Diagrammatic representation of phytomer growth.

Start from here: SPL Theory links regenerative and life history strategies

The best chance of growing to maturity is to have the best start. A new theory connects seed and leaf size with phytomers to describe the relationship between regeneration and growth through a plant’s lifetime.

There’s a joke about a tourist asking for directions. He flags down a local and asks how to get to a village on the coast, to which the local replies: “Well sir if I were you, I wouldn’t start from here.” In this context, it’s a joke but for plants aiming for a productive life, the best start would give them a competitive advantage. Are there common traits that can link regenerative strategies with life histories?

Hodgson and colleagues think there is a way to connect the two and it has further uses. They consider the worldwide leaf economics spectrum. This is an analysis of leaf economics, particularly size and the local mineral nutrients. It is, as the name suggests, a spectrum with a great deal of variation and it’s been a puzzle why there isn’t an obvious optimal solution for a given level of nutrients.

Hodgson et al. propose a seed-phytomer-leaf (SPL) model. The connection between seed size and leaf size might seem plausible, but the extra feature is the phytomers connecting them. The phytomers are repetitive blocks that make up the stem. If you can think of a phytomer as a leaf, a node connecting it to the stem and the internode, the part of a stem that connects down to the next node. Effectively you can then grow a stem by adding phytomers on, a bit like building blocks.

Diagrammatic representation of phytomer growth.
Diagrammatic representation of phytomer growth. (A) Early stages of ‘juvenile growth’ (Phyt 0–2), with sequential phytomers increasing in size. (B) ‘Adult’ vegetative growth (Phyt n onwards), with sequential phytomers more or less even in size. Each phytomer consists of an internode (coloured black) and, above this, a node bearing a leaf (green) with an axillary bud (red).

The authors propose a model where the size of the ‘adult’ phytomer (≅ leaf size) is the product of

  1. the size of the first phytomer produced,
  2. the speed at which each phytomer grows
  3. the number of iterations of ‘juvenile growth’

They find this holds up well. Like a lot of models where it breaks down isn’t so much a failure but more a signpost to where something unexpected and interesting is happening. For the Seed=Phytomer-Leaf (SPL) theory, the problem is a photosynthetic stem. How does this affect the relationship? The authors state that there needs to be more work taking into account variations like this.

The value of SPL theory, Hodgson and colleagues argue, is that it connects traits that are already studied in ecosystems. There’s ongoing debate about how the regeneration of a plant connects to its established life, and the authors think the SPL relationship is a model to describe it. SPL theory can also describe trade-offs between regeneration and vegetative growth and why some plants might have very short maturity compared to others.

Finally, they argue that SPL theory might help explain the climatic distribution of plants. Different areas have different growing seasons, as light and water vary. Different climates will allow a different variety of SPL relations. These relationships might also help inform ecologists as to why some plants can survive changes in climate better than others.

This paper is part of the Special Issue on Morphology and Adaptation. It is FREE access for a limited period to the end of January 2018. It will then be free access from November 2018.


The Annals of Botany Office is based at the University of Oxford.

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