**1. Introduction**

Rice (*Oryza sativa* L.) is considered as one of the most essential food crops around the world, especially Asia, Latin America, and Africa [1,2]. It constitutes nearly 20% of overall calorie intake worldwide [2], with up to 80% of calorie ingestion in Asia [3]. Global rice production is direly needed to increase at a growth rate of 1.0–1.2% and grain yield must increase by 0.6–0.9% annually to feed the rapidly growing population, comprising a projected increase of nearly 2 billion people up to the 2050s [4,5]. Agricultural crops are more prone to abiotic stresses due to irregular and unsteady climatic changes [6–9]. Atmospheric temperature is one of the most critical variables determining the seasonal

**Citation:** Zafar, S.A.; Arif, M.H.; Uzair, M.; Rashid, U.; Naeem, M.K.; Rehman, O.U.; Rehman, N.; Zaid, I.U.; Farooq, M.S.; Zahra, N.; et al. Agronomic and Physiological Indices for Reproductive Stage Heat Stress Tolerance in Green Super Rice. *Agronomy* **2022**, *12*, 1907. https:// doi.org/10.3390/agronomy12081907

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

Received: 26 May 2022 Accepted: 12 August 2022 Published: 14 August 2022

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**Copyright:** © 2022 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/).

growth and geographic cultivation and distribution of crops [10–12]. An increase in the global mean surface temperature of 0.85 ◦C was observed between 1880 to 2012 and future projections forecast a 3.0–5.0 ◦C increase by the end of this century [13] and 2.0 to 4.0 ◦C until 2050 in Southeast Asia, specifically [14]. Relative to the period from 1900 to 2000, the climatic observations through various models have projected a high (more than 90%) probability of temperature-increase during crop growing season in the tropical and subtropical regions of Asia, such as China, by the year 2100 [15,16].

According to projections, it is expected that environmental fluctuations, especially high temperature stress, may cause a 41% yield decline by end of this century [17,18]. High temperature stress destructively impacts the rice metabolism in all growth phases [19–21]. Rice seedlings are very sensitive to the critical high temperature of 35 ◦C [22]. Further elevation beyond the critical high temperature can be destructive and may lead to plant death at respective growth stages [18,23]. The frequent occurrence of extreme climatic events, such as high temperature, leads to adverse impacts on rice growth and development.

Flowering in rice is one of the most critical phases in the context of high temperature stress because it could reduce the grain yield due to pollen sterility, poor grain-filling, low grain weight accumulation, and undermined seed setting [24]. Rice is sensitive to heat stress and the threshold temperature for rice at the anthesis and flowering stages are 33.7 ◦C and 35 ◦C, respectively [25,26]. High temperature stress also impacts the physiological processes of rice, such as chlorophyll contents, photosynthesis, respiration, and RuBP carboxylase activities [27]. High temperature stress above 38 ◦C inhibits the spikelet formation associated with the decomposition and synthesis of cytokines [28], and spikelet differentiation aggravates spikelet degeneration and reduces the overall number of spikelets through peroxide accumulation, which destructs the cell division and construction [29–31]. Additionally, the incidence of high temperature stress inhibits the anther filling and panicle initiation phase, which may lead to a decrease in pollen activities inducted by the impeded development of pollen mother cells and abnormalities in the decomposition of the tapetum [32,33]. Recently, studies have shown that high temperature stress could cause spikelet sterility due to a reduction in pollen vitality, vigor, and viability, and also due to the inhibition of anther dehiscence [34,35] and obstruction of pollen tube germination [36,37]. High temperature stress could also cause the insufficient accumulation of nutrients in pollens, which may lead to a reduction in pollen activities, sugar transport, accumulation of peroxides, and carbon metabolism [38,39]. High temperature stress at the flowering stage also effects the stigma vitality and pollen tube elongation [37].

Self-adaptability in the rice plant and responses to high temperature stress greatly depends on several factors, such as the intensity and duration of high temperature stress, growth period, plant size and age, and rice genotypes. To address this challenge, natural variations in rice germplasm under drought stress could be utilized to evaluate the associated traits of stress-tolerant genotypes [40,41]. Target breeding programs could be exploited as an important and essential genetic breeding resource to stimulate the genetic variations through hybridization. Aiming at this, studies have been initiated focusing on green super rice (GSR), an elite and highly water- and nutrient-use efficient rice type [42–44]. Based on the information on cloned green genes and loci, large-scale cross- and backcross-breeding was conducted to generate IL populations and lines abundant in green traits with wild rice, core germplasm, and specific local varieties as the donors [45]. The GSR lines were developed by combining genes from different native and non-native sources and required fewer fertilizers, pesticides, and irrigations. These lines also have greater stress tolerance without compromising the high yield and quality [46].

This study was conducted with the aim of investigating and evaluating the mechanisms of high temperature stress-tolerance through the identification of high temperatureresponsive morpho-physiological traits of different GSR lines along with local rice cultivars. GSR lines showed several genotypic differences on high temperature stress tolerance; however, the physiological and biochemical heat-tolerance mechanisms are rarely considered. Other major aim of the study was the investigation of mechanisms that how high

temperature incidence on flowering impacts the growth, yield, and quality of rice. The acquired research knowledge will be the basis for sustainable GSR production systems and the breeding of novel rice genotypes in order to optimize the net grain yield and nutritional quality, ultimately moving towards human health by decreasing poverty in densely-populated rice regions.
