Stephen P Bonser
Many plant traits are plastic. While plasticity in plant traits is often interpreted as adaptive, plasticity will also be observed where a trait changes with body size, and/or when growth rates differ across environments. This plasticity has been termed apparent plasticity. This protocol outlines:
- how to test for significant plastic allometry (plasticity in size-dependent traits); and
- how to control for apparent plasticity and interpret potentially adaptive plasticity.
Many plant traits are plastic. Individuals of the same or similar grown in different environments can have different phenotypes. Plasticity is often interpreted as having adaptive value. However, the expression of many traits is size-dependent (allometric). In other words, trait expression in large plants differs from trait expression in small plants. In such cases differences in traits can simply be due to differences growth rates across environments (see below).
For plants following a common developmental trajectory across sizes, plants compared at the same point in time will express plasticity for traits that are size-dependent if growth rates differ across environments (Fig 1). This has been termed “apparent plasticity” (Weiner 2004).
Fig 1. When a plant’s growth rate is different across environments (e.g. Environment A is low resource, and Environment B is high resource) and allocation to a given trait is allometric (it changes with changing body size), then allocation will be different for individuals compared a common time (the arrow), even allocation at a given size is the same across environments. Figure redrawn from Weiner (2004).
Apparent plasticity is a null hypothesis for plasticity, particularly plasticity in allocation traits (e.g. biomass of reproductive tissues, roots, stems and leaves). In such a situation plasticity may not have functional or adaptive value but is a product of differences in the rate of growth and development.
While plasticity in size related traits may represent a null expectation, these traits can also express plasticity as adaptive or functional responses to environmental variability. Examining relationships between size and allocation at a common developmental stage (e.g. first reproduction, senescence, etc.) rather than at a common age can allow appropriate tests for meaningful plasticity. The example below (Fig 2) shows significant allometry in reproductive allocation (large plants from favourable environments have proportionally higher allocation to reproduction than small plants from unfavourable environments). In this example, plasticity is not due to differences in developmental age, rather plasticity in reproductive allocation has been interpreted as a response correlated to adaptive growth form plasticity (Bonser and Aarssen 2001, 2003).
Fig 2. Regression of an allometric relationship of reproductive allocation at final development across individuals ( – ) grown in different environments (resulting in individuals differing in size). The dashed lined represents a slope (equal to one) of an isometric relationship.
For many longer-lived species, it may not be possible in the time frame of an experiment to record a common developmental stage such as the age of first reproduction. In such instances, developmental stage can be defined using a plastochron index (an index based on the rate of production of modular units such as leaves or ramets on an axis of plant growth). See Birch and Hutchings (1992) and Huber and Stuefer (1997) for simple methods to calculate a plastochron index.
Plants in different environments may also follow different developmental trajectories. For example, environmental adversities (e.g. stress, competition) may tend to induce a reduction in size at reproduction. Thus, plants in different environments follow different developmental trajectories, and plasticity in allocation across sizes could be due to a non linear relationship between size at reproduction and final reproductive allocation (Fig 3; Bonser and Aarssen 2009).
Fig 3. An allometric relationship of reproductive allocation at final development (see Fig 2). The lines with arrows represent different developmental trajectories of size and reproductive allocation. Individuals in more favourable environments reproduce at larger sizes and have proportionately higher allocation to reproduction at final development.
No specialised equipment is needed. However, plasticity and plastic allometry is best examined in experimental conditions where individual replicates are grown across environments (e.g. resources). These experiments can be conducted in the field and/or glasshouses or controlled environment chambers.
Units, terms, definitions
Allometry: In the broad sense, the effects of size alone. Large individuals have a different shape or proportions than small size individuals
Apparent plasticity: Plasticity in a trait due to plasticity in growth rates across environments.
Isometry: Size independent allocation or proportions. Parallel to “similar” in geometry. Large and small organisms have the same shape or proportions.
Phenotypic plasticity: The ability of a given genotype to produce different phenotypes in different environments.
