3.5.9. QTL for Length of Staminal Column (LOS)

Three QTL, s*qLOSa1, qLOSa2,* and *qLOSa3*, were detected on chromosome number 18 for length of staminal column. The LOD values were 3.78, 3.76, and 3.07 respectively. Results showed positive additive effects of 0.52, 0.52, and 0.30 for the three QTLs, while 16.30, 15.84, and 14.57 were the values for phenotypic variance for all QTLs. When the three QTL's were fitted together simultaneously the phenotypic variance was 47% (Table 6).

#### 3.5.10. QTLs for Length of Petal (LOP)

One QTL, *qLOP1*, was identified that influenced Length of petal trait. The QTL was located on chromosome number 2. The LOD value for QTL was 3.56. Phenotypic variance was 19.46. The QTL showed a positive additive effect of 0.45 (Table 6).

#### **4. Discussion**

The current study was carried out to identify the genetic basis responses of cotton plants under heat stress. The data collected were at the parental line and then after by F2 generation from which heat-susceptible and heat-tolerant genotypes were selected for the screening process. Initially, the emergence of the first sympodial branch at lower nodes determined the early maturity of cotton plants. Theoretically, it is implicated for the 1st sympodial branch to appear on lower nodes as it is highly correlated with earliness and heat tolerance [50,51]. The strong relationship between early maturity and lower sympodial branch node number was reported in previous studies [52]. It was reported that there was a strong association of the 1st sympodial branch node number and heat tolerance. Highly significant differences were found in analysis of variance for the 1st sympodial node number (Table 4). The data of correlation (Figure 3) showed a positive correlation of the 1st sympodial node number expressed with all the traits except sympodial node number, present boll set on first position along sympodia, cell injury, and length of pistil. Node number to set the initial fruiting sympodia is a reliable and realistic morphological trait of heat tolerance [53]. Minimum and maximum temperature significantly affected the first sympodial branch with 1st boll [54]. All genotypes under study differed significantly for this trait (Table 4). Hussain et al. (2000) revealed similar results for plant height under heat stress, presenting a familiar correlation among traits that plant height has a positive correlation with the morphological traits under study [55]. Boll development was affected by the high temperature stress as compared with vegetative phase and a similar reduction in boll weight was observed when the temperature fluctuated [56]. Morris (1964) also reported a reduction in cotton boll maturity time at high temperature stress [57]. After screening the genotypes on morphological parameters, one genotypes was selected as tolerant against heat stress and another one was selected as heat susceptible, among others, on the basis of physiological characteristics, i.e., relative water contents, water potential, osmotic potential, cell injury, and proline contents. Highly significant differences were perceived by analysis of variance for all the physiological traits among the genotypes, except photosynthesis rate, which is significant (Table 3). The membrane structure of plant cells was distorted under severe temperature stress, which caused the increased permeability of membrane. As a result, electrolyte leakage increased and eventually led to cell death [58]. Azhar et al. (2009) measured the heat tolerance in term of relative cell injury percentage in cotton and found that thermal stress-tolerant genotypes were more stable

for seed cotton yield and also maintained fiber quality as compared with heat-susceptible genotypes. A significant decrease was observed in leaf relative water content % (RWC) for heat-susceptible genotypes when exposed to heat stress, and similar findings were also obtained by Rahman et al. (2000), Siddique et al. (2000), and Parida et al. (2007) under stress conditions [59–61]. Higher leaf relative water content (RWC) could be a criterion for selection of a parent for hybridization to develop stress-tolerant genotypes [62,63]. On the basis of grand mean attained from normal and heat-stress situations, the protein contents was variable among genotypes and Raison et al. (1982) revealed that for temperature conditions above the optimum, significant reticence of photosynthesis takes place, resulting in substantial reduction in protein formation [64].

Finally, it was observed that high heat tolerance is a multigenic trait and its expression is controlled by many QTLs. Almost all the vegetative and floral characteristics of cotton plants were affected adversely because of this stress. The identification of QTLs activated to combat heat stress allowed the estimation of genetic architecture and improvement of heattolerance traits by molecular marker-assisted selection (MAS). A total of 1450 markers were applied, among which 175 SSR markers were observed to be polymorphic and were found to be significant; the observations were also in accordance with some other researchers [65]. In order to dissect the genetic basis of heat tolerance, two upland cotton cultivars (MNH-886 and MNH-884) were selected as parents and an F2 population was developed. A high LOD (logarithm of odds) value provided strong evidence that the reported QTLs are actually associated with the respective traits. We only reported QTLs whose LOD score values were greater than three and which showed a significant additive or dominance genetic effect. A total of 17 QTLs with different effects on ten morphological and physiological traits such as First sympodial node height (FSH), sympodial node height (SNH), Percent boll set along sympodia on 2nd position (PBS), total no. of sympods (TNS), total no. of nodes (TNN), number of bolls (NOB), total no. of buds (TNB), length of bracts (LOB), length of staminal column (LOS), and length of petal (LOP) were detected in the present study. These QTLs were mapped on chromosome numbers 2, 3, 5, 6, 15, 16, 18, 19, 23, and 26. QTLs for length of petal and length of bracts were located on Chr. 2 while QTLs for total no. of buds and length of staminal column were located on Chr. 18 [66,67]. Likewise, QTLs for Boll no. and Percent boll set along sympodia on 2nd position were located on Chr. 26. Our findings are in accordance with work carried out by [68,69].
