Variation in temperature during the trials

The maximum temperatures recorded for 90 days during both trials using the Google Weather application are presented in Fig. 2 and Supplementary Table. During the heat stress season, the temperatures ranged from 36 °C to 39 °C degrees. The highest daily maximum temperature under heat stress condition (39 °C) was recorded in March during the tillering stage The average maximum temperatures under heat stress during the flowering stage was 35 °C under heat stress conditions. The average relative humidity during heat stress was 40%.

Fig. 2
figure 2

Temperature variation in normal condition and under heat stress

Effect of temperature on growth traits

Analysis of the variance (Table 3) revealed that the variety had a significant effect on plant height and leaf length at 30 days after transplanting and on leaf length at 56 days after transplanting (P ≤ 0.0001), panicle leaf length (P ≤ 0.001), plant height at 56 days after transplanting and panicle leaf width (P ≤ 0.01).

Table 3 Significance of plant growth variables

Temperature had significant influence on plant height and plant diameter at the base point of tiller at 30 days after transplanting, number of tillers at 30 and 56 days after transplanting, number of internodes at 30 days after transplanting, internode length at 30 and 56 days after transplanting, and plant vigor at 30 days after transplanting (P ≤ 0.0001), plant height at 56 days after transplanting and panicle leaf width (P ≤ 0.001) (Table 3).

The temperature by variety interaction had a significant effect (P < 0.01) on plant height at 30 days, leaf length 56 days after transplanting, and internode length measured at 30 days after transplanting (Table 3).

Effect of temperature on yield traits

The analysis of the variance (Table 4) showed that the variety had a very highly significant effect (P ≤ 0.0001) on the days to 1% flowering, the days to 50% flowering, the days to maturity of grains, the width of grains, the weight of 1000 grains and the average yield per plant. We observed a significant effect (P ≤ 0.01) of variety on length of panicles, average number of filled grains and the yield per plant (Table 4).

Table 4 Analysis of variance of yield and yield components

The analysis of variance (Table 4) showed that the temperature had a significant effect (P ≤ 0.0001) on days to 1% flowering, the days to 50% flowering, the days to maturity of grains, number of secondary branches, and the weight of 1000G.

Variety by temperature interaction had a significant influence (P ≤ 0.01) on the days to 50% flowering date, days to grain maturity, number of secondary branches number, and 1000G weight.

Effect of temperature on physiological traits

The analysis of variance (Table 5) showed that variety had a significant influence only on the initial value of fluorescence taken at 30 days after transplanting.

Table 5 Significance of physiological variables

Similarly, temperature had a highly significant influence (P ≤ 0.001) on the ratio of variable to initial fluorescence (Fv/Fo) taken at 40 days after transplanting. Temperature had a significant effect (P ≤ 0.01) on the initial fluorescence value (Fo) at 30 days after transplanting and the variable fluorescence (Fv) at 40 days after transplanting. In contrast, temperature had no effect on the rest of the physiological parameters.

The interaction between variety and temperature had no effect on the physiological traits. It is therefore concluded that no physiological variable explained the influence of temperature on the varieties.

Comparison of varieties under the two environmental conditions

The variety with the highest plant height under normal conditions (73.96 cm) and heat stress (60.80 cm) was BRIZ-8B (Fig. 3a). BRIZ-9B had the shortest plant height under both optimal and heat stress conditions.

Fig. 3
figure 3

a Boxplots of plant height under normal condition (left) and heat stress (right). b Boxplots of leaf length under normal condition (left) and heat stress (right). c Boxplots of internode length under normal condition (left) and heat stress (right). d Boxplots of flowering date under normal condition (left) and heat stress (right). e Boxplots of the maturity grain date under normal condition (left) and heat stress (right). f Boxplots of the number of secondary panicles. g Boxplots of the weight of thousand grains

Under optimal conditions and heat stress, BRIZ-8B recorded the longest leaves at 56 days after transplanting (Fig. 3b).

Under normal temperature regimes and heat stress, NERICA-L14 and NERICA-L20 had the highest averages of internode length at 30 days (Fig. 3c).