RMA Regression: (Reduced major axis (RMA) regression): A regression analysis appropriate for describing relationships between two traits or biological variables (variables with associated measurement error). The slope of an RMA regression is calculated by the slope of an ordinary least squares regression divided by the correlation coefficient. RMA regression is also referred to as a standardised major axis (SMA) regression
Individual plants should be grown across experimentally established or natural environmental gradients (e.g. resource availability, neighbour density, environmental stress). See: Experimental Treatments
Traits of interest (e.g. leaf allocation, reproductive allocation, branching intensity, etc.) should be measured at common developmental stages (e.g. age at first reproduction, final development).For most traits, e.g. leaf allocation pattern, more insight can be provided if we follow the fate of an individual throughout growth and development.This means not just relying on one measurements at the final developmental stage but taking sequential harvests from the early seedling stage to maturity.
Plastic allometry in the traits of interest can be assessed using Log Trait versus Log Size relationships (RMA regressions). Resources for testing significant allometric relationships are provided below.There is a growing concensus among researchers (e.g. Ryser and Eek, 2000; Navas and Garnier, 2002; Japhet et al. 2009; Forster and Bonser, 2009) that considering a wider range of plant traits at the same time will give better insight than relying on very broad categories such as root versus shoot biomass. In this case plasticity of the traits can be assess by analysis of covariance (ANCOVA) using a measure of plant size, such as total biomass as a covariate in the model. Where there is a significant plant size effect on any traits, adjusted means correcting for the effect of size should be performed before interpreting the results.
The modular organization of a plant suggest that plant traits may differ in thier magnitude of plasticity in response to different resources. Therefore it will be helpful if traits are ranked in relation to thier plasticity (Navas and Garnier, 2002; Japhet et al. 2009). The Kendall’s tau rank correlation coefficient can be used to test wether trait ranking on the basis of thier plasticity is comparable among treatments.
Notes and troubleshooting tips
Species on which this protocol has been used: Brassicaceae Arabidopsis thaliana and Brassica rapa; Caryophyllaceae Arenaria serphyllifolia; Scrophulariaceae Chaenorrhinum minus.
Growth forms: Herbs (protocol can be conducted on other growth forms but short lived herbaceous species are easiest since they quickly reach each developmental stage).
Setting in which the protocol has been tested: Glasshouse, controlled environment chambers.
Links to resources and suppliers
SMATR: Standardised major axis tests and routines – Free software that will allow you to test for significant differences in the slopes of allometric relationships. http://bio.mq.edu.au/research/groups/comparative/SMATR/index.html
Excel spreadsheet for testing for Allometry using RMA regression (developed by Stephen P Bonser): attached at base of wiki page
Birch CPD, Hutchings MJ (1992) Analysis of ramet development in the stoloniferous herb Glechoma hederacea using a plastochron index. Oikos 63, 387-394.
Bonser SP, Aarssen LW (2001) Allometry and plasticity of meristem allocation throughout development in Arabidopsis thaliana. Journal of Ecology 89, 72-79.
Bonser SP, Aarssen LW (2003) Allometry and development in herbaceous plants: functional responses of meristem allocation to light and nutrient availability. American Journal of Botany 90, 404-412.
Bonser SP, Aarssen LW (2009) Interpreting reproductive allometry: Individual strategies of allocation explain size-dependent reproduction in plant populations. Perspectives in Plant Ecology, Evolution and Systematics 11, 31-40.
Huber H, Stuefer JF (1997) Shade-induced changes in branching pattern of a stoloniferous herb: functional response or allometric effect Oecologia 110, 478-486.
Warton DI, Wright IJ, Falster DS, Westoby M (2006) Bivariate line-fitting methods for allometry. Biological Reviews 81, 259-291.
Weiner J (2004) Allocation, plasticity and allometry in plants. Perspectives in Plant Ecology, Evolution and Systematics 6, 207-215.
Forster MA, Bonser SP (2009) Heteroblastic development and optimal partitioning of traits among contrasting environments in Acacia implexa Annals of Botany 103:95-105
Navas M, Garnier E (2002) Plasticity of whole plant and leaf traits in Rubia peregrina in response to light, nutrient and water availability. Acta Oecologica 23: 375-383
Wisdom Japhet, Daowei Zhou, Hongxuan Zhang, Hongxiang Zhang & Tian Yu (2009) Evidence of phenotypic plasticity in the response of Fagopyrum esculentum to population density and sowing date. Journal of Plant biology 52:303-311
Ryser P, Eek L (2000) Consequences of phenotypic plasticity vs. interspecific differences in leaf and root traits for acquisition of aboveground and below ground resources. Am J Bot 87:402
Health, safety & hazardous waste disposal considerations
No health, safety and hazardous waste considerations.