Archive for the ‘Physiology’ Category

During summer, growers experience a lot of problems with tomatoes. This article deals with the effects of temperature on tomatoes – on pollination and fruit set and also on ripening.  I will deal with diseases in another post.

Tomatoes are affected by high temperatures in a number of ways. Some sensitive varieties are affected when average daily temperatures exceed 25°C, whereas more heat tolerant cultivars are not impacted until daytime (maximum) temperatures exceed 32°C. There are even some cultivars are able to set fruit at temperatures above 35°C.

Under marginal conditions fruit may set without adequate pollination but the internal fruit segments will contain few seeds and the tomato will be flat sided and puffy. Irregular pollination can also cause ‘cat facing’ (http://vric.ucdavis.edu/veg_info/catface.htm).

In general fruit set is adversely affected when temperatures fall below 10°C or rise above 27°C. Optimum temperature for fruit set is 18° to 24°C. Even moderate increases in mean daily temperature (from 28/22°C to 32/26°C day/night) result in a significant decrease in fruit set.

As a general rule, the 8 to 13 day period prior to flowering is the most critical phase. If the average maximum temperature in that time exceeds 29°C, pollination and fruit set are impacted. However as pointed out earlier, this does vary according to cultivar.

Why aren’t my tomatoes ripening?

In hot weather people expect fruit to ripen faster. But with tomatoes the optimum temperature for ripening is 21 to 24ºC. When temperatures exceed 29 to 32ºC, the ripening process slows significantly or even stops. At these temperatures, lycopene and carotene, the pigments giving the fruit their typical orange to red appearance cannot be produced and so the fruit stays green.

For tomatoes light has very little to do with ripening. Light is not needed for ripening and fruit exposed to direct sunlight can heat to levels that inhibit pigment synthesis (As explained above). Direct sun can also lead to sunburn. Do not remove leaves in an effort to ripen fruit. Also, soil fertility doesn’t play much of a role. High magnesium and low potassium can cause blotchy or uneven ripening or yellow shoulders. But slowness to ripen is generally not due to poor nutrition and adding more fertilizer won’t help.

You can remove fruit which are just showing the first colour changes (mature green), and store them at 21-24ºC in the dark, preferably in an enclosed space or in the presence of fruit that give off ethylene gas such as bananas. This may speed up the process by up to five days.

References and further reading





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This post is a bit longer and more technical than usual.  I originally wrote it for a different audience and this is my attempt at making it user friendly for most people.  Like many things in nature it’s complex.

Healthy soils teem with microbes. Inoculating soil with with microbes to boost crop production is all the rage at the moment with companies sprouting – well, like microbes! Usually they are being sold to the commercial grower but more and more they are also being marketed into the home garden market where cost is less of an issue (ie home gardeners don’t have to make a profit).

Whether you can parachute in foreign organisms and expect them to live and prosper is really open to debate.  In most studies it doesn’t happen.  And in sandy soils with virtually no organic matter there is nothing there for them to feed on so it is even less likely.  Sure you can add composts and manures but these don’t tend to hang around much in sands, they burn off very quickly unless some clay is also added.  And large amounts of compost and manure provide phosphorus far in excess of plant needs which means it leaches down into the groundwater, potentially getting into waterways and causing algal blooms.

I have seen some papers showing how mycorrhizae have increased plant growth, even without fertiliser being added.  Those papers carefully omitted to cite a soil analysis!  One thing to bear in mind – I certainly don’t dispute that some substances can increase plant growth but they can’t do it forever without, at some stage, the food supply having to be replenished.  What you are doing is mining the soil.  It might work once or twice but then you run out of food.  Plants can’t feed on fresh air!  OK I can hear you saying they can fix nitrogen from the air – yes – but not phosphorus or potassium!

Mycorrhizae may alter root architecture – ie the way roots branch and proliferate.  That can make plants take up nutrients more efficiently by exploring more of the soil.  So you might see a growth spurt – but again, that nutrient will have to be replenished if that response is to be repeated and maintained.

So lets look at microbes.

A changing population

Populations of fungi are not constant. They change frequently in response to a whole range of factors. Even those associated with a single plant, change with the growth stage of that plant. Figure 1 shows the relative amounts of different fungal species over time for a pea plant. In the vegetative stage, Fusarium is most common but diminishes over time in contrast to Heliotales which increases as the plant matures.

The relative amounts of each fungus also change with fertility (Figure 2) and crop health (Figure 3).

