homogeneous PSII populations, and superior recovery capacity. Conversely, Zora showed reaction-center depletion, elevated energy dissipation, impaired electron transport beyond Q_A, and persistent PSII heterogeneity even after rehydration. Drought tolerance in the studied genotypes was associated with the maintenance of PSII structural integrity, efficient photochemical function, and rapid recovery mechanisms. These physiological markers—particularly early PSII photochemistry kinetics and reaction-center stability—provide valuable selection criteria for breeding programs, targeting drought resilience under changing climate conditions.