Posted by Phytopath on Mar 26, 2010
Most gardeners are aware of the importance of light for plant growth, but just how important is it?
Anyone that has ever tried to grow a plant in dimly lit conditions indoor’s, knows that the plant suffers and usually declines or dies. There is no such thing as an indoor plant. Plants grow outdoors, but we choose specific plants that are capable of growing in low light situations. These are usually plants from rainforest areas where the canopy is quite thick and minimal light reaches the forest floor.
Plants need sufficient light (intensity) to photosynthesize and different types of plants require different amounts (sun lovers vs. shade lovers).
Photosynthesis is quite an amazing thing. Chloroplasts within the plant (mainly in the leaves), absorb energy from sunlight, mostly the blue and red spectrum of light (where solar energy has its maximum output). Think of a rainbow and the colours in it, remember ROYGBIV? Red, orange, yellow, green, blue, indigo and violet. Well, plants don’t much like the green part of the light spectrum so they don’t absorb the colour. It is reflected back from the leaf surface or passed straight through the leaf tissue, and that is why plants appear to be green to our sense of sight (unless you are colour blind).
This absorption of light energy is transferred into electrical and chemical reactions which in turn produces organic compounds such as sugar and starch, or carbohydrates (this is an over-simplified description) that the plant uses as fuel for growth.
The second important plant/light relationship is phototropism. This is when plants bend toward a light source. When a seed germinates and the new shoot heads for the sky, it had better not be under an overhanging rock. But what if it is? That’s when phototropism comes into play. Or have you experienced an indoor plant that leans toward the window or source of light and you have to give it a quarter turn every week or so? That’s phototropism.
Plants contain hormones (yes, they can get hormonal just like humans) and one hormone in particular is called ‘Auxin’. This is a growth promoting chemical, also known as IAA, you may have seen it on labels when using products for striking cuttings. Anyway, this auxin hormone is responsible for giving plants that lean-too or banana bend look. When the plant senses that it is receiving light from one direction only, the movement of auxin is initiated and all the little hormones move to the dark side of the plant. Because auxin is a growth hormone, the excess amounts on the dark side of the stem, causes that side to grow faster than the lighted or sunny side. Hence, a curvature in the stem. This can be a real problem for florists who like to work with straight stemmed flowers but keep them in a dark cool room with the door being opened frequently to expose light.
The third plant/light relationship is called photoperiod. Yes, you guessed it. It is the period of light a plant receives. Why is this important? Well it could be why your vegetables bolt to seed when you don’t want them to.
Plants are divided into three groups according to their response to the length of day.
- Long day plants (majority of vegetables)
- Short day plants
- Intermediate (or, ‘I don’t care’) day plants
The long day plants require 12 to 14 hours of uninterrupted daylight to produce flowers and set seed. These vegetables will not set fruit in the glasshouse during winter. Or if its flowers you are growing, they will not flower in the glasshouse during winter (unless you artificially light the glasshouse). Some examples of long day vegetables include: lettuce, spinach, silver beet, beetroot, potato, radish and flowers: Hibiscus syriacus, henbane and Rudbeckia bicolour.
The short day plants require long nights (or short days) to initiate flowering. The critical period seems to be 8 to 10 hours of daylight. Chrysanthemums are a good example of short day plants. They always flower around Mother’s Day in May (autumn) in the southern hemisphere, when the days are getting shorter and the nights are getting longer. To have Chrysanthemums all year round, growers place black plastic ‘curtains’ inside glasshouses to mimic an extended night period. This tricks the plants into thinking that autumn has arrived and they duly initiate flowering.
The intermediate group of plants will flower and set seed regardless of the length of day. Examples of day neutral vegetables are: peas, tomatoes and French beans.
So if you are still having problems with vegetables bolting to seed or plants not flowering, then there is another reason, for another blog post.