Under optimal conditions, NERICA-L20, NERICA-L14, IR841, BERIZ-9B, and BRIZ-8B had the longest days from sowing date to 50% flowering with an average of 98 days, 97 days, 97 days, and 94 days, respectively. While under heat stress, IR841, NERICA-L20 and NERICA- L14 flowered late (115 days, 115 days, 114 days, respectively). BERIZ-10B had the shortest number of days to 50% flowering under both optimal and heat stress conditions.

Under heat stress, it is observed that NERICA-L20, NERICA-L14, and IR841 matured late at 142 days, 141 days, and 137 days, respectively (Fig. 3e). Under optimal and heat stress conditions, BRIZ-9B showed the shortest days to grain maturity (100 days) (Fig. 3e). Regardless of the conditions, IR841 had the highest variability in days to maturity. Overall, the varieties exhibited higher variation and took longer to mature under heat stress compared to normal conditions (Fig. 3e).

Under optimal conditions, NERICA-L20 had the highest number of secondary branches (30.00), followed by NERICA-L14 (29.00) and BERIZ-8B (27.33). The variety IR841 had the lowest number of secondary branches under optimal conditions with an average of 23.66; while under heat stress the variety BERIZ-8B recorded a low number of secondary branches (12.00).

Under optimal temperature and heat stress conditions, two varieties BERIZ-8B and BERIZ-9B had the highest weight of 1000 grains. Under optimal conditions, BERIZ-8B and BERIZ-9B recorded 37.44 g and 36.78 g, respectively while under heat stress, they recorded 34.67 g and 32.41 g, respectively.

The hierarchical ascending classification (HAC) of rice varieties under both temperature conditions

HAC was used to assess the similarity and dissimilarity between the varieties based on agro-morphological, phenological and physiological data collected. A dendrogram was thus generated according for each type of temperature regime.

Relationship between the varieties under optimal condition

The varieties were clustered into two classes and each cluster is made up of three varieties (Fig. 4). The first group consisted of BRIZ-8B NERICA-L14 and NERICA-L20 while the second group included BRIZ-10B, BRIZ-9B and IR841. Group 1 had the tallest plants, the biggest diameter at the base point of the stem or tiller, the longest and largest leaves, best vigorous plants, the longest internode, longest panicle, highest number of secondary branches, the highest days to maturity and the highest average yields per plant. The variety in group 2 longest grain. Finally, class 3 had varieties with a considerable number of tillers and internodes, longest flowering dates, and highest thousand grain weight compared to the other classes.

Fig. 4
figure 4

Clustering of the varieties under normal condition

Relationship between the varieties under heat stress

The clustering analysis grouped the varieties into three classes (Fig. 5). Class 1 consists of two varieties BRIZ-10B and BRIZ -8B. Class 2 is also made up of two varieties BRIZ-9B and IR841. Finally, the last class is constituted by the varieties NERICA-L14 and NERICA-L20. Individuals in class 1 showed the tallest plants, longest leaf, leaf width at 56 days after transplanting, biggest diameter at the base point of the stem or tiller, longest internode, the highest number of internodes, best vigorous plants, longest panicle leaf width, the shortest days to 50% flowering, the lowest Fo30, and Fm30, the highest Fv/Fm at 40 days and as well as the highest weight of 1000 grains. The varieties of class 2 had the shortest plant height, the largest number of tillers and internodes, the shortest panicle length, the highest values of initial and variable fluorescence, the highest ratio between the initial and variable fluorescence. As for the varieties in class 3 they had the smallest diameter at 30 days after transplanting and number of internode at 30 days, the lowest number of tillers at 30 days, longest panicle leaves, highest value of initial fluorescence, longest panicles, highest number of secondary branches, longest days to 50% flowering and maturity, and longest grains with the highest yields per plant.