                     Vegetative                                                    Flowering





Figure 1.  Change in microbial populations with growth stage in the rhizosphere of pea. (Note the rhizosphere is that thin coating of soil left on the roots after the rest of the soil is shaken off).

OM rates

Figure 2  Relative amounts of fungal species in pea roots grown with either nil or three levels of organic fertiliser (OF)


Figure 3.  Relative abundance of fungal species associated with the rhizosphere of healthy and diseased pea plants.

You will note that in all the examples above there is a mixture of ‘good’ (e.g. Glomus sp.) and ‘bad’ species of fungi (e.g. Olpidium sp.).  This is normal and disease only occurs when this balance is disrupted in favour of the pathogen and other conditions in the environment and host are right for infection and disease development.

Mycorrhizae are one group of fungi known to have beneficial effects on plant growth in some circumstances, but this is highly variable. The term mycorrhiza covers a large number of genera. Glomus species are some of the most prevalent. Examples of their effects on plant growth are provided below.

Highly specific effects

Example – Three mycorrhizal species were studied on basil.  None affected plant phosphorus level.  Only one significantly affected plant growth and increased the amount of one essential oil produced while the other two increased the amount of a second oil and decreased that of a third.

Microbes can also have adverse effects such as growth suppression

Example ‑ A trial on onion and plantago showed varying effects of mycorrhizae on plant nutrient levels.  The authors expected the mycorrhizae to increase host nutrient levels, however in some cases they reduced them.

Example – Twenty-three different mycorrhizal strains were evaluated for their symbiotic response with Piper longum (long pepper).  Almost all resulted in increased plant growth, biomass and nutrient content (nitrogen and potassium) over the control, however six species depressed growth.

It’s Complicated!

Example – A comparison of mycorrhizal versus non-mycorrhizal roots showed phosphorus uptake doubled and was independent how much was in the soil. There was no additional benefit of the mycorrhizae on plant growth other than that due to increased P uptake.

Example – The effects of a mycorrhiza on growth and photosynthesis of cucumber were studied using different rates of nutrient supply, phosphorus ratio and different forms of nitrogen.


Plants inoculated and given full-strength nutrient solution showed a 19 per cent reduction in total biomass compared with mycorrhizal (AM) plants (Figure 5).

The highest percentage of mycorrhizal infection in cucumber was found at the low P treatment, however a 90 per cent reduction in total nutrient supply almost totally counteracted the potential positive impact of a low concentration of P on mycorrhizal infection.

The extent of mycorrhizal infection in cucumber was correlated with a low root P concentration, which agrees with other studies that plant P status influences mycorrhizal infection.

Not all plants are mycorrhizal

Some plants do not form relationships with mycorrhizae even when inoculated. Brassicas, beetroot and spinach are among those.

Local species usually prevail

This is probably the most important consideration of all.  Any microbe placed in a field situation will face competition from local species and may eventually be displaced.

Example.  A Turkish study took 70 soil samples from 25 different plant varieties grown in local fields.  Arbuscular mycorrhizal fungi (AMF) were found in 59 soil samples; 58 of these were identified as Glomus and one as Gigaspora.

The effect of the local Glomus sp. was compared to a commercial preparation on tomato and cucumber plants.

The local Glomus species increased cucumber and tomato plant growth, but the commercial mix did nothing.  The local Glomus species colonised plant roots at almost twice the rate of the commercial one.

The relationships are very specific

Each plant species, and even variety, tends to have a preference for certain mycorrhizal species. It is difficult to manipulate relationships and so the effect of inoculating a soil with mycorrhizae depends on many factors.  If there are already successful mycorrhizal associations present, those existing association may be stronger and the introduced species may fail to displace those already present, or it may displace a proportion of the existing associations.  Inoculation with mycorrhizae does not necessarily mean more root colonisation in terms of either numbers or species.

And lastly – Beware trial results!

Many research results, when you read the fine print, are from work that has trialled mycorrhizae under unnatural situations, whether using sterile media or in a laboratory. Those results are not often reproducible in field situations.  They work because the introduced (foreign) microbes have no competition.  A better approach and one more likely to succeed, would be to use local species, bulk them up and inoculate back.

