*3.3. Mean Performance of Green Super Rice under Heat Stress*

Heat stress significantly reduced the GY and affected other yield-contributing agronomic traits, namely TPP, SY, HI, and CMS, in most of the studied GSR and local genotypes (Figures 2–5). However, the traits PH, GW, GL, PFP, and NDVI were not affected in most of the studied genotypes and probably do not play a role in grain yield variations (Figures 2–5). Regarding the PH, we did not observe a very strong effect of heat stress on the tested genotypes (Figure 2A). Although it decreased overall, PH was considerably decreased in NGSR-1, NGSR-19, IR-6, and Kashmir Basmati under heat stress (Figure 2A). TPP and GY were decreased significantly under heat stress in most of the genotypes, except for Kashmir Basmati (Figure 2B,C). Overall reduction in the TPP was more obvious in

GSR lines as compared to controls and Kashmir Basmati was the genotype with maximum (42.33) TPP (Figure 2B). For the GY, we observed a 5.68–86.52% decrease under heat stress. Interestingly, NGSR-16 and Kashmir Basmati showed no or little reduction in GY under heat stress and proved to be relatively heat-tolerant genotypes. Of all the genotypes, type IR-6 (11.83 g) was the worst performer and showed the maximum reduction (81.86%) under heat stress.

SY is another important agronomic trait that determines the overall plant biomass. We observed a sharp increase in SY in most genotypes under heat stress compared to the normal conditions (Figure 2D). The GSR lines NGSR-13 (197.17 g), NGSR-14 (178.5 g), and NGSR-16 (169.67 g) were the highest SY producers and showed a 39.84%, 46.47%, and 41.42% increase in SY, respectively (Figure 2D). The increase in SY suggest that plants have increased their vegetative growth and reduced their reproductive growth, which could be an avoidance mechanism.

HI is an important indicator of genotype performance, and a significant reduction in the HI was observed in all the studied genotypes except Kashmir Basmati (Figure 3A). It reduced from 0.40 to 0.07 but an overall less reduction was observed in GSR lines as compared to non-GSR checks (Figure 3A). The GSR lines NGSR-21 (0.40), NGSR-19 (0.34), NGSR-4 (0.33), and NGSR-2 (0.33) were the best performers. Highest reduction was observed in our high-temperature sensitive checks IR-6 (0.07), followed by Kisan Basmati (0.14) and NIAB-B-2016 (0.18).

Previously, drought- and heat-susceptibility indices have been widely used for the identification of tolerant genotypes, and genotypes with low values are considered tolerant ones [4,24,50]. Based on the heat susceptibility index (HSI), the GSR lines NGSR-16 and NGSR-18 showed the minimum HSI values 0.38 and 0.65, respectively, showing their maximum heat tolerance level. While the control variant Kashmir Basmati showed the minimum (0.11) HSI (Figure 3D). In contrast, the highest HSI was observed for NGSR-13 (1.72) and IR-6 (1.63), indicating the least heat tolerance in these genotypes. Overall, GSR lines performed well as compared to studied controls.

Pollen fertility has been used as an important indicator for the identification of heattolerant genotypes because it influences the seed setting and, ultimately, the grain yield. However, we observed the significant effect of heat stress on pollen fertility in only NIAB-B-2016, while all the remaining genotypes showed a higher fertility under heat stress. Most of the GSR genotypes were completely fertile under heat stress, and maximum sterility was observed in NGSR-1 (10.66%) and NGSR-13 (9.07%) (Figures 4 and 5A). These results suggest that these genotypes used some tolerance mechanism to avoid the deleterious effect of heat stress on PFP, and there were some other factors responsible for the reduction in grain yield under heat stress.

#### *3.4. Normalized Difference Vegetative Index (NDVI)*

To monitor the health of a plant, remote sensing is widely used, and data collected from NDVI can be utilized in the identification of tolerance. In this study, we observed that there was no significant reduction in GSR lines as well as controls. The genotypes NGSR-2, NGSR-11, and the control variants IR-6 and Kissan Basmati showed the maximum (>0.7) value of NDVI. Minimum NDVI was observed in NGSR-3 and NGSR-13, while the NIAB-B-2016 showed the minimum NDVI among the control variants (Figure 5B). These results indicate that the GSR lines have the ability to maintain the growth rate under heat stress.
