**1. Introduction**

The world's population has been projected to cross 10 billion by 2050, which will significantly increase the demand for food supply [1], while climate change has already threatened food safety. Abiotic stresses have reduced the productivity of the staple crops, which has multiplied the existing challenge of food and nutritional security [2,3]. According to NASA [4,5], the first decade of the 21st century was the hottest in human history, which had huge impacts on agriculture productivity. Various climatic models predict that there will be a significant rise in the Earth's average annual temperature due to the increasing CO2 concentration [6] that will lead to a significant reduction in crop yield and reduced the farmer's income drastically [7]. Heat stress could impart numerous phenological, morphological, and physicochemical changes in crop plants. Considering all the climatic

**Citation:** Waraich, E.A.; Ahmad, M.; Soufan, W.; Manzoor, M.T.; Ahmad, Z.; Habib-Ur-Rahman, M.; Sabagh, A.E. Seed Priming with Sulfhydral Thiourea Enhances the Performance of *Camelina sativa* L. under Heat Stress Conditions. *Agronomy* **2021**, *11*, 1875. https://doi.org/10.3390/ agronomy11091875

Academic Editors: Channapatna S. Prakash, Ali Raza, Xiling Zou and Daojie Wang

Received: 15 August 2021 Accepted: 16 September 2021 Published: 17 September 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

challenges, heat stress at seed formation had the primary role in affecting final yield and quality of oilseed crops.

In the wake of rising temperature, the emphasis should be on crop health [8] because crops require optimal temperature for proper development, and camelina is no exception. The effects of high temperature may hamper the performance of the photosynthetic apparatus that could lead to reduce carbon assimilation to reduce crop growth. Chemical signaling mechanisms in the thylakoids and carbon metabolism are more prone to heat stress damages [9], as high-temperature damages photosystem II that affects the electron transport chain and glycolate pathway due to the overproduction of reactive oxygen species (ROS; such as hydrogen peroxide, H2O2) [10]. High temperature severely influences the mineral and water transport system of plant tissues, which results in mineral deficiency and decreased turgidity [11,12]. Crop yield may decrease up to 10–15% due to each degree ( ◦C) rise in temperature above the optimum [13]. Plants have developed several mechanisms to reduce ROS levels in plant cells [14] by activating enzymatic and non-enzymatic scavenging systems [15]. Heat stress at any crop stage can cause substantial yield losses in Brassica crops [16]. Camelina has shown a substantial reduction in photosynthetic efficiency and crop yield when grown under high-temperature stress [17]. Ahmad et al. [16] have reported a reduction in the performance of camelina under heat-stressed conditions due to the impairment of plant physiological attributes. Temperature above 32 ◦C is critical at the reproductive growth stage and tends to decreased crop yield by the pod abortion, decreasing the number of seeds per plant, and seed weight [17]. Innovative and sustainable methods need to be introduced to improve the performance of crops under heat stress. Recently, thiourea (TU) emerged as one of the effective approaches to enhance high-temperature stress tolerance in plants by regulating metabolic balance, plant growth, and development [18].

Seed priming is a controlled hydration technique that accelerates the key metabolites for osmotic up-regulation [19]. Priming is one of the most feasible and economic technologies enabling the efficient uptake of nutrients, boosting water use efficiency, breaking seed dormancy, promoting early maturity, and improving crop physiology that ensures successful crop production [20,21]. It has been reported that osmopriming enhanced the antioxidant defense system under stressful conditions [22] to improve the defense system against heat stress. In addition, TU modulates the activity of numerous biological compounds such as plant growth regulators, polyamines, enzymes, mineral nutrients, and produces many derivatives, which have the potential to mitigate heat stress damages [23]. It has a major role in the production of proteins, vitamins, enzymes, and chlorophyll in plant cells and tissues from vegetative growth to maturity [24]. Interaction of TU-containing compounds with various biological compounds produces specific derivatives essential for enhancing thermo-tolerance by modulating the ROS scavenging system [25]. The applications of TU may upregulate enzymatic activities in different plant parts that help to remove the ROS, by activating the ascorbate-glutathione cycle to alleviate heat stress [26,27]. Nonetheless, optimal quantity is vital for TU-induced increase in antioxidant activities that contributed to reverse the high temperature stress [28,29]. The heat stress-induced damages can be ameliorated by the pre-sowing seed treatments with different chemical agents, which may be useful for the generation of heat-stress tolerance in plants.

Camelina [*Camelina sativa* L. Crantz] is an emerging oilseed crop with unique characteristics in relation to its high adaptability against abiotic stresses [30]. Its oil constitutes a vital product for the bio-based industries, as its distinctive composition permits multiple applications [31]. It is a rediscovered oil crop that belongs to the family *Brassicaceae*, having a seed oil content of 26–43% [32,33] with high percentage of unsaturated fatty acids. The seeds of camelina are unique compared to other members of *Brassicaceae* due to their high amount of polyunsaturated fatty acids and low level of erucic acid. Regular ingestion of camelina oil reduces the level of cholesterol in blood and the presence of tocopherols prevents the oil from rancidity [34]. According to Zubr [35], Camelina can be adapted to various environmental conditions including limited water conditions, high and low temperatures, etc. Camelina is popularly known as false flax, which is typically a cool-season crop with a temperature that seldomly exceeds 30 ◦C [36]; however, it can be grown in the winter and spring seasons [37].

However, the role of TU has been documented under abiotic stress tolerance, while the role of TU to alleviate heat stress damages in camelina needs further investigation. The study was hypothesized that TU priming regulates the heat stress tolerance in camelina. The objective of the study was to evaluate the impact of seed priming techniques on growth and yield parameters on physiological basis in camelina under heat stress conditions.
