Life's a Gas - if you're a plant
A bigger harvest from shorter plants (board 1).
Life on Earth is sustained by photosynthesis - plants’ ability to trap the energy in sunlight and use it to power the conversion of water and carbon dioxide into oxygen and sugar. Annually photosynthesis produces 100,000 million tons of plant material.
Photosynthesis is a very efficient process and making it more efficient is
a difficult challenge. An alternative is to change the way that the products
of photosynthesis are distributed around the plant, so that a greater proportion
are stored in parts of the plant we harvest. Reducing the height of plants
(dwarfing) is one way to increase their yield, as less energy is put into
producing stems and leaves and more into making grain
Making plants work harder (board 2).
Some plants, maize for example, use carbon dioxide and water more efficiently than other plants, for example wheat and oilseed rape. Understanding the difference between these so-called C4 and C3 plants is important as more efficient use of carbon dioxide and water can increase crop yields.
Differences in the anatomy of the leaf and the metabolism of the leaf cells
accounts for the greater efficiency of C4 plants. The plant family Moricandia
includes some species that are C3, and others that are intermediate between
C3 and C4 plants. Studying these plants is giving us clues as to how the C4
characteristics are controlled, as the first step towards developing C4 versions
of common crop plants that may be higher yielding and require less water.
Fertiliser from fresh air (board 3).
Nitrogen is an important plant nutrient and supplying nitrogen is one of the main reasons for fertilising crops. 70% of the atmosphere is nitrogen gas, but it must be ‘fixed’ – converted into ammonia or nitrate salts – before plants can use it. The only biological process for fixing nitrogen is carried out by a few microbes; plants are unable to do it themselves.
Legumes (peas and beans) have evolved the ability to create an intimate symbiotic relationship with soil bacteria called rhizobia. The bacteria invade the legume’s roots, causing small nodules, where they are able to ‘fix’ nitrogen gas and supply it, in a useable form, to the plant with which they are associated.
Making all plants make their own fertiliser (board 4).
To be able to make any plant ‘fix’ its own nitrogen (see ‘Fertiliser from fresh air’) has been a dream of scientists for many years, but the plant-rhizobia relationship is a complex one.
Recent research suggests that achieving the dream may not be so difficult as we imagined. Many plants form intimate relationships with ‘mycorrhizal’ fungi, where the fungus helps the plant take up nutrients from the soil. Some of the genes controlling these common mycorrhizal associations are also needed for the much less common rhizobia association. Thus much of the genetic instruction book, needed to build the nitrogen-fixing relationship, may already be present in a wide range of plants.
The poison is in the dose (board 5).
Nitric oxide is a toxic gas that quickly damages living tissues, yet it is important in controlling plant growth, development, and defence. Controlled production of nitric oxide is an essential step in switching on the expansion of cells that underlies the growth of plant shoots, roots, leaves and buds. It also has a key role in switching on a variety of plant defence systems in response to physical damage or insect and pathogen attack.
Nitric oxide is a by-product of normal metabolism and has to be ‘mopped up’ before it accumulates to a level that will damage cells. This is done by molecules called antioxidants. Plants are a rich source of these protective chemicals, which through our diet help protect us against the effects of nitric oxide and other damaging natural chemicals produced by our metabolism.
Plant talk? (board 6)
The sweet smell of new mown grass is part of the alarm and defence system of the wounded leaves. Many plants (including alders, grasses and tomatoes) produce volatile chemicals when they are damaged. In some cases these signals spread through the plant, activating the plant’s defences, but there is increasing evidence that some are also carried through the air and stimulate the defence systems of nearby plants.
The odourless gas ethylene is used by plants as a hormone to control their development. Among its many effects ethylene stimulates growth of dormant potato buds, germination of grain, and aging of flowers and leaves. Its action is most easily seen in the ripening of some fruits, for example tomatoes and bananas, where the maturing fruits produce ethylene, which stimulates the ripening process.