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.