Azolla, the Wonder Plant R
Total Page：16
File Type：pdf, Size：1020Kb
Azolla, the wonder plant R. L. Bieleski1 It was a bit prophetic that the very first branching thread, with leaves and colour photo I ever took, number 0001 roots coming off at the nodes. The in my collection, taken with my trusty small overlapping scale-like leaves Kodak Retina camera and a spectacle have large air spaces, giving the lens, was a close-up of Azolla plant its buoyancy. Reproduction is rubra at Bethell’s Beach, Auckland. largely vegetative, with side branches Prophetic, because 25 years later breaking off from the main plant and I was to spend 9 months on study growing to form a new plant, from leave at the University of California, Fig. 1 Individual plants of Azolla rubra. which further side branches split Davis, probing the physiology of this Photo: Murray Dawson. off, and so on. What this means in fascinating plant. But before I get to effect is that given ideal conditions, my own small contributions I want to Azolla has an exponential growth take you for a small tour highlighting pattern, and we can talk about its some of the things we know about doubling time as a measure of its this remarkable plant. growth rate. This is not a theoretical concept: in my experiments I grew First, the taxonomy. Despite the Azolla mexicana under near-optimal small size of an individual plant, conditions, and exponential growth 1–2 cm across (Fig. 1–2), and its was maintained over five generations, habitat, floating on the top of ponds Fig. 2 Close-up of a dense Azolla rubra mat, till space in the growth tubs became (Fig. 3–4) usually in company with growing in a pond at Halswell Quarry Park, Christchurch. Photo: Murray Dawson. limiting, causing crowding of the the monocotyledenous Lemna fronds and therefore limiting the (duckweed, ducksmeat), Azolla exposure of individual fronds to light. is actually a fern in its own family, The original 500 mg starting weight Azollaceae, though current research of inoculum consistently reached now places it in the Salviniaceae ca. 16 g in 11–12 days, giving a together with another floating fern, doubling time of 2.3 days. Salvinia. Azolla has a worldwide distribution in warm, subtropical and In one stage my growing went tropical regions. Taxonomist’s views through three iterations, meaning vary a little, but most centre about Fig. 3 Azolla rubra plants (red) competing that after the 12-day period I took there being six species worldwide with Lemna minor plants (green) in a rāupo 0.5 g samples from that 16 g to seed wetland, Flaxmere. Photo: Rod Bieleski. (Fig. 5), with the scientific name for new tubs, and then again 12 days New Zealand’s single native species later, with exponential growth being having done a taxonomic do-si-do maintained right through. If each time from A. rubra through A. filiculoides I had been able to use the full amount and A. filiculoides var. rubra and in seeding tubs, I would have had 32 back to A. rubra (its main Māori tubs at the second iteration and 1024 name is kārearea). To complicate at the third, with a final tissue weight things in New Zealand, in recent around 16 kg. In the field, comparable years an exotic species, A. pinnata, growth rates are sometimes reached, has come into our country and has Fig. 4 Azolla rubra growing in a drainage as doubling times are often spoken of been taking over from our A. rubra in channel alongside Pegasus Bay Walkway, as being as low as 3–5 days. It’s hard areas north of Auckland. The jury is north of Christchurch. to comprehend the power of such Photo: Murray Dawson. out as to whether A. pinnata entered growth, so can I pose a question for naturally (such as on waterbirds’ you? Let’s begin with my standard feet) or accidentally through human inoculum, 500 mg of Azolla, about actions, but it has enjoyed our climate as much as will fit on the end of a and waterways enough to become teaspoon. If we were to allow that classed as an invasive species. Azolla to keep up its exponential growth without limitations of light, The typical fern structure has nutrients and space coming into play, been modified in Azolla to a point how long would it take for the weight where the stem and rhizome have Fig. 5 Azolla on Lake Macquarie of our Azolla crop to equal the entire become reduced to a frequently- (NSW, Australia). Photo: Rod Bieleski. 