Why do acacias (wattles) have different leaves?
A keen observer will notice that plants in the genus Acacia tend to grow dramatically different types of leaves. Known as compound leaves and phyllodes, these leaves not only occur on different parts of the tree but can even occur on the same branch. Even a single leaf can be of the two types! What is going on here? Is it a hideous mutuation? A disease or illness? Is it caused by some kind of fungus?
The short answer is that the development of different types of leaves on acacia trees, and many other species for that matter, is quite normal and not related to any bad mutation or illness. The adaptive significance of the different leaves, and the advantage, if any, it confers on tree growth and survival, is still largely unknown or disputed.
In this article I explain:
- why plants develop different types of leaves
- the different types of leaves acacias species develop
- why traditional explanations about acacia leaves aren’t quite correct
- a new hypothesis based on a Frankenstein style experiment from the 1960’s
Compound leaves and phyllodes on an acacia sapling
Heteroblasty and Heterophylly
types of leaves is heteroblasty and heterophylly. Heteroblasty is a plant that, as it grows, dramatically changes from one leaf shape or form to another. For example, eucalypts (gum trees) tend to have large leaves on saplings and smaller, narrower leaves, on mature trees. The transition from the large, juvenile leaves to narrow adult leaves is heteroblastic development.
Heterophylly, on the other hand, describes plants with dramatically different leaf types at the same developmental stage. For example, a single branch of a mature tree may have oval shaped leaves and elongated leaves growing side by side.
If we want to be extremely pedantic we could define just about every tree or plant as showing heterophylly if even two leaves are slightly different sizes or shapes. However, heterophylly is reserved for leaves that are obviously and dramatically different from another. For example, the compound leaf and phyllode of acacias are obviously and dramatically different.
Heteroblasty in Eucalypts: All of these leaves were from a single tree
Why do plants bother with different types of leaves?
Plants develop different types of leaves often for very good reasons. As plants are sessile organisms they often develop a different leaf type to cope with a different environment. An obvious example is the development of sun and shade adapted leaves. Sun adapted leaves grow on the outside of a canopy and are designed to cope with the high intensity light, higher temperatures and drier air on the outside of the canopy. Shade adapted leaves grow on the inside or beneath the canopy. Shade adapted leaves tend to be larger than sun adapted leaves and have a different physiology to cope with a low light environment.
Plants also have different types of leaves when they are young compared to their adult stage of life. Young plants are primarily concerned with growing so they have leaves with relatively large areas and small structural mass, geared towards high photosynthetic rates and carbon uptake. Leaves on adult plants tend to have relatively smaller areas with greater structural mass. These leaves have lower photosynthetic rates but they are stronger and last longer – important traits for adult plants that are more concerned with survival and reproduction rather than growth.
A particularly celebrated case of heteroblasty comes from New Zealand. Approximtely 10% of New Zealand shrubs show a particularly dramatic form of heteroblasty where the juvenile growth stage consists of tangled branching and small leaves (called divaricate growth) and the adult stage shows “normal” leaves. Many scientists hypothesised that the small leaves in tangle branches was an adaptation to prevent herbivory by the extinct giant birds called moa. Once these plants grew above the height of the largest moa (about 3m tall) they start to develop “normal” leaves. Other scientists hypothesise that this adaptation is due to cold environments or variable light environments.
So what’s the story with acacias?
While the examples given above readily explain why some plants develop dramatically different leaf types, the phenomenon in acacias is harder to explain.
The first explanation usually given is that phyllodes have evolved to cope with dry environments and compound leaves are adapted to wetter environments. Scientists have shown that photosynthesis in phyllodes is greater than compound leaves when plants face drought stress. Many other adaptations of phyllodes also suggest it is a drought adapted type of leaf. While many species of acacia with phyllodes do grow in harsh Australian deserts, many other species with compound leaves happily exist in equally dry environments – think of those acacias in Africa that giraffes like to eat. Furthermore, a number of phyllodinous acacias grow in rainforest or tall forests of eastern Australia.
Most of the phyllodinous acacias occur in Australia where soil fertility is notoriously low. Therefore, other scientists hypothesised that phyllodes are an adaptation to low nutrient soils. Phyllodes are tough leaves similar to species known as sclerophylls. This class of species have leaves that are often small, tough and leathery, and typically grow on low nutrient soils. As leaves require a large amount of investment from the plant to grow, in low nutrient environments leaves are reinforced or protected from would be predators. Perhaps this is the case with phyllodinous acacias.