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There are many myths about potassium. The biggest one being you need it for flowering.  WRONG!  You need potassium no more or less than any other nutrient for flowering.  Sure, some commercial growers might change the ratio of nitrogen to potassium as the plant moves from vegetative to fruiting but it has nothing to do with the formation of flowers and fruit, it’s more about flavour.  Too much nitrogen can produce fruit that is big but watery, tasteless and low in sugar and other flavour components

Potassium is important in that it helps plants tolerate stressors such as cold/hot temperatures, drought, and pests.  It is a catalyst for many plant enzymes and helps regulate water use in the plant by affecting the opening and closing of the stomata in the leaves and water movement in and out of cells.

Potassium is a funny nutrient in some respects because we often see little response to it in trials.  But palms and other plants that clump, may respond to it by increased clumping and branching.  We’ve seen that response in some native plants as well, such as Stirlingia.  Too much potassium can make stems very brittle so they snap easily.

Signs of potassium deficiency can be quite dramatic and also species specific.  Sweet corn gets a sort of burning – a band around the leaf margins which is dried out and dead looking.  Carnations also get necrotic spotting at the tips of older leaves.  Hoyas end up with necrotic spots all around the leaf edges.  The symptoms are always on the older leaves because potassium is mobile and will move to the place of greatest need – we call that a sink and is often the fruit or flowers, or at the very least a growing point.

Potassium is supplied in many products as potassium nitrate – 36-38% potassium and 12-13% nitrate depending on formulation and purity.  Potassium sulphate can also be used but is more acidifying – an effect we often wish to avoid in our soils.  The cheapest source of all is muriate of potash or potassium chloride which we don’t really recommend at all because of the high chloride (salt) content.

Good organic forms of potassium are wood ash, dried seaweed and blood and bone, though the phosphorus content of blood and bone is rather high relative to the nitrogen (N) and potassium (K) and plants fed with blood and bone will need both N and K supplemented.

Potassium is readily leachable in our sands but is held on slightly better than nitrate in the presence of clay or compost as it is a positively charged ion.  Nevertheless, we see high rates of leaching of potassium from newly applied compost.

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A frequent cry from gardeners relates to lack of flowering and very often the response is to add potassium. But potassium is NOT responsible for flowering, any more or less than any other nutrient and the real reasons for flowering problems can be many.

No flowers at all? Ask yourself how old is the plant? Is it a seedling which may not flower and fruit for several years? Many eucalypts are in this category. I have an Illyarrie (E. erythrocorys) which is now seven years old and hasn’t yet flowered – I’m not happy but there’s not much I can do about it! Passionfruit from seedlings are similar and may not fruit for several years.

Sometimes pruning is the problem. Particularly with fruit trees, cutting off the wrong wood means no flowers and no fruit. Pears, plums and cherries produce fruit on spurs, lateral growth is often minimal. Japanese plums fruit on 1-3 year old spurs. European plums fruit on lateral spurs on 2-3 year old wood. Peaches and nectarines produce fruit on the new season’s fruiting wood. Apples produce their best fruit on fruit on wood two-years and older, though some varieties such as Sundowner™ also bear fruit on one-year wood. Figs produce their main crop on current seasons growth and an early lighter crop on the previous years wood.

Sometimes the weather is responsible. A cold snap at a critical time can prevent flowers from initiating or abort them very early. Drought can do the same. Occasionally plants don’t flower because the light hasn’t been right. Some plants such as Zygocactus will not flower if their night time darkness is interrupted by light from a street light or inside a house.

The mechanisms for flowering in plants can be complex and are generally a combination of temperature and daylength and one of these may be more important than the other. So anytime you grow a plant out of its natural habitat, you run the risk of problems depending on how pernickety it is. Banksia coccinea is one plant like this that tends to not flower well, if at all, around Perth. Banksia coccinea is also prone to the ‘pruning effect’ because it actually is initiating the next years flowers around the same time as it is flowering. So prune after flowering and you’re apt to be pruning off all the next years flowers!

It needs to be understood that flowers may have quite separate requirements for flower initiation as compared to flower development. Obviously flower development can’t happen without first having flower initiation but you can have flower initiation and no flower development afterwards. Most European bulbs require a period of chilling during winter to initiate flowers but the soil temperature during spring can also be too high and those flowers will abort. Conversely, many South American or South African bulbs that are autumn flowering have high temperature requirements that must be met for flower initiation and flower development to occur. Sometimes these requirements can be met by other means such as putting bulbs in the fridge for a few weeks but sometimes this doesn’t always work well because of these other factors at play.

Sometimes uneven flowering or flowering on one side of plant can happen and that’s usually for the same set of reasons as above.

