Tag Archives: phenology

Pollinators as selectors for evolution

Following from yesterday, do pollinators act as selectors for evolution? A pollinator shift explains floral divergence in an orchid species complex in South Africa by Peter and Johnson tests this idea.

The orchid in question is Eulophia parviflora. This is a deceptive orchid found in Africa, and deceptive means it doesn’t offer a reward to pollinators, it merely looks like it does. The aim is to entice insects in when they look for food and hit them with their pollinaria to carry to other orchids. To do this they need to look and smell convincing, but they also need to make things as easy as possible for the pollinators. The orchid’s problem is that there are so many insects that it could build its flowers in all sorts of ways.

This is indeed what happens.

Eulophia parviflora forms

Floral morphology of Eulophia parviflora. Left, Short-spurred morph. Right Long-spurred morph. Image © Peter and Johnson.

Peter and Johnson identified two forms of Eulophia parviflora. In the image above, the one on the left is the short-spur morph. This grows tall from the ground with plenty of flowers. The one on the right is the long-spur version. This opens when the stalk is barely out of the ground. They look different and they smell different, but they’re both E. parviflora. So what is it that makes the same plant grow long or short spurs?

The answer seems to be the pollinators. The short-spur plants are pollinated by the beetle Cyrtothyrea marginalis who can get in close to the orchid. The long-spur orchid is pollinated the bee Amegilla fallax. However, simply watching and seeing that the plant has two forms pollinated by two different creatures isn’t enough. There might be some other cause, like local climate that explains the spurs and the presence or lack of an insect. So Peter and Johnson have done some experiments.

Are bees deliberately picking long-spurs in flowers? If they are then that would show the bees are selecting flowers and helping drive the morphological change. The experiment is simple. Reduce the size of the spurs in some of the long-spur flowers. If the spurs matter, then the bees will pick the long-spur plants and ignore the short spur plants. Sure enough, the bees went for the long-spur flowers.

Another experiment was to see how the scent attracted insects. They tried it with both beetles and bees, but found the bees weren’t cooperating, so there were just results from beetles. The experiment is simple and elegant. You have a Y shape. At the top of each arm of the Y you have a fan pushing out the scent of a flower. Put a beetle at the bottom and where does it go to? In this case, it picks the scent of the short-spur plant.

In fact the paper notes the experiment wasn’t quite as simple as I made out. It wasn’t just the scent that attracted beetles, they’d also pick a tunnel depending on the position of the sun, so they found they had to calibrate the tunnels properly before they could sensibly test the beetles.

Peter and Johnson also show that the two forms of the plant are not just diverging in shape but also in time. It makes sense to flower when the pollinators are about. The short-spur flower doesn’t start till after the winter frosts in October (remember South Africa is in the southern hemisphere). This is when the beetles emerge. In contrast the long-spur flower can get going sooner in July when A. fallax starts getting active.

The isolation in time for exchanging pollen, and the specificity of the pollinators means that the pollinators seem to be definitely acting as selectors for the plants. Peter and Johnson say that the two forms might already be considered two sister species given the genetic differences.

You can pick up the paper from Annals of Botany.

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Duration of shoot elongation in Scots pine

Duration of shoot elongation in Scots pine

Duration of shoot elongation in Scots pine

The within-season timing of shoot growth in trees has often been considered independently of shoot growth rate. Schiestl-Aalto et al. study lateral shoot growth in Scots pine (Pinus sylvestris) over 7 years and find that daily maximum growth rate correlates positively with growth duration, expressed as thermal time. Higher July–August temperature of the previous summer also prolongs the growth period. The results suggest that the thermal-time requirement for completion of lateral shoot extension in Scots pine may interact with resource availability to the shoot, both from year to year and among shoots in a crown each year. If growing season temperatures rise in the future, this will affect not only the rate of shoot growth but also its duration.

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Some Like It Cold

Modelling temperature, photoperiod and vernalization to predict flowering

Modelling temperature, photoperiod and vernalization to predict flowering

Brunonia australis and Calandrinia sp. are Australian native herbs with commercial potential as flowering potted or bedding plants. Both species are best grown as annuals and flower naturally during spring and early summer. However, many ornamental plants are grown outside their natural flowering period to align flowering with peak market demand, which requires the capacity to predict flowering date under changing or different environments. Scheduling crop production using quantitative flowering time models can have considerable advantages as they can be tailored for individual requirements, unlike traditional scheduling methods that are typically based on calendar date and have no particular reference to the environment.

