Category Biology

            The timing of leaf loss varies with species, site and season. Day length and temperature are the two triggers for colour change and leaf loss.

            The timing is usually species-specific but is also related to site conditions. For example, a fairly dry season would result in some trees leaves dropping early, before they had turned, in a reaction to the drought stress; leaves may also die on the tree but hang on until much later. Species variations are also important. Norway maples normally have green and fully functional leaves that keep on photosynthesizing until two or three weeks after leaves of sugar maples have turned. If both are on a cramped site, Norway maples, with extra weeks of energy storage, may outgrow and outlive sugar maples.

            Oaks keep their leaves much longer than many other species because a layer of cells that forms where the leaf stem is attached, called the abcision layer, does not form a complete barrier. In the beech trees, which are in the same family, an incomplete layer is seen in younger trees, but mature beeches, 25 to 30 years old, form a complete layer. There are also sex differences; leaves of female ginkgo trees usually colour and drop earlier than those of males. And trees near street lights may be affected by the longer light exposure and keep their leaves longer.

    The red portion in the stem of cane is due to a fungal disease called red-rot caused by the organism Glomerella tucumanensis. The organism attacks during the conidial stage (imperfect stage) when it is known as Colletotrichum falcatum.

            The pathogen infects the host mainly through the leaf scars at abscission or immediately thereafter, enters the parenchyma, grows intracellularly in the early stages, and forms an intercellular mycelium in the later stages. The fungal hyphae penetrate the host’s cell wall during the progressive stage of the disease forming minute penetration pegs. These pegs expand to the normal hyphal diameter immediately after reaching the other side of the cell wall. This mechanical pressure causes the dissolution of the tissue. Thus the tissue dissolution is not due to enzyme action, but due to mechanical pressure.

            But hydrolyzing enzymes are produced at a later stage when the tissues begin to die and the pathogen grows on the dead cells of the host, that is, in the saprophytic phase of the fungus. Only at this time reddening of the stem vascular tissue occurs followed by the formation of lysigenic cavities. At this stage when the affected canes are split open, the tissues of the internodes which are normally white or yellow-white will become red in one or more internodes usually near the base.

            The reddening is conspicuous in the vascular bundles and progresses towards the pith. When such diseased shoots appear in the field, secondary infection is caused by conidia which are produced in aierouli (asexual reproductive bodies) and transmitted through insects, wind and water.

  The bipinnate compound leaves of Mimosa pudica, touch-me-not plant, have a swollen base called pulvinus which has two distinct halves. The lower half below the vasular strand is made of thin walled parenchyma cells with larger intercellur spaces and the upper half has slightly thick walled parenchyma cells with a few small intercellur spaces.

            Under normal conditions, the cells of both the halves remain turgid. When the touch stimulus reaches the pulvinus the osmotic pressure in the lower half of pulvinus falls. As a result they release water into the intercellur space and become flaccid. But the upper half maintains turgidity the pressure excerted by which causes the leaves to drop down.

            The leaflets also have similar swollen bases but are smaller and are called pulvimules. The touch stimulus is first perceived by these pulvimules. Here also the process occurs which results in the folding of the leaflets. When the stimulus is passed on to the stalk base the entire leaf droops down.

            The touch-me-not plant shrinks within a few minutes of being touched. This is due to the loss of turgidity by cells within the pulvini-specialized motor organs at leaf joints. Upon stimulation the leaf cells lose a potassium ion which causes water to leave the cells by osmosis. It takes about 1 o minutes for the cells to regain turgidity and the leaflets to open out.

  Crotons are ornamental plants grown for their variegated leaves. The different coloured patches in these leaves are due to the presence of chromoplasts in the leaf cells. Chromoplasts contain coloured pigments, other than chlorophyll, which can reflect or transmit light, or both.

            The colour of a pigment depends on its selective absorption of certain wavelengths of light and its reflection of others. Carotenoids are a group of red, orange, and yellow pigments and contain many catalytic members. Some carotenoids act as accessory pigments in photosynthesis, transferring the light energy they absorb to chlorophyll for conversion to chemical energy.