Poor nutrition is seldom responsible for a complete lack of flowering, although excessive nitrogen can keep a plant vegetative. Poor nutrition though, can be responsible for some peculiar flower development such as no petals/empty flowers. Calcium or boron are most often the culprits when there are fruiting or flowering problems but first check for periods of water stress because interruptions in the transport of these nutrients can be the reason. Both calcium and boron are essentially immobile (so when in short supply they can’t be moved from older leaves to places of greater need) so that is why symptoms are seen on the growing tips and why symptoms of deficiency show up in flowers/fruit or any growing points (in palms death of the growing point will kill the plant). Bent neck (should be self explanatory) is usually a calcium problem but often more to do with water stress/periods of reduced transpiration such as high humidity) rather than actual lack of calcium in the soil.

Sometimes flowers go green (gerberas show this symptom). This is usually due to infection by a mycoplasma and is incurable. Get rid of affected plants.

Occasionally flowers go mad and start producing flowers within flowers. Or whole little plants within flowers. Usually this sort of secondary development is physiological and due to erratic weather conditions. Rarely is it due to herbicides or any other chemical agent. I have seen this on banksias, roses and carnations to name a few.

You may have flowers but they remain stubbornly closed. This is often wind damage – we call it blasting. Sometimes, it’s also disease such as grey mould caused by Botrytis cinerea.

Finally, you may have had flowers but they may have aborted or dropped for some reason. And depending on when that happened, it may not have been that evident. Most often this is due to moisture stress or a sudden increase in soil salinity (either because the soil dried out temporarily or because you threw on a heap of fertiliser). Some plants also have inbuilt mechanisms for controlling flower (and hence fruit) load to ensure that all fruit set can fully mature. All cucurbits are like this (pumpkins, melons). Other fruit trees can be biennial bearing – that is they tend to have one year when they crop heavily, the next year is a lighter load.

While we are talking flowers in this blog, don’t forget the importance of pollinators where fruit is concerned. Bees, flies, moths or other insects. Lack or, or poor or uneven pollination can also cause a range of problems from no fruit to malformed fruit. But that will be a whole other episode!

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I covered the question of growing plants from seed or cuttings in another blog. Now I’d like to explain a little more about how to grow plants from seed.

Germinating seed can be as simple as scattering some seed on the ground or in a pot and Voila! It all happens. But often its not that easy and in fact can be down right frustrating. Why?

Well there’s a whole heap of factors that affect seed germination. If you’re buying packaged, branded seed such as vegetable seed then you can be fairly sure the seed will germinate provided its not out of date and you haven’t killed it! You see seed doesn’t last forever – although some lasts for many years, some species need to be germinated within about 12 months of being harvested because viability drops off really quickly (viability means it is capable of germinating ie the embryo in the seed is still alive).

Seed storage
For most seed of most European varieties, keeping the seed in the fridge is a good idea but recent research with many Western Australian species has shown they need to go through a process called after-ripening and that means room temperature storage is best. And when you think about it, Western Australian species are not used to going through a period of winter chilling so that sort of makes sense.

Other requirements and treatments
Some species require light for germination and so you should not cover the seed while it is germinating. This includes species such as lettuce and many other small seeds. Many hard coated seeds such as Acacias need to be scarified in some way – dropping them in boiling water is the easiest way but sandpaper can also be used. For larger seeds nick the seed coat to allow moisture in. Some seed contains inhibitors in the seed coat that have to be leached out. That can be done under running water. Nature does it over winter in the rain. Often seed may be dropped in acid to help it germinate – this is just another form of scarification. As is passing through the digestive tract of an emu, bird or other animal!

There are also a number of other specific chemical treatments that may be used on seed. For example priming seed with potassium nitrate often helps germination. Sometimes hormones can be used (gibberellic acid for example). If you have collected the seed yourself then there are a number of other considerations. Was the seed harvested at the correct stage of ripening? Is the seed even viable ie does it have an embryo and is that embryo mature? Many native plants have a large degree of seed predation – insects may bore into the seed and eat part of the inside. Most of this can be answered by cutting the seed open and examining it under a magnifying glass.

If you’re a native plant lover then you’ve probably heard of smoke treatment. There are certain compounds in smoke that help trigger germination in native plants. Extensive work has been done on that at Kings Park in Western Australia and also in South Africa. Both these places have plants that have evolved in an environment prone to bushfires.

