*2.3. Data Analysis*

Analysis of variance was conducted with replication, genotype, and position as main effects. The interaction effect of genotype and position was also analyzed to identify sources of variation [28]. Statistix 8.1 (An Software, 2003) was used to calculate ANOVA and Tukey's test [29]. Tukey's test was applied to test the significant difference of variation between the cotton genotypes and variation between the two positions for selected traits [30]. Cluster analysis was carried out using the statistical software package of Minitab ver.17.

#### **3. Results**

Genotypes were significantly different for all recorded parameters, and plant position was also significantly different for all parameters except boll weight (Table 3). Of the sources of variation, plant position had the largest effect on PV, PG, chlorophyll content, and boll retention. The effect of genotype was largest for the cell membrane thermostability and boll weight parameters. The mean values for each measure of heat tolerance in the top and bottom plant positions of the genotypes are provided in Table 4.

**Table 3.** Mean squares for six measures of heat tolerance in cotton grown under field conditions.


\*\* *p* < 0.01 and NS = Nonsignificant; DF = Degree of freedom; PV = Pollen viability; PG = Pollen germination; Chl. Cont. = Chlorophyll content; CMT = Cell membrane thermostability; Boll Wt. = Boll weight; Boll Ret. = Boll retention.


**Table 4.** Mean values with standard errors for six measures of heat tolerance in the top and bottom positions of 13 cotton genotypes grown in 2019 in Faisalabad, Pakistan.

PV = Pollen viability; PG = Pollen germination; Chl. Cont. = Chlorophyll content; CMT = Cell membrane thermostability; Boll Wt. = Boll weight; Boll Ret. = Boll retention.
