Adequate Light and Air

Plants need a good supply of water, a well aerated soil and enought  light – they are also sensitive to the daylength as this    triggers behavioural changes – particularly  the onset of flowering – or bolting.

Plants need enough energy from sunlight so that they can turn CO2 and Water into sugars and other cell molecules.

Gardens often have shady patches where you may need to choose plants that suit those conditions.  However most plants, including most vegetables need full sun to grow well – at least in the UK – as long as they are kept well watered. Interestingly in areas where the sun is stronger – such as Turkey – salad crops which suffer from too much heat are grown in the shade of trees – there is a kind of traditional permaculture  which uses very thin poplars planted in a band round the outside of the plot to provide intermittent shade.  This is the same principle that we use with our leaf crops in the nursery in full summer where we use thermal screens pulled half across to temporarily provide some shade and traditional glasshouse owners used limewash in the summer.

The main problem that the plant faces is how to do enough photosynthesis to build the food reserves it needs to flower and fruit without running out of water. Since plants can’t move about they’ve evolved various ways of doing this which mainly centre around leaf designs that allow control of the loss of water.

As gardeners, we have put plants into un-natural conditions for our own purposes so we have to manage their environnment with respect to light, water, nutrients and  competition (that’s weeding to you and me!). In fact production horticulture these days frequently supplies extra CO2 in glasshouses to stimulate the rate of photosynthesis and hence bump up the yield of crops like tomatoes.

The one thing you really musn’t do is let them dry out.  Any check will stunt the growth and force the plant to move on to the next stage, age and die prematurely.  When the roots start to go dry they produce a plant hormone called abscisic acid which is responsible – amongst other things – for leaf fall in the autumn.  It also acts to close the leaf pores – or stomata – so reducing water loss – but also restricting the amount of CO2 the plant can take in – and so limiting photosynthetic production of the goodies that the plant needs.

So it’s good to be able to keep track of light and moisture levels around your plant. Fortunately our wonderful combination meter does just this. Pause for commercial before we go on with the science.

propellorheadHere’s the Science

How the plant gets its CO2

The plant makes glucose via photosynthesis which is then converted into the other molecules that the plant makes. The rate of photosynthesis depends mainly on the intensity of the light and the availability of CO2 and water.
The plant takes in CO2 through its leaf pores which are found on the underside of the leaf. They are called stomata and they open and close according to how much water pressure there is in the “guard cells” which form the rim of the pore.
There are usually 100 to 1000 stomata per square mm depending on the plant species and the environmental conditions during development. More stomata are made on plant surfaces under higher light, lower atmospheric carbon dioxide concentrations and moist environments. Grasses typically have lower stomatal densities than deciduous trees.

It’s been found that they tend to be spaced at about 10 times their diameter. Because this lets the maximum amount of CO2 in. However, if the plant starts to dry out, then the change in the water pressure or turgor in the plant causes the guard cells to reduce the size of the pores. This has a much greater effect on the loss of water than it does on the intake of CO2 so the plant can carry on functioning effectively in the early stages of drought.

Light and Daylength

Plants are affected by  daylength.  Strictly speaking its the length of the night that matters – so  commercial growers will sometimes give plants a burst of the light (which needn’t be very long) in the middle of the dark period to convince the plants that they are still in a short night regime.

The main practical issue for gardeners is to be aware of which plants are short day and which are long day plants.  There are a number of vegetables – especially chinese leaves like Pak Choy which if planted in the spring will be very liable to bolt as the day length gets longer. The remedy is to regard them as a crop for the second half of the year – planting in late May and June so that when the plant starts to put on leaf it’s already under a regime of shortening days. More details in the section on why plants bolt.

Finally, the amount of photosynthesis depends on how much light you actually get at the leaf surface.

Recent research on De-leafing

Some recent research that’s been carried out for commercial growers may be interesting for you.  Basically,  plants like raspberries and peppers are grown close together and the lower leaves are quite shaded.  It’s been found that under these circumstances the lower leaves are not contributing very much in the way of photosynthetic production.  Removing the lower leaves has the effect of reducing the water burden on the plant (since it cuts the amount that the plant can transpire through the missing leaves) while not affecting the plant’s ability to be productive very much.

An article in December 2009’s “Commercial Greenhouse Grower” reports that in peppers the top 40cm of the plant do most of the work while the second 40cm makes a reasonable contribution with the bottom 60cm being pretty useless.   Removing the leaves from the bottom 80 cm of a 2.4m plant made no apparent difference to either plant growth or yield.

The effect on transpiration was to reduce humidity in the glasshouse by between 6-9%.  As managing humidity (due to danger of fungal infection) is important in a commercial glasshouse environment there are obvious savings to be made in terms of water costs and the energy costs of the fans which reduce humidity especially during September.

An interesting practical piece of research – how far these observations could be used to improve cultivation of  crops like runner beans or glasshouse tomatoes and cucumbers  is of interest to the practical gardener.

The golden rule is to water well – but not necessarily often. You need to make sure that you use enough water to soak the roots. If you only water a little – the roots will stay at the surface and the plant will be weakened. The amount of water needed to wet the soil to its maximum capacity will vary with the type of soil you have and it’s texture.

As climate becomes more unpredictable, its like that we will start growing more drought resistant plants as ornamentals and it is likely that there will be development in lawn varieties.

However, for vegetables we need succulence so a study of what you really need to do is worth making.  More information about what to do here.

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