An acacia tree displaying heterophylly
An experiment to find out
I was interested to test these ideas so I performed an experiment to determine which environmental factor influenced leaf development in an acacia species (Forster & Bonser 2009a). I also decided to test a third environmental factor: light. I could not find any reference to light influencing acacia leaf development however I decided to test in based on a hunch. I remembered a comment a friend made when he visited Australia from England. He commented about how bright and harsh the Australian sun was which is also the cause of the high rates of skin cancer in Australia. So I hypothesised that phyllodes develop to protect the plant from high sunlight.
My experiment found that drought and low nutrients had no effect whatsoever on acacia leaf development. That is, plants growing in the drought and low nutrient treatments developed phyllodes at the same time as plants growing in well watered and high nutrient treatments.
A scientific experiment on acacias
However there was a dramatic effect in the light environment. Plants grown in a shade environment developed more compound leaves whereas plants grown in a high sunlight environment quickly developed phyllodes. Compound leaves are large, flat and have a horizontal orientation – perfect traits to optimize light interception in a low light environment. Phyllodes, on the other hand, are long and narrow, and have a vertical orientation – perfect traits to avoid high sunlight and the damaging effects of too much light.
Subsequent tests confirmed that light did indeed have a significant effect on whether this species of acacia developed phyllodes or compound leaves. Interestingly, the origin of the seeds I used in my experiment also produced different results. I sourced seeds from trees growing in three different environments – a low light, forest environment; a medium light, woodland environment; and a high light, sparse woodland environment. All the plants were grown in a glasshouse with the same light conditions however seeds from the low light population developed more compound leaves than seeds from the high light population (Forster and Bonser 2009b).
Simulated shade treatment affects acacia growth
The story, however, is more complicated
After four years of grinding scientific research, strongly believing that the light environment could explain everything about compound leaves and phyllodes, I began to have doubts. For example, my seedlings would always eventually form phyllodes even if they were constantly growing in the shade environment. It was just the plants growing in a shaded environment would form phyllodes at a much later stage than plants growing in a high light environment. Additionally, a different species, Acacia rubida, has both compound leaves and phyllodes on the same branch which could be either growing in the sun or the shade.
There was also a suggestion that when acacia plants are mechanically damaged, such as the snapping of a branch, the new shoot that can spring from the damaged area would invariably have compound leaves even if it was growing in full sunlight.
And then there was this image from a paper published in 1964:
The effects of giberellic acid on compound leaf development in acacias
The image is a result of a Frankenstein style experiment conducted on a poor, hapless acacia sapling (Borchet, 1964). The scientists applied a plant hormone, called gibberellic acid, to the growing tips of the sapling. Once applied, the plant would then form compound leaves. The scientist wasn’t satisfied with applyig the hormone once but did so repeatedly to prove his point. Regardless, the implication is that the hormone giberellic acid appears to play a role in the development of compound leaves in acacias.
Giberellic acid and a new hypothesis
Giberellic acid is one of many plant hormones occuring in all plants. Among its many functions, it is known to increase or develop when, wait for it, plants are growing in the shade and when plants have faced some form of mechanical damage.
So I have a new hypothesis about compound leaves and phyllode development in acacias. This hypothesis is untested so I do not know if it is true but, for mine, there is logic to it. That is, an acacia plant is grown in the shade and produces giberellic acid. This hormone then produces compound leaves which, coincidentally, are a perfectly suited leaf for the shade environment. Having produced the compound leaf in the shade, this confers growth advantages to the plant to assist it in growing faster and growing away or above the shade.
This hypothesis certainly leaves a lot to be explained. For example, why did phyllodes evolve in the first place? Why do some species of acacia never revert to compound leaves once they have phyllodes and are growing in the shade? And many other questions besides.
Although the development of different types of leaves in many species is readily explained, the occurrence of compound leaves and phyllodes in acacias is still largely a mystery. We do know a lot about the function of compound leaves and phyllodes and why these functions are better adapted to different environments. However, there is still no unifying explanation that satisfies everything about the occurrence of compound leaves and phyllodes.
Borchert, R. 1964. Gibberellic acid and rejuvenation of apical meristems in acacia melanoxylon. Naturwissenschaften, 52: 65-66.
Forster, MA., Bonser, SP. 2009a. Heteroblastic development and the optimal partitioning of traits among contrasting environments in Acacia implexa. Annals of Botany, 103: 95-105.
Forster, MA., Bonser, SP. 2009b.Heteroblastic Development and Shade-avoidance in Response to Blue andRed Light Signals in Acacia implexa. Photochemistry and Photobiology, 85: 1375-1383.
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