If you’re having trouble germinating something, here are some references for you. And always try a search on the internet for your species, you may be surprised at the amount of research work out there that has been done.


Australian seeds: a guide to their collection, identification and biology By Luke Sweedman, David Merritt, Kings Park and Botanic Gardens, 2006 CSIRO Publishing

Ball Culture Guide: The Encyclopedia Of Seed Germination by Jim Nau, 1999, Ball Publishing.

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This may be short but it is topical for me at the moment. And not without relevance for the home garden.

I am lucky I guess because I know my block was bush before I bought it. And still is in part. So I can grow veges and have chooks and eat their eggs without worrying what they might contain. But many people on newer blocks, or even not so new, may not know their history. Land around me that was orchard for years is being turned into housing. There is certainly land in the metro area that was a dumping ground for rubbish in the past. If you’re intending to grow veges and eat them, or run animals on your land and eat them or their products whether they be eggs or meat, it may just be worthwhile getting a soil test done first to ensure the land you’re on doesn’t have any nasties in it. Old farming land may have organochlorines. Rubbish dumps – who knows what!

Then there’s the question of water. If you’re drawing water from a dam or bore also consider having it tested. Especially if there’s any sort of commercial activity nearby.

Also consider what you bring on your property. Those manures/composts you bring in may also be contaminated via the land they are from. Particularly if the composts are not made to the Australian Standard which does test for all those sorts of things. Even just simple soil may contain almost anything and its hard to go back once you’ve got any sort of contamination, especially disease. Look at the sting nematode debacle around the metro area at the moment! And of course dieback.

Finally consider the things you grow. Some crops accumulate nasties. Brassicas are known to be good at accumulating heavy metals, spinach also accumulates cadmium, tea- aluminium and so on. Plants that take up lots of heavy metals can be used for bioremediation and that is a good thing but you wouldn’t want to eat them!

So surely if you’re buying a new place its worth doing a bit of checking and testing to see if there are any potential issues lurking in your land.

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So many times this comes up in conversation. Saving seed of something to grow. And how often it is a waste of time! For a start so many plants grown from seed will not resemble their parent. This goes for almost all fruit trees and all roses for starters. Anything you buy with a name like Babyface, Gala, Red Globe, Hickson, Black Beauty etc will be a cultivar and unless you grow it vegetatively from a cutting it won’t be anything like the original. Even if you can grow it from a cutting its still not that easy. Apart from Plant Breeders Rights legislation which means its illegal to propagate, there is about a 99% chance it is grafted onto a rootstock that also gives the scion (what’s on top) special characteristics. Often resistance to a particular disease but it may make it more compact or more vigorous for example. Many conifers are selected cultivars grafted onto a rootstock.

Most ornamentals you buy from the nursery are vegetatively propagated. So camellias, grevilleas, lavenders, you name it, they are all grown from cuttings.

Another advantage of growing from cuttings is that the material is usually adult. That means it will flower or fruit almost immediately. Plants grown from seed often take years to flower or fruit. That mean 5-6 years to find out that lovely pink tree you took the seed from is now white! The exception to this is some plants that have juvenile leaves – like some eucalyptus species. You can get cuttings of juvenile material to strike but the resulting plant will still be juvenile and will take some time to flower.

Of course there are some plants that are relatively true from seed and some that can’t be grown from cuttings so seed is the only way. Most banksias, dryandras (OK these are now renamed banksias but you know what I mean), hakeas and acacias are like that.

Some other plants are usually grown by other means, such as dividing them up or tissue culture. Most ferns you buy are tissue cultured, some are grown from spores. Many house plants are tissue cultured, also orchids. Kangaroo paws are mostly tissue cultured, a few are still grown from seed and division is possible but too slow for commercial use.

There are some differences in structure between seed and cutting grown plants. Most seedlings have taproots that head straight down for the water table, they aren’t present in cutting grown plants, though over time some roots may take over, become dominant and head downwards to a source of water. But in any case, in most plants, seed or cutting grown, 85% of the roots that take up water and nutrients are in the top 30 cm regardless. In proteaceous plants eg banksias, the specialised proteoid roots form when its wet (so usually winter unless you are irrigating) and they can be extremely shallow – that means don’t cultivate or spray herbicide around the plant if you want to be safe.

By the way – we pretty much hit 10 mm/day evaporation on those last two hot days. I hope you took account of that in your irrigation. My citrus are sizing up and I made very sure to give them two drinks on those days.

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