            Chemically, pigments fall into a number of minor groups, arbitrarily divided into 2 major groups. The first group comprises pigments that contain nitrogen; it includes chlorophyll and dark coloured pigments called melanin.

            Related to melanins are the indigoids, of which the well known plant pigment indigo is an example. Riboflavin, also known as vitamin B12, is one of a number of pale yellow to green pigments produced by several plant groups.

            The second group is formed of pigments without nitrogen. Carotenoids are members of this group, as are the important plant pigments called flavonoids. In leave, flavonoids selectively admit light wavelengths that are important to photosynthesis, while blocking out UV light, which is destructive to cell nuclei and proteins.

            Bright colours are produced by the conversion of colour less flavonoids, called flavonols, into coloured forms, called anthocyanins. Quinones provide many yellow, red and orange pigments.

            Venus fly-trap, an insectivorous plant, normally grows in swamps and moist soils characterized by lack of sufficient nitrogen (as nitrates). Their root system is also not so well developed. As a result these plants tend to trap insects and ‘digest’ them to augment their nitrogen supply. These carnivorous plants do not have any special mechanisms or honey secretions to attract insects but only modified leaf traps (Dionaea muscipula), vase-like leaves (Nepenthes Khasiana), leaf hairs having glue on their tips (Drosera) and leaf surface having a sticky coating (Pinguicola alpina) to trap them. In Venus fly-trap plant, the two halves of the leaf blades can swing upward and inward as though hinged.

            Inside the hinged portion of each leaf are several long trigger hairs. As the insect walks along the leaf surface and touches these hairs, it stimulates a hydraulic response in the leaf-cells and makes them lose water rapidly. This causes the leaves to close. Long projections along the leaf margins help in trapping the insect.

            Once an insect is trapped, digestive enzymes are secreted by the hairs which ingest the insect and absorb the contents. After a meal, the trap opens again only after several days. Generally each modified leaf is used to trap only 3-4 insects before it falls.

            These plants also have chlorophyll by which they can photosynthesis to cater to their energy (food) requirements. Hence these plants are not obligatory carnivorous forms. But they can grow exuberantly to produce flowers and seeds, if insects are available, as they supplement their nitrogen supply.

           Plant tissue culture (PTC) is the art and science of multiplying plants or plant parts (such as organs, tissues, cells, pollens, sores and embryos) under controlled conditions of light, temperature and humidity in an optimal nutrient medium under aseptic conditions in a glass vessel.

            Even a single cell has the potency to perform all the metabolic activities to form an independent plant. This phenomenon is known as totipotency. This is successfully used in tissue culture. Infinite number of plants can be produced from single explants in a span of time, irrespective of natural conditions. The seedlings will be genetically ‘true copies’ of the mother plant. That means, genetic purity can be maintained as far as required, in every seeding, which is almost impossible in conventional means of propagation.

            PTC has been successfully tried in almost all plant varieties. Some plants like orchids produce millions of non-endosperm us seeds in a single fruit, which cannot be cultivated in natural conditions. When grown through PTC, they have more than 75 percent germination. In the medium in which the plants are cultivated all the required micro and macro nutrients and vitamins are added. Since the growth inside the glass vessel is heterophic, a carbon source in the form of sucrose is added to the medium. PTC has been commercialized and is a lucrative business.

            In PTC, a very small tissue from a parent plant called as explants is placed in a test tube in a nutrient medium. The tissue may be taken from any part of the plant, that is, root, stem, leaf, anther or embryo. This is because all plant cells possess totipotency meaning a single cell can give rise to an entire plant.

            The nutrient medium used in tissue culture consists of sucrose apart from mineral salts and vitamins. Plant hormones such as Auxins are used to help growth and cell division. The solidifying agent, agar makes the medium semi solid otherwise the culture is done suspension. The inoculated tubes are kept in an incubator to maintain sterile conditions and controlled temperature and light. After 2-3 weeks of incubation an irregular mass of cells called callus develops, which on sub culturing gives rise to small plantlets. These are potted and maintained in a green house and subsequently transferred to the field. PTC is aimed at engineering crop plants for good traits.