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Into Horticulture

Issue 9   August 2012

Plant & Garden News

Inland cycad not a rainforest remnant
Analysis of the MacDonnell Ranges cycad (Macrozamia macdonellii), suggests its ancestors arrived in central Australia in only the last 2 million years or so. If it had been isolated from its eastern relatives for 30 million years, as previously thought, greater divergence should have been observed. This means that the cycad is probably not a survivor of an ancient Eocene era rainforest. Source: Cycads in central Australia are not ancient relics

Invention promises economical P recovery from sewage
Recovering nutrients from sewage and food processing waste has the potential to conserve precious agricultural phosphorus and reduce pollution problems. Now German scientists have developed an electrolytic process employing a magnesium anode that precipitates magnesium-ammonium phosphate from wastewater. The crystalline product can be used directly as fertiliser. Importantly, little energy and no additional chemicals are required. Source: Using wastewater as fertilizer

Evolution in action
A new plant species has evolved in Scotland, far away from the home of its ancestors and facilitated by horticulture. Two ornamental species of Mimulus introduced from the Americas in the 1800s have since escaped into the wild in Britain, but their hybrids are routinely sterile due to to an equal compliment of DNA from the two parents. Duplication of the entire genome in a hybrid, however, has given rise to a new line capable of reproducing itself sexually. It's distinct enough to be classified as a new species, Mimulus peregrinus. While it is hypothesised that many new species arose in this way in the past, this provides an opportunity to study the process in action. Source: Rare glimpse into the origin of species

Where the rubber hits the roses
The UK's RAC Foundation has estimated that seven million British front gardens have been converted into parking for cars, about a third of dwellings that originally had a front plot: Seven million front gardens disappear to make way for cars

Why are heirloom tomatoes tastier?
Californian researchers have identified genetic factors that contribute to the colour and flavour of heirloom tomatoes. These were related to the darkness of the green fruit. With selection of uniformly light green fruit to facilitate harvest, associated production of sugars and lycopene (the red tomato pigment) had been compromised. This knowledge could assist the improvement of flavour and health properties in farmed tomatoes. Source: Discovery may lead to new tomato varieties with vintage flavor and quality

Lignin digestion the end of coal
Timber brown rot fungi can break down cellulose but not lignin. White rot fungi can attack both. A genomic study of wood-rotting fungi has estimated appearance of white rot fungi 290 million years ago, at the end of the Carboniferous period. This may have been a major reason that coal formation largely stopped at that time. Previously, fungi did not posses the enzymes required to digest the lignin component of wood, so it accumulated and eventually became coal. Source: White Rot Fungi Slowed Coal Formation

Tree-soil-microbe interaction creates limestone
Scientists have discovered a bacterium that can incorporate carbon dioxide into limestone, with the the help of a tropical tree and a fungus. Under certain conditions, the tree combines soil calcium with atmospheric CO2 and the bacterium creates the conditions under which it can be converted into calcium carbonate, depositing limestone around the tree's roots. This is a way of both improving the soil for agriculture or reforestation as well as locking away carbon in the soil and is already being trialled in several tropical countries. Source: Bugs in key role of CO2 storage method

Pitcher plant's springboard of death
Nepenthes species are well known for their ability to trap insects who slide to their doom on the slippery surfaces of the its fluid-filled pitcher. However, another method of catching prey has been observed in N. gracilis. Insects sheltering or feeding on the underside of the pitcher lid could be catapulted into the pitcher by the force of raindrops hitting the lid. Further investigation revealed specialised wax crystals coating the underside of the lid. These seem to give insects just enough footing to be able to walk under the lid, but ensure they are easily dislodged. The large amounts of nectar secreted by N. gracilis may be a way of enticing insects into this novel trap. Source: Pitcher plant uses rain drops to capture prey

Ladybirds aren't bluffing
The colour of ladybirds acts a warning to birds that they aren't good to eat. An Australian-UK collaboration has confirmed that there is indeed a relationship between the intensity of the red coloration and the toxicity of the insect, proving a definite incentive for predators to avoid the most brightly coloured individuals. Furthermore, producing the colour and taste comes at a cost to the ladybird and is affected by the quality of its diet. Source: I'm bright red and I taste foul - the message behind colour and the ladybird's spots

Turn up the heat on weeds
Most of us have heard of using boiling water to kill weeds in paving, but studies at the University of Copenhagen have helped define how such heat treatments (flame, steam or boiling water) can be used most effectively. It was found that the leaves must collapse completely after each application, starving the roots. Also, the treatment must be repeated frequently enough to prevent recovery. Superficial or infrequent treatments permit regrowth and can actually promote grass weeds. With the right strategy, however, this low-toxicity method can eliminate even stubborn weeds. Source: Blanch your weeds


Grow for Gold

Now the real games begin, as vested interests maneuver for more public money to buy gold medals.

