Introduction
A fundamental goal in evolutionary ecology is understanding the timing
of life history in variable environments (Gremer et al., 2020). As one
of the crucial events in the life cycle of plants, the time of
germination or seedling emergence often determines subsequent plant
performance and success (Harper, 1977; Weiner, 1988). Short delays in
germination can be magnified into large differences in final biomass and
reproduction (Burghardt et al., 2015; Donohue, 2005; Galloway &
Burgess, 2009; Wilczek et al., 2009), especially under competitive
conditions (Dyer et al., 2000; Kelly & Levin, 1997).
Many annual herbaceous plants can germinate within a wide range of
period, and subsequently face contrasting environmental circumstances,
which may be of great complicacy. Plants germinating in different
periods can be exposed to different environmental cues or growing
conditions, such as day length, temperature, water availability, and
interactions with other individuals and species (Donohue et al., 2010;
Lortie & Turkington, 2002; Verdu´ & Traveset, 2005), which regulate
plastic responses in life-history traits such as reproductive timing
(Huang et al., 2016; Wilczek et al., 2009). The individuals that
germinate early have more time to acquire resources to grow for
reproduction, but may face an increased risk of mortality, since
stresses often occur earlier in the season such as drought, frost or
predation (Donohue et al., 2010; Lortie & Turkington, 2002; Verdu´ &
Traveset, 2005). Most of the previous studies believed early emergence
benefits plant performance and survival (Abe et al., 2008; Afonso et
al., 2014; Bianchi et al., 2019; Cogoni et al., 2013; Verdu´ &
Traveset, 2005), the ecological significance of delayed emergence may
have been underestimated (Verdu´ & Traveset, 2005; Wu & Owen, 2014).
However, late germination can also confer higher fitness benefits (than
neighbors) for plants (Castro, 2006; Leverett et al., 2018; Wu & Owen,
2014), by contributing more to seed production, in comparison to the
more contribution to competition of the earlier germinants (Grundy,
2003). Therefore, selection may favour either early, intermediate, or
late germination (Donohue et al., 2010; Kalisz, 1986; Verdu´ &
Traveset, 2005), depending on specific circumstances, or there may be
conflicting selections on germination timing (Akiyama & Ågren, 2014).
An alternative perspective is that germiantion time can be influenced by
a combination of different abiotic and biotic factors (Verdu´ &
Traveset, 2005), and plants may evolve responses to these environmental
cues to promote time emergence, growth and reproduction to coincide with
favourable conditions (Andrés & Coupland, 2012; Blackman, 2017).
Consequently, despite either advanced or delayed germination may have
both advantages and disadvantages, there may be an optimal germination
timing for plants in terms of overall performance in adaptation to
unpredictable environments (Gremer et al., 2020; ten Brink et al.,
2020). Unfortunately, we know little about the optimal germination time
of wild plant species and its influences on plant subsequent
performance.
Phenotypic plasticity, defined as the ability of a genotype to produce
different phenotypes under different environmental conditions, allows a
plant to make morphological and physical adjustments during the entire
life cycle (Bradshaw, 1965). Plants germinating at different times may
be able to cope with environments of different biotic and abiotic
complexity later in their lives, by modifying life-history traits. For
instance, spring-germinating plants may have prolonged vegetative growth
and life cycle with delayed reproduction, as a strategy of competitors
(C), whereas summer-germinating plants tended towards the strategy of
ruderals (R): having a compressed vegetative growth and initiating
reproduction at a smaller size, due to shortened life cycles, than early
germinants (Zhou et al., 2005). However, the phenotypic response at the
whole-plant level constitutes integrated responses of plant modules and
characters (de Kroon et al., 2005), the local responses may be different
and even contrary (Wang & Zhou, 2021). Shifts in germination timing can
cause cascading effects on different modules or traits and thus fitness
(Gremer et al., 2020), and different local responses may interact to
determine the final phenotype of plants. It is unclear how plants
regulate a number of traits in contribution to their strategies of
coping with variations in germination timing (Verdu´ & Traveset, 2005),
and the strategy of plants that germinate with in the optimal time.
Another important aspect should be the effects of plant ontogeny. Most
studies on effects of germination timing have examined plant performance
in morphological traits at the final growth stage (Afonso et al., 2014;
Wang et al., 2006; Zhou et al., 2005), lacking the information on their
dynamic changes or comparison on their performances in the same growth
period. For example, by comparing final performances, it is easy to find
early germinants can perform better than late germinants since they use
more time for growth; whereas late germinants may accumulate greater
biomass and reproduction within the same growth period as early
germinants. And the pattern of biomass allocation varies with different
stages due to allometric growth (Weiner, 2004), the responses of plants
to germination timing may largely depend on specific growth stages,
which may contribute to the discrepancy in relevant results. To address
the effects of germination timing, requires the information on dynamic
pattern of plasticity in traits (ten Brink et al., 2020).
To better understand how plants respond to germination timing via
morphological plasticity at different stages of plant growth, we
conducted a field experiment, with an annual weed species ofAbutilon theophrasti , by growing plants in four times of growth
season including spring, late spring, summer and late summer, as four
germination treatments, and measured a number of mass and morphological
traits on plants at three (or four) stages. We ask the following
questions: 1) is there an optimal germination time for plants? If yes,
which time of germination can lead to the best performance of plants? 2)
How plants respond to germination timing via plasticity in different
mass and morphological traits? And 3) do these responses vary with
different stages of plant growth?