4.1 The early rice-cultivation season is a stressful environment for weedy rice
Analytical results of 10-year climate data showed different variation patterns of air-temperature and daylength between the early and late rice-cultivation seasons in Leizhou, and the 10-year average air-temperature of the early season was significant 1.7 ℃ lower than that in the late season, particularly during the initial rice growth stages (4.6~8.2℃). In addition, the 10-year average daylength of the early season was significant 0.45 hours longer than that in the late season, and it was up to 0.98~1.81 hours during the reproductive stages. Generally, low temperature and long daylength are environmental restrictions on rice performance, because rice is a short day and cold-sensitive plant (do Amaral et al., 2016; Wei et al., 2020). As for a conspecific weed that belongs to the same biological species of cultivated rice, weedy rice has the same responses to temperature and daylength. Therefore, these findings indicate that the low temperature and long daylength may be the primary stressful elements affecting weedy rice growth and development in the early-season environment.
The optimal temperature for rice cultivation is between 25 and 35 °C (Hussain et al., 2019), and temperature beyond optimum is harmful to rice and negatively affects growth, development and ultimately reduces the grain yield. In this study, lower average air-temperature was found during the seedling stage (20.9 °C) in the early season, as well as during the tillering stage (24.2 °C), than the optimum. By contrast, the average air-temperature was 29.1 and 28.8 °C, which fit the optimum, during the corresponding stages in the late season. Therefore, the early-season weedy rice had to experience the low temperature stress at the germination and seedling stages in the early season, and the low temperature may be a strong selective pressure to affect the growth and development of the early-season weedy rice in early stages. Altogether, above findings suggest that the early rice-cultivation season is a stressful environment with lower ambient temperatures compared to the corresponding late season, especially during the initial growth stages.
Generally, rice growth at the early stage is important for stable seedling establishment and subsequent vigorous vegetative growth (Sharifi, 2010), because seed germination, seedling emergence, stem elongation and individual survival are typically temperature dependent (Krishnan et al., 2011). During this period, the occurrence of low temperature stress inhibits seed germination, seedling establishment, and causes the retardation of rice growth. For example, apparent reduction in final seed germination ratio and the length of coleoptile and radical were observed when rice seeds were germinated at a temperature below optimum (Sharifi, 2010), and evidently reduced seedling growth and vigor of rice were also found under a low temperature condition (Rahul et al., 2017). In this study, significant reduction in plant height and number of tillers was found in almost all weedy rice populations during the early stages in the early-season common garden, compared to the same stages in late-season common garden. Therefore, these findings indicate that low temperature stress has direct negative effects on weedy rice growth, most likely caused by retardation in germination and seedling establishment during the initial stages. Low temperature stress not only lengthens the vegetative growth of rice, but also causes lagging in reproductive growth through delaying flowering time (Song et al., 2012). Our common garden experiments provided solid evidence of significant reduction in the number of seeds per plant, seed setting ratio and 100-seeds weight in weedy rice populations during the early-season common garden, compared to the high-temperature late season. Therefore, the early rice-cultivation season is a relative stressful environment for weedy rice growth and development because of its low ambient temperatures.
In addition, the daylength was significant longer in the early season than that in the corresponding late season, especially during the reproductive growth stages (~2 hours), based on the 10-year daylength data analyses in this study. Noticeably, the evident lagging in flowering time was found in almost all weedy rice populations in the early-season common garden, and it was about 10 days later in flowering time than that in the late-season common garden. Therefore, this finding supports the photoperiodical flowering response of rice in a long day environment, because rice is known as short day plant and long daylength environment usually delays the flowering time of rice (Wei et al., 2020). Therefore, long daylength is another constraint environmental factor affecting weedy rice growth and development in the stressful early rice-cultivation season. Previous studies suggest frequently interactions between temperature and photoperiod in plant flowering time regulation, and low temperature and long day environments are crucial causes of late flowering in rice (Franklin, 2009; Song et al., 2012). Altogether, we can conclude that the early rice-cultivation season is a stressful environment for weedy rice growth and development, because of its low temperature and long daylength.