Meanwhile, in Adelaide, it's hoped that volunteers will help keep the city's botanic gardens in shape after a budget cut and resultant loss of staff [1], but tropical plants will be lost from their Bicentennial Conservatory because the administration can no longer afford the heating bill [2]. The Australian National Botanic Gardens in Canberra are also calling for volunteers, and asking us to remember them in our wills [3].

If physical activity and general health in the wider population has been part of the rationale for taxpayers funding "elite" sports, then return on investment over recent decades has been rather poor [4],[5]. If we need to try something else, wouldn't improving our urban landscapes encourage people to get out and moving?

Well maintained and well-appointed parks and gardens provide a destination for family days out as well as an attractive spaces for exercise and recreational games. Clean, safe and well-lit footpaths and cycleways are necessary for greater use of human-power instead of petrol, but street trees and regular verge maintenance could make these facilities even more appealing to potential users.

If local residents can then be encouraged to enhance their own properties - through horticultural education (children and adults), garden competitions or plant/mulch giveaways, for example - your walk to the train station could become positively delightful!

Such things come at a cost, of course. But then, so do sporting trophies [6]. Perhaps it's time to reconsider our investment strategy?

[1] Adelaide Botanic Gardens receive unwanted pruning Australian Broadcasting Corporation
[3] Support the Gardens Australian National Botanic Gardens, Canberra
[4] Overweight and Obesity in Australia Australian Government Department of Health and Ageing

Queensland Garden Events

With the arrival of spring comes a multitude of garden shows and garden openings. If you live in Qld, be sure to check out the diary at to help plan the weeks ahead.

By the way, it would be great to see more events (plus other garden-related news) from North Queensland. If you're organising something in the tropical north, please do take a minute to send in your info. More information on the page.

The Science of Horticulture

Plants and Iron (Part One)

The relationship between iron (chemical symbol: Fe), soil and plants is very complex, but vitally important. This series of short articles is intended to give a broad overview of the topic, highlighting aspects that have practical implications for gardeners.

Why do plants need iron?

Although associated with a good green colour in leaves, iron is not actually a part of the chlorophyll molecule. It is, however, essential for chlorophyll synthesis.

Iron is also crucial to the electron transport system of photosynthesis, via which light energy is converted into food and (ultimately) structural materials. Later, the release of energy from food (respiration) required for biological functions throughout the plant also employs iron.

Furthermore, a number of important enzymes involved in a variety of synthetic and protective processes in plant tissues require iron in order to function. Consequently, iron deficiency can affect plant growth in a myriad of critical ways.

The role played in symbiotic nitrogen fixation is particularly interesting. The iron-containing molecule leghemoglobin helps regulate the oxygen level inside legume root nodules, such that the nitrogen-fixing bacteria can survive without inhibiting the oxygen-sensitive nitrogenase enzyme (which also requires iron).

Leghemoglobin has a very similar structure to the oxygen oxygen carrier hemoglobin in our blood. It's also red like hemoglobin. If you cut open an active root nodule, you'll see it's pink inside. Casuarinas, which also fix nitrogen with the help of a different group of bacteria, also produce a hemoglobin-type protein in their nodules.

Deficiency symptoms

An important feature of iron utilisation in plants is that it can't be re-mobilised from old tissues and directed to the more critical growing points if an iron shortage develops. Consequently, deficiency symptoms usually manifest in the youngest leaves first. This is an important criteria for visual diagnosis of iron deficiency.

The classic iron deficiency symptom is yellowing (chlorosis) of leaves between the veins. The veins themselves tend to remain green and contrast strongly with the chlorotic interveinal tissue. In cases of extreme deficiency, the leaves may become uniformly pale to nearly white. Youngest leaves are affected first.

These symptoms may be referred to as "iron chlorosis" or "lime-induced chlorosis" in some literature, the latter because iron tends to become chemically unavailable on high-pH soils.

Other possible symptoms include brown or dead (necrotic) spots or margins on leaves, stunted growth, leaf drop or shoot die-back. A tree may not be uniformly affected. The quantity and quality of fruit may also suffer. Ultimately, the whole plant may die.

Note, however, that a deficiency (of any nutrient) is difficult to diagnose conclusively by sight. Growers of commercial crops also use soil and tissue tests for guidance because observed symptoms may be confused with other nutrient deficiencies/toxicities, herbicide injury or diseases. Expression of symptoms may also vary between plant species/varieties and be influenced by co-existing conditions. Not surprisingly, a plant weakened by deficiency is likely to be more vulnerable to diseases and other disorders. Conversely, conditions that inhibit nutrient uptake can lead to deficiencies.

Before going to the trouble and expense of treating an iron deficiency with fertilisers, gardeners should consider whether interacting factors such as soil alkalinity may need to be addressed. Such issues will be discussed further in Part Two. In the meantime, refer to the links on if you'd like to learn more about iron nutrition in plants.

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