Dynamic morphological plasticity in response to emergence timing in Abutilon theophrasti (Malvaceae)

Selections on emergence time might be conflicting, suggesting the
existence of the optimal emergence time for plants. However, we know
little about this and how morphological plasticity contributes to the
strategies of plants in response to emergence timing. To better understand
this issue in a dynamic perspective, we conducted a field experiment by
subjecting plants of Abutilon theophrasti to four emergence treatments
(ET1~ET4) and measuring a number of mass and morphological traits on them
at different growth stages (I~IV). At day 50, 70 and/or final harvest,
among all ET treatments, plants germinated in late spring (ET2) performed
the best in total mass, spring germinants (ET1) and ET2 performed better
in stem allocation, stem and root diameters than later germinants (ET3 and
ET4); summer germinants (ET3) had the highest reproductive mass and
allocation, while late-summer germinants (ET4) had the greatest leaf mass
allocation, with greater or canalized leaf number and root length traits
than others. Plants that emerged in late spring can maximize their growth
potential, while those with either advanced or delayed emergence are still
capable of adaptation via allocation and morphological plasticity. Early
germinants (ET1 and ET2) preferred stem growth to leaf and reproductive
growth, due to sufficient time for reproduction in growth season. With
limited time for growth, plants emerged late may prefer to quicken leaf
growth (indicated by increased leaf mass allocation and leaf number) at
the cost of stem or root growth for complete life cycle, reflecting both
positive and negative effects of delayed emergence.

针对出苗时间的选择存在矛盾，这表明植物存在最优出苗时间。然而目前我们对这一现象，以及形态可塑性（morphological plasticity）如何参与植物响应出苗时间的适应性策略，仍知之甚少。为从动态视角更好地理解该问题，我们开展了一项田间试验：将苘麻（Abutilon theophrasti）植株设置4组出苗处理（ET1~ET4），并在不同生育期（I~IV）测定其多项生物量与形态性状。在播种后50天、70天及/或最终收获期，所有出苗处理中，晚春出苗植株（ET2）的总生物量表现最优；春出苗植株（ET1）与ET2的茎分配比例、茎径及根径均优于较晚出苗植株（ET3与ET4）；夏出苗植株（ET3）的生殖生物量及分配比例最高，而夏末出苗植株（ET4）的叶生物量分配比例最大，其叶片数与根长性状较其他处理组更为突出或处于稳态化水平。晚春出苗的植株可最大化其生长潜力，而出苗提前或延后的植株仍可通过资源分配与形态可塑性实现适应。早出苗植株（ET1与ET2）倾向于优先进行茎秆生长而非叶片与生殖生长，这是因为其生育季内拥有充足的生殖生长时间。而生长期受限的晚出苗植株则可能以牺牲茎秆或根系生长为代价，加快叶片生长（表现为叶生物量分配比例与叶片数提升），以完成完整生活史，这体现了出苗延后带来的正反两方面效应。