Heat-tolerant cultivars are required to mitigate the adverse impact of changing climate without the reduced harvest exhibited by conventional, non-heat tolerant cultivars21. As expected, HT has significant effect on physiological and agronomical traits2,5,18, and duration of seedlings survival among cultivars can be ranged. Thus, duration of pepper seedlings survival was 5 days in both cultivars, but on the 7th day were revealed significant reduction of heat tolerance, and the threshold of heat tolerance for cv. “New Bigarim” was detected on the 10th day of HT, which was heat susceptible compared to the tolerant one cv. “Chyung Yang”. This kind of the difference can be related with development stages, genotype features and structural changes of plant11,16,22−23.
We determined the physiological and biochemical status among pepper cultivars at HT, while physiological parameters are fundamental for the understanding of plant responses to the environment, where an increase in maximum temperature above the thermal optimum should have a negative effect on physiological performance6,24−25.
The diurnal temperature and duration of heat treatment influence the physiological parameters of plants 16,24−25, that were found in our study, where values of CHL, EC, PRL, PN, Ci, Gs and Tr were ranged depends on the duration of treatment. So, at heat-stress condition, the CHL tends to decrease in leaves, whereas the magnitude of the change in heat-tolerant plants have to be smaller than in non-tolerant plants6,23,26−27, resulting in premature chlorosis28. While, greener leaves with low LHD under elevated temperatures contribute to better photosynthesis, which is an important agronomical characteristic of stay-green that contributes to higher yield under heat stress29. However, in our case the CHL was higher in heat-susceptible cv. “New Bigarim” compared to the heat tolerant cv. “Chyung Yang” during HT, which indicate that the response of genotypes on stress is different24. And, we assume that the appearances of leaf heat injury symptoms are important in comprehensive evaluation of heat tolerance, whereas in heat tolerant seedlings cv. “Chyung Yang” were observed yellowing of leaf parts but in heat-susceptible cv. “New Bigarim” were determined wilting and desiccating without yellowing- degradation of CHL, resulting in premature drying. While, it is known that chlorophyll fluorescence induction method such as Fv/Fm can often correlate with the state of photosynthetic apparatus30,31.
Our results are not in accordance with previous reports6,16,23, where indicated that the photosynthetic apparatus in heat-tolerant cultivar with high CHL have better stability than that of the heat-susceptible. Since, in our present study the HT significantly increased the PN in both cultivars compared to the NT seedlings, where heat tolerant cv. “Chyung Yang” showed low CHL. Also, in our previous work we identified high concentration of CHL in tomato leaves, even though PN values were significantly reduced in low temperature condition32, and this kind of differences can be explained by the fact that the response of the genotype differing in heat stress condition.
While, Gs is regulated to accommodate the need for Ci for PN. As a consequence of being able to maintain high rates of PN, the higher Gs and Tr during heat stress improved leaf cooling in the heat-tolerant genotypes for protection, if temperatures are close to the threshold for heat damage6. However, our results contradict some conclusions, where the high rate of PN was detected in heat tolerant cv. “Chyung Yang” on stress day 2 but on day 7 it reduced significantly, whereas cv. “New Bigarim” maintained high rate of PN than initial rate at NT yet. It can be explained that in heat susceptible plants protection mechanisms still were more activated than tolerant one to avoid the death of plants from heat stress and decrease in PN values was the result of non-stomatal factors2,8,16.
Heat treatment made to significantly increase the Tr in heat susceptible25 cv. “New Bigarim” seedlings on stress day 2 and day 7, and this pattern were also revealed in measurement of Gs than in heat tolerant cv. “Chyung Yang”. If air temperature is very high and the plant’s regulatory mechanism could not maintain the leaf temperature at its thermal optimum25, then a decrease in Ci, maximum PN and Gs would be caused by an increase in stomatal limitation, but in our results were identified increasing of trends of Gs, Ci and Tr32. There no identified significant interrelation between the increasing of Gs and Tr with heat tolerance that allows the better cooling of the leaf with limits PN in the sensitive cv. “New Bigarim” plants at HT6. Since, Ci was slightly increased with no significant difference at HT among cultivars and unaffected.
In line with this, heat tolerant cultivar had more stable2 or higher membranes22 in comparison with a heat-susceptible one, and cell membrane thermo-stability has often been linked to PN and Tr at HT12, however these latter two traits do not seem to be affected much by moderate heat-stress1,33, while in our results we did not find any significant relation between membrane stability and reproductive traits.
The plants response mechanism to environmental stress is the accumulation of osmoprotector solutes, such as PRL20,34 and generally found in large amounts under any stress conditions15,22,35. In the present study regardless of cultivars, high accumulation of the PRL in all LHD plants on recovery day 5 was observed, where the mean fold increased significantly over NT plants. It indicates that the plants were under stress condition and protection mechanisms were activated yet. These tendency indicated that pepper plants subjected to high temperature in seedling stage experienced high stress levels by recovery, as accumulation of PRL is considered a strong indicator of abiotic stress15,35, and we agreed with conclusion of some researchers36,37 that high accumulation of PRL in plants during HT is detrimental for plant growth at recovery, which is not always a compatible solute during environmental stresses and high doses will impart toxic effects and may significantly delay the growth rate of plant. Thus, PRL surplus is probably channeled and used in other metabolic pathways instead of producing of specific hormones or may hinders the growth activates.
Evaluation of effect HT on vegetative parameters of seedlings showed that regardless of cultivar fresh weight of seedlings and roots were decreased than NT seedlings, where we could not find any significant difference of biomass between heat tolerant and sensitive cultivar. While, a decrease in PN rate during HT, was reflected in seedling growth parameters, where photosynthesis deficit may disrupt the metabolic pathways and reduce the growth rate and biomass, whereas the tolerant genotypes accumulated more biomass, with lower heat injury index and higher fruit yield6,38.
In the plants of pepper with different LHD significantly reduced both FS and NFR per plant regardless of cultivars. The fruit setting is the main indicator for screening the response of genotype on abiotic stress condition1,5,32, and heat stress may significantly reduce or increase the FS and number of flowers5,17,18,22.
We observed in the current study that the plants which were subjected to HT in seedling stage and had a different LHD may a FS to a large extent, on par with NT plants regardless of cultivars. However, the NFR per plant was drastically reduced.
NFR can be used as predictor for FS in screening of the heat tolerant genotype since it positively correlates with FS in HT and that NT16,18. The plants of the cv. “New Bigarim” with different LHD identified with more positive percent of differences in FS and NFR, which were screened as heat susceptible in seedling stage than cv. “Chyung Yang”. We agreed with the hypothesis that genotypes with good FS under NT can be pre-selected simply for screening on heat tolerance5,16,18.
The different LHD caused a decrease in TY of both pepper cultivars, where over 25% of the LHD was critical for producing economic harvest per plant. And, as mentioned above the cv. “Chyung Yang” distinguished as a heat tolerant in seedling stage showed the biggest negative differences in index of yield between NT and LHD plants. There are no identified positive relation between PRL and reproductive parts 39, whereas heat tolerant genotype have to accumulate high PRL and produces the higher fruit yields22, and we could not find any the linkage between the seedlings and reproductive growth stages in heat tolerance, which confirmed in other results5.