Conclusions
Our results showed that there is an optimal germination time, and it was late spring in this study, when plants can maximize their growth potential in relatively favorable conditions. Plants that germinated earlier in spring used a longer time for vegetative growth and did not outperform late-spring germinants, probably due to exposure to spring drought and response to competition. Delaying germination into summer led to a faster growth, increased leaf allocation, decreased stem allocation, advanced reproduction and shorter life cycle, but further delay of germination into late summer led to insufficient reproduction and incomplete life cycle due to extremely short growth period. In response to delayed germination, plants may first increase root morphological traits before enhancing leaf morphological traits, in order to achieve stable reproductive efforts. They may increase the leaf allocation by producing more leaves than increasing individual leaf mass, at the cost of reduced stem allocation, rather than root allocation. These suggested root plasticity plays a predominant or fundamental role in plant response to environments, probably orientating, regulating and determining the responses of above-ground organs. Therefore, it is more crucial for plants to maintain root allocation stable, whereas mass allocated to stem or leaves can always be deliberated and balanced depending on specific situations. In spite of conspicuous disadvantages, plants with advanced and delayed germinated were still able to use different strategies to better adapt to subsequent environments, via modifying a number of allocation and morphological traits. Plasticity in these traits may play an important role in determining plant survival and success in face of the environmental challenges due to inopportune germination timing.