Most development rate models for ornamental species predict flowering time in relation to temperature, photoperiod and/or daily light integral as observed for the above models. However, there are few flowering time models for ornamental plants that include a vernalization function. Vernalization is important for early and complete flowering of many traditional herbaceous crops. Plant responses to vernalization have been incorporated into some models for field crops and arabidopsis, which reportedly improved accuracy. A new paper in Annals of Botany quantifies temperature and photoperiod or vernalization responses of B. australis and Calandrinia sp. and model development for the purpose of scheduling year-round flowering. The effects of temperature and photoperiod or vernalization on plant quality characteristics, including flower and branch number, were defined.

 

Modelling temperature, photoperiod and vernalization responses of Brunonia australis (Goodeniaceae) and Calandrinia sp. (Portulacaceae) to predict flowering time. Ann Bot (2013) 111 (4): 629-639.
doi: 10.1093/aob/mct028

Crop models for herbaceous ornamental species typically include functions for temperature and photoperiod responses, but very few incorporate vernalization, which is a requirement of many traditional crops. This study investigated the development of floriculture crop models, which describe temperature responses, plus photoperiod or vernalization requirements, using Australian native ephemerals Brunonia australis and Calandrinia sp.
A novel approach involved the use of a field crop modelling tool, DEVEL2. This optimization program estimates the parameters of selected functions within the development rate models using an iterative process that minimizes sum of squares residual between estimated and observed days for the phenological event. Parameter profiling and jack-knifing are included in DEVEL2 to remove bias from parameter estimates and introduce rigour into the parameter selection process.
Development rate of B. australis from planting to first visible floral bud (VFB) was predicted using a multiplicative approach with a curvilinear function to describe temperature responses and a broken linear function to explain photoperiod responses. A similar model was used to describe the development rate of Calandrinia sp., except the photoperiod function was replaced with an exponential vernalization function, which explained a facultative cold requirement and included a coefficient for determining the vernalization ceiling temperature. Temperature was the main environmental factor influencing development rate for VFB to anthesis of both species and was predicted using a linear model.
The phenology models for B. australis and Calandrinia sp. described development rate from planting to VFB and from VFB to anthesis in response to temperature and photoperiod or vernalization and may assist modelling efforts of other herbaceous ornamental plants. In addition to crop management, the vernalization function could be used to identify plant communities most at risk from predicted increases in temperature due to global warming.

 

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Phenology and resource allocation in Ruellia

Phenology and resource allocation in Ruellia

Phenology and resource allocation in Ruellia

In plants that produce both closed, obligatory self-pollinated (cleistogamous) and open, potentially out-crossed (chasmogamous) flowers, suboptimal conditions typically favour production of cleistogamous flowers. Munguía-Rosas et al. study the effects of shade and drought on Ruellia nudiflora and find that cleistogamous flowers are produced earlier under shaded conditions whilst chasmogamous flowers are produced for shorter periods; however, resources are preferentially allocated to those chasmogamous flowers receiving larger pollen loads. The results demonstrate complex interactions between environment and reproduction in cleistogamous plants.

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Genome size and DNA base composition (Research in Context)

Genome size and DNA base composition (Research in Context)

Genome size and DNA base composition (Research in Context)

Genome size is known to affect various plant traits such as stomatal size and seed mass but these associations are not well understood for species with very large genomes, which are largely represented by geophytic plants. Veselý et al.  survey genome size across 219 geophytes and find that it is associated with species’ ecology and phenology, and analysis also shows an association with changes in DNA base composition. They suggest that although production of larger cells appears to be an advantageous strategy for fast development in seasonal habitats, the drought sensitivity of large stomata may restrict the occurrence of geophytes with very large genomes to regions not subject to water stress.

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Functional significance of style length in Mertensia

Functional significance of style length in <i>Mertensia</i>

Functional significance of style length in <i>Mertensia</i>

Mertensia fusiformis (Boraginaceae) is a spring-flowering perennial showing pronounced intraspecific variation in style length and stigma–anther separation. Forrest et al. show that population-level variation in flowering time, driven by patchiness in timing of snowmelt, causes different populations to experience different temperature regimes during flowering and functionally distinct suites of pollinators. The interaction between plant and pollinator phenology in particular appears to be involved in maintaining style-length variation in this species.

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