Lupinus arboreus - Background information
Commonly called the Tree Lupin, this plant is often found living in the hot, dry, sandy shore ecosystem or as a secondary coloniser in exposed dry soils after clearing of other vegetation. In the sandy shore ecosystem, colonisation of the bare sand begins with marram grass, followed by lupins.
These lupins, living in sand, need to be adapted for both water conservation and the ability to withstand exposure to high temperatures. Summer temperatures on the sand are often in the 50-60 degree Celsius range. Winds blow beach sand to produce a highly abrasive effect.
Lupins are plants which live in, and are well adapted to dry conditions. Many of their adaptations are concerned with water conservation.
Structure and adaptations
These grow rapidly so that the young seedling will be able to quickly reach water, this is of obvious importance in fast draining sandy soils. Seedlings germinate in the late winter while the upper layers of sand still contain some moisture. As the root matures it develops a bark-like layer which helps prevent desiccation.
Root nodules are bump-like growths on the side of the roots. They develop soon after germination and are important in providing nitrogen to the plant. These root nodules contain Rhizobium bacteria which are in a mutualistic relationship with the plant. The lupin provides shelter and nutrients, the bacteria providing nitrates to the plant. The presence of these Rhizobium bacteria imparts a reddish to the inside of the nodule.
A nodule forms when Rhizobium bacteria multiply around a root hair, these bacteria secrete auxins which stimulate the root hair to grow around the bacteria. The bacteria also secrete an enzyme which causes the development of a weakened region in the root hair, they migrate through this region and eventually establish themselves in the cortex of the root. The cortex cells become enlarged to form the root nodule. It is worthwhile noting that bacteria will not invade the root if the soil is acidic - the presence of acidic superphosphate will prevent formation of nodules in other legumes such as clover.
Soon after germination the stem develops a layer of fine hairs. These hairs trap humid air lost from the epidermal cells, lowering the humidity gradient, and therefore reduce further water losses by slowing the rate of evaporation.
In older plants a thick layer of bark forms, this provides protection from desiccation, stem boring insects, as well as decreasing the damage caused by fires. After a fire the plant is often able to re-grow its foliage from the undamaged cells within the stem.
As the plant grows the stem divides into many branches so that it forms a compact, rounded shape. This means the interior regions of the bush are shaded, cooler and more humid than the surrounding environment. These features provide a lower rate of water loss for leaves on the inside of the bush.
The plant has compound leaves made of between 4 and 7 leaflets. They are covered with a relatively thin cuticle (a waxy, waterproof layer) which is a little surprising for a plant living in dry conditions. The leaves are covered in a layer of white epidermal hairs which will help reflect heat away from the leaf and will also trap humid air lost by transpiration from the stomata. This trapped layer of more humid air will slow evaporation and therefore reduce further transpirational losses.
There are two surprising features about the leaves:
i. The upper side of the leaf has a greater density of stomata than the lower surface.
ii The lower side of the leaf has a much higher density of leaf hairs than the upper surface.
Both of these are the opposite of what is found in most other plants. In most plants the upper side of the leaf is hotter and therefore the stomata are normally found on the cooler lower surface. In most plants the greatest density of leaf hairs is found on the surface which has the greatest density of stomata.
The cause of this difference in leaf hair and stomatal distribution is caused by the environment in which the lupin lives. The mostly bare ground gets very hot in the summer sun and re-radiated heat from below so that the lower surface of the leaves may get much hotter than the upper surface. The greater density of white leaf hairs on the lower surface help to reflect some of this heat away. The greater density of stomata on the upper surface means that their are less transpirational water losses than if they had been concentrated on the lower hotter surface.
There is another adaptation which helps prevent excess water losses. The leaflets are folded longitudinally down the length of the leaflet. This fold helps trap humid air lost in transpiration by acting as a barrier it created a wind-shadow which means the humid air tends to stay close to the lead allowing the formation of a humidity gradient against the upper leaf surface. The folded leaflet also means that the light and heat from the sun strike the leaf surface at a more acute angle and so the heat tends to be reflected rather than absorbed by the leaf.
The folding of the leaf varies according to the amount of water loss. When the leaf is losing a lot of water it is more tightly folded than when it is not losing excess water. Leaves in the centre or sheltered side of the bush tend to be flatter, whereas exposed leaves tend to be more folded to slow water losses.
Juvenile leaves have a higher surface area to volume ratio than mature leaves and so would be expected to loose water at a greater rate than mature leaves. Because of this it might be expected that the juvenile leaves will show greater leaf folding than mature leaves under heat stress conditions.
Flowers occur in groups called inflorescences, the grouping of flowers and the fact that the inflorescence is held vertically means that they are better able to attract bees for pollination. The reproductive structures are well protected by the petals and are only revealed when a bee lands of the flower.
The flower has two sets of stamens which mature at different times. The first set mature before the stigma is ready to receive pollen this allows cross-pollination and prevents self pollination. If cross-pollination does not occur then the second set of stamens ripens so that self pollination will occur. This mechanism ensures that every flower will produce seeds and so greatly enhance the survival and spread of the species even in the inhospitable dry sandy shore ecosystem.
The seeds ripen in the pod (the fruit of the plant). At first the pod is green and covered in a dense layer of white hairs which protect from desiccation and browsing caterpillars. After the pod hardens it dries and on very hot days it splits and twists, suddenly flicking the seeds over a considerable distance. The seeds are covered with a very hard testa which provides protection and ensures the seeds remain dormant until the next spring. The primary way the testa maintains dormancy is by preventing the entry of water into the seed, without water the seed will not start to germinate.
If the seed did not have a way of maintaining dormancy then it could germinate in summer after a heavy rain. Once germinated the seed would most likely die in the hot dry summer conditions.