1 INTRODUCTION
Temperature and photoperiod have profound effects on plant growth and development (Franklin, 2009; Ding et al., 2020), and contribute to seed germination, flowering time, and reproductive traits of plants (Penfield, 2008; Jackson, 2009). Generally, each species has a specific temperature range represented by minimum, maximum and optimum (Hatfield & Prueger, 2015). For example, the optimal temperature for rice cultivation is between 25 and 35 °C (Hussain et al., 2019), and temperature beyond optimum may have negative effects on rice growth and development. However, responses to temperatures differ among plant species throughout their life cycle and are primarily the phenological responses (Hatfield & Prueger, 2015). For instance, a prolonged period of cold, called the vernalization response, can promote plant to flower in Arabidopsis thaliana (Kim and Sung, 2014), and a high temperature can shorten the period of grain filling in rice (Kim et al., 2011). In addition, the defined range of maximum and minimum temperatures form the boundaries of observable growth, vegetative development increases as temperatures rise to the species optimum level (Hatfield & Prueger, 2015).
However, temperature is often an unreliable marker of seasonality, most plant species native to areas outside the tropics have evolved a second line of safeguarding them against misleading temperature conditions—photoperiod, which is defined as the developmental responses of plants to the change of daylength over the years (Körner, 2006). In other words, the response to photoperiod has evolved in plants because daylength is a reliable indicator of the time of year (Andrés & Coupland, 2012; Kubota et al., 2014), enabling developmental events to be scheduled to coincide with particular environmental conditions (Jackson, 2009). Photoperiod plays major roles in synchronization of flowering in plant populations and thus ensuring reproductive success, and preventing phenology from following temperature as a risky environmental signal for development (Körner, 2006). Noticeably, interaction between a temperature and photoperiod also plays important roles during the life of plant species (Franklin, 2009; Song et al., 2012). For example, the floral transition of plants always depends on the accurate measurement of changes in photoperiod and temperature, and thus photoperiod and temperature are two pivotal regulatory factors of plant flowering (Song et al., 2012).
Individual plants are sessile, and therefore have to develop the means to detect and respond to environmental changes as they occur. As a consequence, plants continuously monitor their surroundings and adjust their growth to daily and seasonal cues (Capovilla et al., 2015). Weed species, such as agricultural weeds, have been rapidly evolved to adapt to changes during farming practices (Vigueira et al., 2013; Mahaut et al., 2020). In the light of intrinsic capacity of rapid adaptation, weedy species that occur over a relatively short period of time become an appealing system to study evolutionary processes. Generally, the agricultural weed syndrome includes rapid growth, high nutrient-use efficiency, seed dormancy, efficient seed dispersal, crop mimicry, and herbicide resistance (Vigueira et al., 2013). Therefore, agricultural weeds must possess traits that permit them to survive and thrive in the recently created environment. The evolution of herbicide resistance is probably the most emblematic and well-documented case of rapid evolution in weeds (Baucom, 2019). In addition, climate change, such as temperature and moisture fluctuations, has direct effects on the survival, distribution and competition of weedy species in cropping system (Peters et al., 2014). For example, Xia et al. (2011) found seeds of weedy rice can germinate at a lower temperature than its co-occurred cultivated rice, and the germination ratio showed a latitudinal gradient pattern between weedy rice populations from north China down to the Jiangsu Province.
Weedy rice (Oryza sativa f. spontanea , WR, Figure S1a) is a noxious agricultural weed infesting worldwide rice fields (Delouche et al., 2007). It is a conspecific weed that belongs to the same biological species of cultivated rice (O. sativa ), but with strong seed shattering and prolonged seed dormancy. In the typical tropic rice cultivation regions, such as Guangdong, Guangxi, and Hainan Provinces, rice is cultivated for two seasons, namely the early and late rice-cultivation seasons. In both the two seasons, weedy rice was found in the same rice fields (sympatry). Generally, phenological conditions, such as temperature and photoperiod, between the two seasons are considerably different. Differential genetic diversity and considerable genetic differentiation between the two-season WR populations were reported by Kong et al. (2021). Therefore, we believe that such considerable genetic differentiation is accompanied with certain phenotypic divergence between the two-season WR populations in the same rice fields, most likely because of the adaptive evolution in the weedy rice populations.
Common garden experiment is regarded as an efficient tool to study adaptation and the genetic bases of the adaptive traits by growing individuals from different populations in a common environment (de Villemereuil et al., 2016, 2020), and it has been used extensively with plant species (Albaugh et al., 2018; Groot et al., 2018). In this study, we conducted in situ common garden experiments to estimate the phenotypic divergence between the early- and late-season WR populations in early and late rice-cultivation seasons, respectively. The major questions addressed are as follows: (1) What are the patterns of temperature and photoperiod variation in different rice-cultivation seasons in Leizhou? (2) Do the vegetative and reproductive growth traits diverge between the sympatric two-season weedy rice populations? (3) Has the local adaptation developed in weedy rice populations? Answers of the above questions can support the genetic divergence between the sympatric two-season WR populations from another perspective, and provide solid evidence of ambient surroundings associated rapid adaptive evolution in plant species.