Drought conditions, low and high temperatures, increased salt levels, pathogens and insects are common environmental stresses which plants are exposed to. Over time, plants have evolved unique mechanisms to respond to the stresses
Drought conditions, low and high temperatures, increased salt levels, pathogens and insects are common environmental stresses which plants are exposed to. Over time, plants have evolved unique mechanisms to respond to the stresses imposed in them. In response to pathogen infection, plants have two types of defence; constitutive and induced responses. Constitutive defences are those which are passive and always present in the plant. These defences are those such as the cuticle which is composed of waxes, cutin and pectin and the cell wall, composed of cellulose, pectin and lignin, both of which act as structural barriers against pathogens. There are preformed pathogen inhibitors such as saponins which disrupts pathogen cell membranes. For example, ?-tomatine in tomatoes acts against many fungal species. Other preformed inhibitors include alkaloids, phenols and tannins. Defence proteins are also present; lectins bind sugar groups such as chitin, which in fungal species binds to chitin in fungal walls and slows sungal growth. RIPs or ribosome inactivating proteins disrups viral replication by deactivating the plants ribosomes, therefore preventing cell replication and therefore also the viral pathogens replication. Hydrolases are present which break down the components of the pathogens and enzyme inhibitors inhibit enzymes which are used by the pathogen in the infection
Heterosporous Plants
Heterosporous
How plants have adapted or become acclimated to shade.
How plants have adapted or become acclimated to shade. Plants which grow in shady conditions have altered structurally and biochemically in order to cope with the low levels of light and harvest it efficiently. Low level light is rich in far-red light and plants have also adapted to make use of this. Shade plants have also had to make adjustments to cope with sudden high levels of light, for example during sunflecks. Whilst some plants show physiological adaptation, many changes are the result of genetic evolution. * Leaves are thinner with a shallow layer of palisade mesophyll cells and a patchy spongy mesophyll with air spaces (fig 1.). This uses less energy and resources to construct. Many shade plants (such as ferns) do not produce flowers for the same reason. * Shade leaves have more chlorophyll in the antenna systems to feed more energy to the reaction centres. There is also an increase in the number of reaction centres. * Shade light contains more light in the far-red range. The reaction centre of photosystem II (PSII) absorbs more light from the far-red range than photosystem I (PSI); hence there is a greater proportion of PSII to PSI. This is achieved by the presence of wide grana with larger numbers of stacked thylakoids (figure 2.). This enables the photosystems to be excited equally in far-red light. * Plants which grow in shade are often exposed to periods
