Onset of reproduction in plants: Size-versus age-dependency

Abstract

Abstract:Understanding the roles of age and size in the timing of first reproduction or flowering in plants has become a goal for those investigating the evolution of life cycle patterns in general. Here I review the studies that are helping to clarify these roles, and indicate some directions for future research. Article:In plants, the critical life cycle phases are embryo development on the maternal parent, seed growth (or dormancy) independent of the parent, vegetative growth and reproduction ( Fig. I ). Each phase is characterized by a particular rate of resource acquisition and pattern of resource allocation, e.g. to vegetative versus reproductive structures. The time of transition from one phase to the next marks the time when one set of phasespecific acquisition and allocation patterns switches to another set. A plant's zygotic life begins at fertilization. Its vegetative life begins at germination.Time of transition from vegetative growth to reproduction (time of first flowering) has recently received much attention from evolutionary ecologists. This interest began in earnest in the early 1970s at the time that Harper and White published a paper on the demography of plants, which showed that age at first reproduction is delayed as 'lifespan' (really, vegetative life-span) lengthens (Fig. 2). The correlation was clear; the causes of the correlation, however, were not. Because plants often grow indeterminately, vegetative age and size can be strongly correlated. Thus, is time of first reproduction deter-mined by age or size? Size-dependencySize is a good predictor of the onset of flowering in short-lived (mostly biennial) monocarps 2-6 , long-lived monocarps 7-9 , polycarps with long-lived monocarpic ramets 9,10 , other polycarps 11 , and monocarps that sometimes become polycarpic 12 . Size-dependency of onset of reproduction, however, varies greatly among species.Annuals are notably absent from this list. Natural populations of many annuals, even of wild progenitors of crop species, show great variation in the density and size at which they flower 13,14 . In many of these plants, flowering is initiated by an external environmental cue like photoperiod, and individuals respond to the cue independently of size (and independently of density, which strongly determines size).Three factors have probably led to this size-independent behavior in annuals. First, the length of the growing season is usually unpredictable, whether the limiting environmental variable be drought, heat, cold, etc. Second, the quality of the growing season often varies. Third, and most important, annuals by definition lack the ability to survive the period between growing seasons; what is predictable is their death within a year after germination. For all these reasons selection should strongly reduce size-dependency, i.e. reduce the threshold size for flowering such that it seldom determines onset of flowering. This reduction ensures that some seed is set every season, irrespective of the length and quality of the season. Variable seed ...

Evolutionary constraints on adaptive evolution during range expansion in an invasive plant

“…However, two factors make this assumption unlikely. First, there is probably a developmental threshold for flowering time, which is the size at which a plant has enough resources to flower and mature seed (Lacey 1986), resulting in a right skew to the distribution of PC1 as small, early-flowering phenotypes are absent. Second, some invasive populations may lack sufficient genetic variation for small, early-flowering phenotypes.…”

Photomorphogenesis in Dracaena draco

“…Dracaena draco flowers after long period of time (at least 9 years) so it is an autonomous flowerer. It needs long time to gather resources (Lacey 1986). Significant resources must be required, considering the production of about 50,000 flowers (Symon 2004).…”

“…The time the plant needs to gather resources to flower depends on the quality of environment, so reduction in such a quality delays flowering (Lacey 1986). In Draco's natural habitat in the Canaries, the major factor limiting growth seems to be low annual rainfall; for half a year, the tree suffers water stress (Cabrera Pérez 1999).…”

Flooding resistance and shoot elongation in relation to developmental stage and environmental conditions in Rumex maritimus L. and Rumex palustris Sm.

“…Bolting is normally delayed in R. palustris compared with R. maritimus and early-summer flooding caused a further delay in bolting until late in the season or into the following season. This was only partly caused by a delay in reaching a certain plant stage or size which is a common prerequisite for flowering in biennial plants (Lacey, 1986;Klinkhamer, de Jong & Meelis, 1987; see also Van der Sman el al., 1992).…”