

Light & Plants - the science
This week, we've gone science - and we ask:- Why do plants need light?
- Does everything need light to grow?
- Longer light or stronger light - which is better?
- Can plants get too much light?
- Which places get most light?
- What colour light helps plants to grow?
Why do plants need light?

Using water (via their roots) they convert that light energy into sugars, enzymes, and chlorophyll that they use to grow.
They breathe out excess energy as oxygen.
What about plants that aren't green?
- Chlorophyll makes plants green, and helps them absorb more sunlight.
- Carotenoids give leaves orange-yellow colour, and help chlorophyll do its job.
- Anthocyanins, which create purple-red leaves, act as ‘sunscreen’ when plants are stressed from too much light.
Does everything need light to grow?

But new ocean bacteria have been found that can photosynthesise in infra-red light, which means that however dark it gets, they can continue to grow. Pretty amazing! When it comes to plants however, it's a different story (for now) - they need the kind of light we can see.
 
How do plants know where to grow?
Seedlings navigate by moving away from gravity. Once they reach the soil surface, they grow towards the strongest source of light, in a process called phototropism. You'll notice this if you grow plants on a windowsill - they'll need a turn every now and then to straighten them up.Some flowers are known to follow the path of the sun so they always face the light - sunflowers get their French and Italian names - tournesol and girasole - from this habit of turning to the sun. 
Longer Light or Stronger Light?

Etiolation - Too Little Light
This is a fancy word for growing "leggy" - plants that don't get the light they need will grow tall, pale and weak. You can see this in action if you discover a forgotten potato at the back of the pantry, covered in long white shoots. Plants can usually absorb and use as much light as you can give them. What will affect them is how hot that light is - most plants stop growing when temperatures get too high. Grass will stop growing at temperatures over 32C. Lettuce will "bolt" or send up a flowering shoot once it gets too hot - shading plants to reduce daylight hours can slow this process.
Too Much Light
Sometimes plants do get too much light, if they're grown outside their native range in more intense light, or a shade plant is grown in sun. This shows in bleached pale leaves. Full-sun plants usually need at least 25,000 lux - or 6 to 8 hours of direct light each day. Sunlight intensity on a bright summer day can reach 100,000 lux. For comparison, indoor light levels are around 100 lux. Plants that thrive in shade, or indoors, can flourish with just 2 hours of daily full sunlight. Plants often adjust the angle of their leaves, to reduce or increase the light reaching the surface - you can see this on many gums which have a "weeping" look as the leaves hang down. The lithops succulents (above) have tiny "windows" which they open in winter and close during summer, to regulate the light reaching the centre of the plant. (Image: yellowcloud) 
The Sunny Country

Australian cities get around 3000 hours of sunshine a year (twice as much as the average northern European city, fyi). But that can differ, depending on where you are and the time of year. Perth leads the sunshine stakes, thanks to its dry climate.
Inland towns also have drier climates, lower humidity, and so less cloud cover and more sunlight. Coastal areas, especially in the south, have more humidity, more cloud cover, and less sunlight all year round than inland places. During the October-April wet season, there is more cloud in subtropical regions because of the monsoon, while southern Australia gets drier, less cloudy conditions with more sunshine.

North v South
In the May-September dry season there is more sunshine in the subtropics; while southern towns receive winter frontal systems which bring rain, cloud, and less sunshine. North-facing sites in Australia have the most intense light - five times that of a south-facing site in the same location. The same goes for northern versus southern towns in winter. In summer, UV levels are much more equal nationwide. Horticulturalist James Beattie says"South-facing aspects are one of the most challenging spots for gardeners in the southern hemisphere as they get no sun in winter, but face summer's unbridled baking onslaught."
 
One Light Two Light Red Light Blue Light

It's also ideal for seedlings which emerge stronger and sturdier.Here you can see zinnia plants growing under an array of Red, Green, and Blue LED lights, at NASA's Kennedy Space Centre and the International Space Station. Changing the wavelength, and so the colour, of light is not the only way us humans can control when plants flower. Our blog post on Plants & Light explains more about the effects of shorter days and longer nights - or vice versa - on how and when plants flower and grow.