Sunlight and Plants

One of the most profound mistakes we can make as gardeners is to overrate our importance and ability to control our garden.

Sunlight and Plants

Bill Bampton talks about the source of life's energy and its impact in the garden.

We have a vision of how we want our garden to be: bountiful, lush, and blossoming. We read garden books, learn a list of rules, symptoms, cures and pests, we visit the garden centre and return with the appropriate pesticides and chemical fertilisers as recommended by the expert or product label.

Yet, still we get those spots on the rose leaf. We are on an endless cycle using more and more inputs and precious water and still our imagined garden eludes us. The truth is the inhabitants of our gardens, the plants, soil, birds and animals are oblivious to our aspiration; they are too busy getting on with their own lives.

A truly successful organic garden is “a place where we can meet nature halfway” (Michael Pollan). We can do this by improving our understanding of the lives of the plants we tend and the environment they grow in.

This does not mean we need a degree in ecology, it does mean we need to know about the areas where our desires as gardeners to feed ourselves and to create beauty meet with the drives and desires of the plants we tend.

We need to go beyond rote learnt rules and recipes and look at the underlying biology of gardening. This is the first of a series of articles looking at some of the basic “hows” and “whys” of gardening, beginning with the very basics of how plants feed themselves.

Photosynthesis is the key to all lie on earth 

Photosynthesis is the engine of the food we eat and the oxygen we breathe. Yet it is only partially understood and while the process itself has been around since the beginning of life, the awkward word used to describe it has only been around since 1893.

I think we have all been exposed to a lesson on photosynthesis dense with terminology, equations and something called the Calvin Cycle. As gardeners we need not get bogged down in the intricacies of the process, but we do need to understand some of its implications for plant growth. Indeed the majority of our successes and failures with plants can relate to how photosynthesis has either been inhibited or promoted.

Photosynthesis is the process by which plants harness the energy of light to create sugars from water and carbon dioxide, producing oxygen as a by-product. This takes place in the chlorophyll found in the green parts of plants.
Plants absorb water through their roots and carbon dioxide through the pores (or stomata) in their leaves.

Leaf structure facilitates the absorption of gases from the atmosphere, they are permeable and their shape maximises surface area.

Reactions in the chlorophyll convert light energy into chemical energy in the form of sugars. The sugars produced are either used in respiration or stored as starch. This storage of excess for the future is why we should avoid the temptation to cut off foliage from bulbs once flowering is past; they are still producing food for next year's flowering.

These sugars are building blocks for many other plant compounds such as: cellulose, lignin, proteins, vitamins, hormones and enzymes.

Sugars travel through the plant concentrating in shoot and root tips, here growth is fastest. Vegetable gardeners should take note: shoots, tips and fast growth are sweet to eat. For the gardener the most important thing to understand about photosynthesis is its limiting factors.

The range of light intensity for optimum photosynthesis for a plant usually relates to the microclimate in which it evolved. Plants from prairie or desert conditions function and flower best in full sun, plants of the forest floor suiting shade or indoor situations.

This relates to the position we place a plant: Clivia robusta from the forested swamps of Pondoland, South Africa in full shade, Salvia chamaedryoides from the Sierra Madre Oriental Deserts of Mexico, full sun. The Clivia has large strappy green leaves maximising access to limited light. The Salvia has small grey leaves; it is more concerned with sunburn and water loss than maximising light for photosynthesis.

If plants are exposed to light levels beyond their level of tolerance cells are damaged, “burnt” by high levels of ultra violet light.

Photosynthesis can only occur in a temperature range from 0 to about 50 degrees Celsius. Again plants range of tolerances varies according to their origins. Lowland tropical plants obviously have higher temperature requirements than alpine temperate plants. A general optimum temperature is between 20 and 30 degrees Celsius.

As we continue to get more days of extreme temperature in the upper thirties and forties we will experience more plants, especially those evolved in cool temperate zones, displaying heat stress. Gardeners often mistake this for lack of water. For example, many plants from the cool temperate forests of New Zealand survived years of drought conditions in Australia but succumbed dramatically to days of extended heat in the high 30s. In the garden, temperature is one of the limiting factors hardest to control except through plant selection. Just as plants respond to fluctuations in light so too many plants respond to changes in temperature – unsurprisingly, this is called thermoperiodism. This can often be linked to day length. The classic example of this is the chill requirements of fruits like cherries or the onset of autumn leaf colour. While only a small amount of water is required for the actual reactions of photosynthesis, the limiting effect of water comes into play as stomata close to conserve water loss and so limit the entry of carbon dioxide. Water is also vital in maintaining the transportation of essential minerals from roots to leaves.

C4 Plants absorb CO2 in the dark

Some plants use additional photosynthetic pathways to the general form of photosynthesis (C3), these are CAM and C4 plants adapted to drought and high temperatures. CAM plants are able to absorb carbon dioxide at night with their stomata closed during the heat of the day. C4 plants are “faster” at absorbing carbon dioxide allowing them to out-compete other plants when conditions of temperature and water are limiting.

As gardeners we can use this knowledge to guide our plant selection. A good example is when choosing turf grasses for hot dry conditions. Throughout Australia we have seen the replacement of drought sensitive C3 grasses like ryegrass with heat and drought tolerant C4 grasses like kikuyu and buffalo or native kangaroo grass. Those that believe in the powers of GM are currently involved in the C4 rice project that seeks to transform rice, naturally a C3 grass, into a warm season C4 crop.

There are very low levels of carbon dioxide in the atmosphere; about 0.035%. Increasing carbon dioxide can increase the rate of photosynthesis. In the vast glasshouses of industrial tomato production not only temperature and light are controlled, even carbon dioxide levels are raised to 0.1 -0.15%. In the home garden the only lift in carbon dioxide is likely to come as a result of human induced climate change.

By going back to some of the basic requirements of plants I hope to encourage you to look at your garden and its plants — are you fulfilling their needs? Look at where the light falls in your garden, you can use the light meter on your camera if it helps. Then look at the type of plants you are growing there, are their light requirements met? Do a bit of research. What microclimate did they evolve in? Look at the timing of your vegetable plantings; did I plant my garlic late? Is this why there is still only one clove?!

As I am writing I can see a sunburnt Clivia, I’d better go and move it.