Rice Genomics and Genetics 2024, Vol.15, No.1, 36-47 http://cropscipublisher.com/index.php/rgg 39 rice seedling stage encounters continuous high temperatures and lack of water in the rice field, the vigorously growing rice seedlings in the early stage enter the state of ear differentiation in advance. The ears will be heading around 12 to 15 days after transplanting, and the ears will be obviously weak. In production, this is called "banding". "Fetal transplanting", if not handled in time, will affect the growth and development of other tillers, and in severe cases, the yield will be reduced by 20% to 40%. The tillering stage is a critical period when rice begins to form side tillers and reproductive organs. High temperature stress may lead to a reduction in the number of tillers, which directly affects yield. In addition to tillering, high temperatures may also affect the development of inflorescences, thereby affecting subsequent fertilization and grain development. In this case, high temperature may lead to the formation of empty spikes or damage to some spikelets (Wang et al., 2017). In the early heading stage, the rice plant is preparing to enter the reproductive stage, forming inflorescences and flowering. High temperature stress may cause delayed or abnormal heading, thereby affecting the success rate of fertilization. This may result in empty ears or insufficient fertilization, thus reducing yield. High temperature stress may also affect the flowering process of rice. Under high temperature conditions, pollen may be damaged, reducing pollen viability and affecting fertilization. Decreased pollen quality and quantity is a key factor in yield reduction caused by high temperature stress. The grain filling period is the period when rice grains are enlarged and substantial. High temperature stress may cause the development of grains to be blocked, affecting the size and quality of the grains. In addition, high temperatures may cause uneven expansion of the seeds, causing some of the seeds to become smaller or hollow. Understanding the different types and classifications of high temperature stress can help to better address this challenge. Sustained high temperature stress refers to rice being exposed to high temperature conditions for a long time, which may cover multiple growth stages. In this case, the impact of high temperature on yield is usually severe, as it can cause damage to multiple reproductive and growth processes. For example, high temperatures may occur continuously during the seedling and tillering stages, resulting in significant yield reductions. Cyclic high temperature stress refers to high temperature events occurring at specific growth stages, but not necessarily continuously. For example, high temperatures may be exposed to rice during the tillering or heading stages, while temperatures at other stages may be relatively low. In this case, the effects of high temperatures focus on specific reproductive or growth processes. Understanding the cyclical nature of this high temperature stress helps to take targeted management measures (Schneider et al., 2020). Rice usually grows under cooler nighttime temperatures, but if nighttime temperatures rise, the temperature difference between day and night will decrease. This is also considered a form of high temperature stress as it affects the normal growth and development of rice. Rice usually photosynthesizes at night, taking advantage of periods of low temperatures to maximize light energy utilization. If temperatures rise at night, plants may suffer during this critical process, affecting yields. 2.3 Physiological changes of rice under high temperature With the intensification of global climate warming, high temperature stress has become one of the most important stress factors restricting the security of world food production. It is predicted that by 2040, high temperatures will significantly reduce global food production. Therefore, mining high-temperature resistance gene resources, elucidating the molecular mechanism of high-temperature resistance, and cultivating new high-temperature resistant crop varieties have become major issues that need to be overcome (Song et al., 2021). High temperature stress limits photosynthesis in rice leaves. Damaged leaves cannot effectively utilize light energy for photosynthesis, resulting in reduced carbon fixation. In addition, stomatal closure is one of the plant's responses to high temperatures, which reduces the entry of CO2 and further reduces the rate of photosynthesis. Therefore, rice biomass accumulation is limited under high temperature conditions.
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