Another interesting thing about the plants on Malta was the number with adaptations to help them survive when nutrients are very limited. To the casual observer, the number of both legumes and parasitic plants was striking. Legumes are plants in the same family as peas and beans and have the same characteristic flower shape.
Ononis sp., Lotus sp. and Medicago sp, Malta, April 2015
They are fascinating plants – the only higher plants which can use nitrogen from the air, rather than having to rely on nitrates in the soil. Nitrogen is a key nutrient for plants, needed for the manufacture of proteins, so being able to use nitrogen gas gives legumes a big competitive advantage when nitrates are in short supply. Our atmosphere is nearly 80 % nitrogen but most organisms can make no use of it in this form.
So how are legumes special? Rather like lichens, where the fungus harbours algal cells which supply it with carbohydrates (see The wonderful world of lichens), legumes host a bacterium (often Rhizobium leguminosum) in a symbiotic relationship. These bacteria, also found living free in the soil, convert atmospheric nitrogen to ammonia and organic nitrogen-containing compounds which the plant can use, in exchange for a supply of carbohydrate from the plant. The enzymes which allow the bacterium to do this cannot function in the presence of oxygen so the legume, once infected with Rhizobium, grows special nodules on its roots and, inside these, the bacteria can grow and ‘fix’ nitrogen in an oxygen-free environment.
Of course it’s not just in Malta that legumes thrive where other plants cannot get a foothold. Gaps between the paving slabs in my garden provide anchorage for tiny Black Medick (Medicago lupulina) plants. Digging them up, I can see 2 mm long, pinkish coloured nodules on the roots.
Inside these nodules, the bacteria have differentiated into bacteroids – effectively, specialised nitrogen-fixing organelles.
Rhizobium bacteroids inside a bean root. Photo P.S. Poole
Using atmospheric nitrogen is an energetically-expensive process for a plant and even legumes will only do it when they have no alternative. Any gardener will tell you there is no point in adding nitrogen fertilizer to the soil when growing peas and beans – all it does is stop them from fixing their own nitrogen. When nitrogen is in short supply, legumes release chemical attractants to which the Rhizobium bacteria in the soil respond. The bacteria enter the roots via their delicate root hairs. Once inside the root cells, the bacteria are packaged in a membrane derived from the plant cell membrane. They grow and divide as the nodule forms but eventually start to change into bacteroids, dedicated to nitrogen-fixation.
The nitrogenase enzymes which fix nitrogen are very sensitive to oxygen and need to function in an anaerobic environment, so how to deal with the fact that the bacteroids still need oxygen for normal respiration? A protein called leghaemoglobin is the answer. If the name sounds strangely familiar, it’s because the proteins is an oxygen-binding protein with iron at its core, just like haemoglobin. The pink colour of the nodules is no coincidence. Leghaemoglobin has a strong affinity for oxygen and helps transport it to the respiring bacteroids, whilst keeping it away from the nitrogenase, just as haemoglobin transports oxygen around our bodies. Fascinatingly, the organic portion of the protein is made by the host plant, specifically in response to Rhizobium infection, whilst the iron-containing ‘haem’ portion is produced by the bacterium. That poses some interesting questions about coevolution!
So, back to Malta. The island is a rocky outcrop in the Mediterranean where soil nutrients are limited but, for most of the year, there is abundant heat and light to provide energy for expensive nitrogen fixation. Legumes are everywhere, as a result – not just ground cover like the clovers and medicks familiar here, but also shrubs and trees.
Italian sainfoin (Hedysarum coronarium) and other legumes, Malta, April 2015
Perhaps the most spectacular in April, in Malta, is Cercis siliquastrum, which seems to fill the niche occupied by flowering cherry trees in the UK.
And finally this is, apparently, my characteristic holiday pose…. Good thing Martyn has sketching to keep him happy while I botanise!