**1. Introduction**

Cotton is a Kharif season crop grown mainly for feed, fiber, and oil in the Punjab and Sindh regions of Pakistan. These are considered hot regions since the temperature reaches 47 ◦C during the growing season. Environmental stresses such as heat, and drought affect cotton plants by impeding normal physiological processes which lead to morphological abnormalities and yield reduction [1]. Plants mostly invest their defense in the most valuable sections, such as reproductive parts under various stress conditions. Cotton production is vulnerable to abiotic stresses, particularly during the growth stages of

**Citation:** Rehman, A.u.; Rana, I.A.; Majeed, S.; Chaudhary, M.T.; Zulfiqar, M.; Yang, S.-H.; Chung, G.; Jia, Y.; Du, X.; Hinze, L.; et al. Intra-Plant Variability for Heat Tolerance Related Attributes in Upland Cotton. *Agronomy* **2021**, *11*, 2375. https:// doi.org/10.3390/agronomy11122375

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

Received: 25 September 2021 Accepted: 19 November 2021 Published: 23 November 2021

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blooming and boll formation, which have become more frequent as our climate changes [2]. Any stress during this stage abruptly reduces the yield. Numerous efforts have been made to understand the physiological, molecular, and genetic pathways of the cotton plant related to sustaining yield under stress conditions [3,4].

The reproductive efficiency of the cotton crop is negatively impacted by temperatures above 32 ◦C in a variety of ways, including reduced metabolism as well as suppression of photosynthesis, pollination, fertilization, and crop growth rate [5]. Heat and drought stress causes male gametes to undergo metabolic and structural changes that result in meiotic abnormalities or premature spore abortion [6]. It also results in poor pollen germination and short pollen tube growth in cotton [7,8]. It was reported that pollen germination is better in flowers that have been pollinated under the canopy of the plant as compared to flowers that are directly exposed to sunlight and pollinated during high-temperature stress [9].

Yield reduction is also associated with certain changes in metabolic and biochemical pathways in plant cells, i.e., excessive accumulation of reactive oxygen species (ROS) such as H2O2, singlet oxygen, hydroxyl ions, etc., during stress conditions [10]. As a result of a dramatic accumulation of ROS during stress, programmed cell death has been observed in developing pollen grains [11]. Hence, ROS scavenging through the action of antioxidants in anthers has a role in maintaining pollen viability under abiotic stress [12]. Under high temperature or water deficit conditions, the role of the cell membrane in maintaining cell osmotic balance may be impeded due to leakage of electrolytes [13]. In cotton, temperatures over 35 ◦C increased membrane leakage and reduced leaf size [14]. High canopy temperature adversely affects the chlorophyll content in leaf tissues and lowers the rate of photosynthesis and carbohydrate production [15]. Reduction in carbohydrate content is also associated with decreased lint yield [16]. Cotton plants will shed bolls when they are stressed, thus boll retention drops significantly under harsh environmental conditions [17,18].

It was noticed that different portions of a single cotton plant of a cultivar exhibit variable responses to stress during reproductive and vegetative phases. Although every cell in a plant has the same genetic material, the different behavior might be due to epigenetic [19,20] or other effects. Every cell expresses itself according to the stimulus received from the environment. Young leaves are more resistant to insect damage compared to old ones [21]. So, every part of the plant faces a different environment. As a result, these positions phenotypically behave differently. Cultivars also differ in canopy shape and intra-plant morphological features. Moreover, cultivars are grown in the same region exhibit variation among them. Environmental and genotypic effects both contribute to the phenotype. Therefore, the objective of this study was to identify inter- and intra-cultivar variability for physiological as well as morphological attributes associated with the yield of seed cotton under heat stress conditions.
