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?