**4. Discussion**

Crop productivity is mainly dependent upon environmental growth factors, which suggested that the ever rising CO2 concentration in the atmosphere is the major reason for climate change that causes temperature fluctuations at an alarming rate, leading to the imposition of heat stress. The suboptimal temperature at any crop growth stage imparts deleterious impacts on crops productivity including camelina. In this study, heat stress showed negative effects on crop growth and yield attributes as it reduced the photosynthetic efficiency and stomatal conductance due to reduction in leaf water status in camelina, while TU supplementation remained effective in ameliorating the negative impacts of heat stress in camelina varieties (Tables 1–3; Figures 1 and 2).

The hypothesis of the study has been accepted as results indicated that the TU supplementation has played important role to ameliorate the negative impacts of heat stress by seed priming (TU0 = No thiourea priming, TU1 = Water priming, TU2 = Thiourea priming), which lead to improving the seed yield. High temperature hampered plant growth by causing a reduction in morpho-physiological attributes in camelina varieties. Among the seed priming techniques tested in this study, TU-osmopriming (TU priming at 500 ppm) remained an effective technique to improve the performance of camelina varieties under heat stress, as TU application boosted stomatal conductance that might be attributed to an increase in photosynthetic rate, which conferred the resistance against heat stress through physiological regulations (Tables 1–3 and Figures 1 and 2). Indeed, TU priming ameliorated the negative impacts of heat stress, regulated plant growth, improved the water status to facilitate the stomatal conductance, and also acted as a compatible osmolyte, which assisted plants to cope with heat stress. The present study confirmed a significant reduction in plant height, roots, and shoot lengths along with their fresh and dry weights under heat stress, while more reduction was observed in the control treatment (no TU-priming).

Temperature above threshold level increased the physiological activities, which consequently fastened the crop growth rate that further reduced the growth period of crops [39,40]. This reduction in the growth period led to the premature completion of phenological plant parts that could damage the final yield. Results showed that heat stress reduced the physiological attributes including photosynthetic rate, stomatal conductance, transpiration rate, and intercellular CO2 concentration under control–no priming as compared to TU osmopriming. Results have shown that the transpiration rate was increased due to high temperature stress, which leads to disturb plant water status; however, it modulated higher water loss, which was primarily caused by heat stress. Heat stress reduced the photosynthetic rate, which is more prone to heat damages that lead to reduce crop growth and grain yield [41,42]. Heat stress negatively affected the reproductive growth that disturbed the seed formation process which may reduce the number of seed pod−<sup>1</sup> and seed weight [43,44], because high-temperature stress affects the source-sink relationship and explain the differences in seed yield [45]. Among the gas exchange attributes, results exhibited that heat stress restricted the rate of CO2 assimilation as indicated by lower photosynthetic efficiency, transpiration rate, and stomatal conductance, while intercellular CO2 concentration was increased, which showed the negative effect of heat stress on the stomatal component of photosynthesis; nevertheless, it also hampered the assimilation of absorbed CO2. The reduction in the photosynthetic efficiency resulted in reducing the assimilation formation and translocation towards the sink, which led to reduce the seed yield and related attributes [18]. Heat stress at the reproductive stage could damage the seed formation processes including flowering and seed set that hampered the rate of grain filling and grain yield [46]. High temperature directly affected the crop water relations as water potential, osmotic potential, pressure potential, and relative water content were decreased by 31.9, 6.72, 46.9, and 30.6% as compared to control–no stress with more reduction in no-TU treatment (Figures 1 and 2). Reduction in plant water status effects the turgidity of the cell that directly effects the elongation of the cell, which leads to reduce the crop growth.