Fig. 5
figure 5

Clustering of the varieties under heat stress

Estimation of genetic parameters

Phenotypic and genotypic variation

Phenotypic variance was greater than genotypic variance for all variables under both normal and heat stress conditions. Phenotypic variance ranged from 0.03 to 297.30 under optimal conditions and from 0.01 to 193.30 under heat stress. The plant height at 56 days after transplanting (297.30 optimal conditions and 193.30 under stress) shows the highest phenotypic variance. This variance is followed by panicle leaf length (99.77 under optimal and 72.77 under stress), leaf length at 56 days (70.29 under stress), and grain maturity date (61.27 optimal conditions). Similarly, genotypic variance ranged from 0.07 to 165 under optimal conditions and from 0.01 to 166.36 under heat stress. The traits with the highest genotypic variances were plant height at 56 days (165.03 optimal and 166.36 under stress), length of panicle leaves (85.67 optimal), days to 1% flowering (75, 26 under stress), and grain maturity date (55, 29 optimal).

Coefficient of phenotypic and genotypic variation

For all the traits, the coefficient of phenotypic variation was higher than the coefficient of genotypic variation irrespective of the season (Tables 6 and 7). Under optimal temperature conditions, the number of tillers at 56 days, the length of panicle leaves (23.86%), the average number of full grains (28.43%), the average number of empty grains (55.05%), the average yield per plant (25.62%), and the yield per plot (46.40%) had the highest phenotypic variation coefficients. Under heat stress conditions, plant height at 30 days and 56 days (20.18%), diameter at the base point of the stem or tiller measured at 56 days (30.51%), number of internodes at 30 days (23.80%) and 56 days (27.85%), the length of internodes at 30 days (21.02%), the length of panicle leaves (23, 79%), number of panicles (25.39%), number of secondary branches (24.87%), the average number of empty grains (54.01%), average yield per plant (99.13%), yield per plot (46.40%) which had the highest phenotypic variation coefficient. The characters with moderate phenotypic variation coefficient under optimal conditions are plant height, leaf length measured at 30 days and 56 days after transplanting, number of tillers at 56 days, plant vigor at 30 days, panicle leaf length, thousand seed weight, and average yield per plant. Under heat stress, leaf length at 30 and 56 days, leaf width, diameter at 30 days, number of tillers, number of primary branches, and average number of filled grains had moderate phenotypic variation coefficients. On the other hand, the coefficients of low genotypic variation in optimal conditions are for leaf length at 56 days (9.52%), number of tillers at 30 days (7.85%), number of internodes, number of panicles (1, 98%), panicle length (3.91%), number of primary (2.67%) and secondary branches (7.64), kernel length (5.92%) and kernel width (2.45%), days to 50% flowering (6.94), and days to seed maturity (6.29%). While under heat stress the characters with the lowest genotypic coefficient of variation were leaf width at 56 days (7.97%), diameter at 30 days (6.42), number of tillers at 30 days and 56 days, internode length (8.99), panicle length (7.77), panicle length (7.35%), maturity date (6.29%). Under optimal temperature conditions, panicle leaf length (22.5%), average number of filled (22.73%) and empty grain (30.24), yield per plot (31.85%), had the highest genotypic variation coefficient. While under heat stress, diameter at 56 days (28.75), number of tillers at 30 (20.28) and 56 days (20.76), the length of internodes (24.51%), the average number of empty grain (41.56%), the average yield per plant (40.05%) and the yield per plot (36.54%) are those with the highest genotypic coefficient of variation.

Table 6 Genetic parameters of growth variables
Table 7 Genetic parameters of performance variables

Broad sense heritability of the traits

Under normal temperature conditions, the variables with the highest heritability were leaf length at 56 days (97.98%), thousand grain weight (95.11%), and days to 50% flowering (90.66%) panicle leaf length (85.89%), grain maturity date (85.34%), leaf length at 30 days (84.21%), days to 1% flowering (83.34), plant height at 30 days (78.98%), and panicle length (73.17%) (Tables 6 and 7). Under heat stress, heritability was higher for the following traits: plant diameter at the base point of the stem or tiller (88.79%), leaf length at 56 days (87.04%), plant height at 56 days (86.06%), height at 30 days (72.55%), number of internodes at 56 days (70.00%), days to 50% flowering (69.33%), days to 1% flowering (68.50%).

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