1 33 William Bond Street, Stanley Point, Auckland 0624, New Zealand; [email protected] New Zealand Garden Journal, 2015, Vol. 18(2) 5 plant biomass of the whole world? years ago when they were already biological fertiliser, and its use on rice A million years? A thousand years? using it as a nitrogen fertiliser for their may be declining. However, uses as A century? All wrong. It would take rice paddies (Fig. 6). The earliest supplements for animal and bird feed about 5 months for the doubling known written record of the practice is are being explored and expanded, time of 2.3 days, or 9 months for a in a book by Jia Si Xue in 540 A.D. on and small lots of Azolla growing in doubling time of 4 days. Of course The Art of Feeding the People. Its use canals and ponds as food for pigs and limitations do occur, saving us from in Vietnam dates to the 11th century. ducks are now ubiquitous throughout being buried in Azolla, but this does By the early 17th century, its use as southern China. A factor in the use explain why a pond almost free a compost was being documented in of Azolla as a green manure and of Azolla can become completely many local Chinese records. an animal feed is its low carbon to covered in less than a month. nitrogen ratio of about 10:1, meaning that when used as a green fertiliser its Though the growth is primarily nitrogen supply cannot be swamped vegetative, Azolla can behave like by bacterial activity. As an animal more typical ferns and reproduce feed, it has a protein content between sexually, but it does have to do it in its 13 to 24% on a dry weight basis, own way. The standard fern way is for making it a very high quality plant the diploid generation, the sporophyte protein source. that we recognise as our fern, to shed haploid spores into the wide world, So where did I come into the act? At there to germinate into separate the time, in 1979, Professor D. W. life stages, the haploid prothalli Rains of the Department of Agronomy (small liverwort-like plants) which and Range Science (University of produce the eggs to be fertilised by California, Davis) was heading a large the swimming sperm, creating the program trying to understand the new diploid (sporophyte) generation. biology of Azolla, with a view to using With Azolla, spores are not released; it more skilfully and further expanding instead male and female sporocarps its agricultural uses. A problem that are formed on the underside of Fig. 6 Chinese farmer inoculating a rice showed up was that Azolla seemed the frond, held there, and the paddy with Azolla 1500 years ago. to have a high requirement for fertilisation occurs on the parent Under field conditions Azolla can phosphorus, with phosphorus supply diploid sporophyte. In this respect, accumulate up to 2–4 kilograms often limiting growth. A typical finding where there is no physically separate of nitrogen per hectare per day, was that even when supplemental gameophyte generation, the life cycle 1.1 tonnes of nitrogen per hectare phosphorus was provided, less resembles that of higher plants! per year, and almost three times than 20% was actually utilised by the Azolla, so that even though the But that is not what makes Azolla the performance of legumes such in-plant N:P ratio was around 10, the really special, it is its ability to fix as clover at around 400 kg of yield of N fixed to P supplied was nitrogen, which it achieves by nitrogen per hectare per year. It is more like 1:1 to 2:1. As an old hand supporting colonies of blue green no wonder, therefore, that use of at studying phosphorus nutrition of algae (cyanobacteria), Anabaena Azolla has become a major tool in plants, I was given the job of trying azollae, living in the air chambers of the growing of rice in China and to uncover some basic facts about its fronds. And it is an association Vietnam. A major push for expanding phosphate transport and utilisation in that puts all other nitrogen-fixing the use of Azolla began in those Azolla, to see if ferns were “different” systems to shame. The growth two countries in the early 1960s. In to the higher plants I had worked on rates I reported, of a doubling time 1980, Azolla was being grown as a up till then. The basic setup I had of 2.3 days, were achieved in the green manure on about 1.3 million available in which to grow my plants absence of any nitrogen supply in hectares of rice in China alone. In was a growth cabinet which gave the nutrient medium that the Azolla the paddy fields, the typical pattern near-optimal conditions for Azolla: was growing on. Every last skerrick is for some of the Azolla to die, as 27oC, 16 hour day, about 25% of of the nitrogen was fixed by the the mat becomes fully shaded by full daylight, standard plant nutrient Anabaena and swapped to Azolla in the rice, sink and decompose – its solution minus any nitrogen source, the form of NH + (ammonium), with rapid decomposition means that 4 in 14 cm × 14 cm × 20 cm plastic the Azolla providing Anabaena with its nitrogen and other nutrients are tubs, with aeration.