Pre-sowing seed treatment was the foundation for the early activation of seed metabolism that in combination with other elements might be helpful to the proper vegetative growth and higher seed yield. The survival of plants under stress conditions could be possible by the supplementation of stress alleviating chemical compounds [47,48], like TU, that can potentially upregulate the plant defense to improve plant tolerance under stressed conditions [49]. The applications of TU manifolds growth regulatory roles in plant species including camelina varieties. Our results depicted that exogenous application of TU as a seed priming treatment improved the seed yield of both camelina varieties under heat stress conditions compared to control (Table 3). The TU priming significantly improved the yield attributes including plant height, root and shoot lengths along with their fresh and dry weights indicating higher biomass accumulation triggered by TU which is in line with the findings of Asthir et al. [40]. The sulfhydryl TU not only increased the root length but also increased branching in roots under heat stress that tends to increase the root fresh and dry weight. The ascribed TU-induced increase in growth could be due to the mediation of a number of important metabolic functions.

Thus, seed priming with TU offered a promising and economical solution for improving crop resistance against heat stress [17]. Plants treated with TU exhibited maximum biomass accumulation as compared to control–no TU applied, indicating the positive role of TU in boosting plant growth by alleviating the adverse effects of heat stress (Table 1). In addition, osmoprimed crops could timely complete all the phenological events, and this phenological plasticity can be helpful when integrated with high-temperature stress to avoid their negative effects on crop growth and development during early and later reproductive stages without yield penalty. TU supplementation reduced heat stress-induced oxidative stress by upregulating the important phenomenon of photosynthesis, and assimilate translocation which was also reported by Patade et al. [27] to enhance the defense system in camelina plants to impart heat stress. The TU-supplementation improved the seed weight and consequently gave higher seed yield per pot in both varieties under heat stress conditions as compared to control–no TU priming at the same conditions. This can be attributed to the improvement in plant metabolism, which enabled the plant defense against heat stress [50]. In the current study, TU priming (500 ppm) helped to increase photosynthetic rate and stomatal conductance [48]; however, the net CO2 assimilation rate was more with TU priming than control–no TU applied.

The possible reason for this variation in photosynthetic rate may be that TU application increased the leaf growth which in turn up-folded the photosynthetic rate by increasing the harvesting of photosynthetically active light (Table 2). Thiourea supplementation at any growth stage and through any methods of applications may improve the photosynthetic apparatus in plants that helped the plants to maintain photosynthetic rate [51,52]. In the present study, exogenous use of TU as pre-sowing seed treatment was found effective in decreasing the damages caused by heat stress. Additionally, the TU-applied reduction in intercellular CO2 concentration rate might be due to the effective role of TU metabolites in the regulation of activities of antioxidative enzymes (Table 2). Available reports support the present findings where seed priming with different compounds was found effective in improving the plant physiological attributes [53–55]. Our results are in line with Orman and Kaplan [56], who reported that TU application increased the biomass of tomato plants by 6–8% grown in sandy loam soil. TU at either stage performed well to alleviate heat induced damages, while, TU supplementation at vegetative stage improved plant height, root length, and dry weight compared to TU applied at the vegetative growth stage [48,57]. In addition, TU supplementation upregulated the plant water relations which played significant role to improve the stomatal conductance as compared to no-TU application in line with the findings of Ahmad et al. [18].

The results have shown variability among two varieties of camelina under heatstressed conditions, as 8046 has shown more resistance to the deleterious effects of heat stress as compared to variety 7126. Variety 8046 has shown better performance in relation to plant growth attributes as compared to 7126, as plant height and root length were maximum in 8046 and minimum in 7126 (Table 1). The impact of heat stress-induced damages is cultivar-specific depending on the extent of tolerance based on various tolerance mechanisms including the cellular oxidative defenses in terms of the enzymatic and nonenzymatic antioxidant compounds [58–60]. The endogenous content of photosynthetic pigments with CO2 assimilation rate was also affected in heat susceptible variety (7126)

as compared to heat resistant variety (8046), which is in line with the findings of [61]. Almeselmani et al. [62] and Balla et al. [63] have also noted that the activities of plant defense system upregulated the plant physiological attributes including photosynthetic rate and stomatal conductance while decreasing intercellular CO2 concentration intolerant variety (8046), but downregulated insensitive variety (7126) under high-temperature stress. However, variety 8046 was not so affected and sustained its biomass, photosynthetic rate, plant water status, and seed yield as compared to 7126 (Figures 1 and 2).
