**3. Results**

### *3.1. Evaluation of Physiological Responses of CSSLs of KDML105 under Drought-Stress Conditions*

Selected CSSLs, namely CSSL97, CSSL104, CSSL106, and CSSL107, were evaluated for drought tolerance by growing the seedlings in soil with 100% or 50% field capacity. In normal growth conditions (100% field capacity), all of the lines were similar (Figure 1A,C), but they differed under 50% field capacity (Figure 1B,D). CSSL104 displayed the most drought-tolerant phenotype, with the lowest leaf dying score and the highest photosystem II (PSII) efficiency (Fv/Fm). This was similar to the performance of DH103 (the drought-tolerant parental line). The highest leaf-death score was detected in CSSL106, while CSSL97 had the lowest PSII efficiency under drought stress. These data sugges<sup>t</sup> that CSSL104 is the most drought-tolerant line, while CSSL97 and CSSL106 are the most susceptible. Therefore, CSSL104 was selected for further characterization.

CSSL104 and its parental lines, KDML105 and DH103 rice, were grown in nutrient solutions corresponding to normal growth conditions and in a solution supplemented with 15% PEG for the drought-stress treatment, which caused about a 50–180% reduction in leaf water potential (Table 1). After nine days of drought stress, we measured the highest reduction of leaf water potential for all lines. Under these conditions, CSSL104 had the lowest leaf water potential at −7.75 MPa, which was about threefold lower than the LWP of the plants grown in normal conditions. The parental lines, KDML105 and DH103, had about a twofold reduction in LWP. Drought stress reduced all parameters of photosynthesis, including net photosynthesis rate, stomatal conductance, transpiration rate, intercellular CO2 concentration, ΦPSII, and electron transport rate in all lines. However, after six days under drought stress, CSSL104 had a significantly higher photosynthetic rate than the KDML105 parent. In addition, CSSL104 had a greater tendency than KDML105 toward higher values for all photosynthetic parameters after nine days of drought treatment (Table 1).

**Figure 1.** Response to drought stress in chromosome segmen<sup>t</sup> substitution lines (CSSLs) and parents. CSSLs, CSSL97, CSSL104, CSSL106, and CSSL107, and their parental lines KDML105 and DH103, were compared for leaf death (leaf dying score) and photosystem II efficiency (Fv/Fm) under (**A**,**C**) normal (100% field capacity) and (**B**,**D**) drought-stress (50% field capacity) conditions. The mean + 1 standard error (SE) was derived from four replicates. Means with a different lowercase letter above them are significantly different (*p* < 0.05). NS demonstrates no significant difference among lines.


**Table 1.** Photosynthetic performance of CSSL104 and parent lines. Net photosynthesis rate, transpiration rate, stomatal conductance, ΦPSII, electron transport

