PLANTING A POISON FROG TERRARIUM

When people first see my vivaria I am usually asked why my plants grow so well. The answer is simple, that you need to understand plant requirements for healthy growth. Many factors affect the biological processes that plants undergo to achieve their maximum potential of Photosynthesis, which is the process by which a green plant manufactures food in the form of carbohydrates such as sugars and starch, using light as energy. Requirements for photosynthesis:

1) For a plant to build up organic compounds it must have a ready supply of carbon dioxide which is present in our atmosphere in concentrations of 330 ppm (parts per million) or 0.03% and can diffuse readily through the stomata in the leaves of plants. CO2 gas moves ten thousand times faster than it would in solution through the roots (which is why carbon rich fertilisers tend to act slowly). The amount of CO2 can drop significantly when planting is dense in the terrarium, or when plants have been forced in a sealed enclosure. This can slow the rate at which the plant develops, but it can be increased artificially by burning propane gas, or releasing pure CO2 to produce an enriched atmosphere up to about 3.4 times normal levels. Such methods produce an increase in growth, provided other constraints do not apply and other factors must be increased proportionately. If any factor is in short supply, then the process will slow down. This principle, the law of limiting factors, sates that the factor in least supply will limit the rate of process, and applies to other non-photosynthetic processes in plants.

2) Light is required for photosynthesis to occur. Energy for photosynthesis is provided by light from the sun or from artificial lamps. The amount of light energy present will directly affect the rate of photosynthesis- the more light absorbed by the plant, the more photosynthesis can take place. Light energy is measured in joules/square metre, but for practical purposes I will express it as the light falling onto a given area, in lumens per square metre (lux). It is difficult to say how much light a specific plant requires, as variation comes with species, CO2 supply, temperature, age, nutrient supply and the general health of the plant. However, it is possible to suggest approximate limits within which photosynthesis will occur; a minimum intensity of 500-1000 lux will enable a plant to keep pace with its own respiration, and thus maintain itself. The maximum light many plants can absorb is approximately 30,000 lux, while good growth in many plants will occur at 10,000-15,000 lux. With these figures in mind we can look at the average daytime light intensity on the rainforest floor of only 1,200-1,600 lux, so it is also possible to over light many tropical plant species. In a vivarium we usually use total replacement lighting, the use of an artificial source instead of daylight. Many different types of lighting can be used to enhance photosynthesis but I will concentrate on fluorescent tubes. These contain mercury vapour, which glows when electrically charged and emits light, mainly in the ultraviolet range. This invisible light is transformed into visible light as it strikes a coating of fluorescent material in the tube.

3) In order that the chemical reactions can occur to create carbohydrates, the plant requires the presence of enzymes that accelerate the rate of reactions. These are temperature dependant with activity increasing with temperature from OșC-36șC and ceasing at 40șC. The air temperature and the growing media temperature have a direct affect on the rate of photosynthesis, with a species dependant optimum in the range of 25-36șC; but at very low light levels there will also be a marked reduction in photosynthesis. This means that an increase in temperature will be wasted if light is limited. The benefits of lower nighttime temperature are documented in many species, e.g. Tradescantia.

4) Water is required during photosynthesis but this represents only a small proportion of the total water uptake of a plant. Water supply through the xylem is essential to retain good leaf formation, to retain fully open stomata to allow sufficient CO2 to pass into the leaf. If there is a shortage of CO2 a 10% loss of water may lead to a 50% decrease in photosynthesis.

5) Minerals are required by the leaf in order to produce the chlorophyll pigment that absorbs most of the light energy for photosynthesis. The production of chlorophyll must be continuous, as it loses efficiency quickly. A plant deficient in iron or magnesium will quickly turn yellow and lose most of its photosynthetic ability. This is known as iron or magnesium chlorosis. Variegated plants have a slower rate of photosynthesis.

6) The chloroplast is the sub cellular unit for photosynthesis that uses the raw components described. The absorption of light by the chlorophyll occurs at one site and the energy is transported to another site where it is used to form carbohydrates, usually in the form of insoluble starch.

7) The leaf is the main organ of photosynthesis, and its structure is maximised to greatly improve its efficiency. A newly expanded leaf is most efficient at the absorption of light and its ability decreases with age.

8) Pollutant gases can cause damage to plants, often resulting in scorching of the leaves that may be wrongly associated with low humidity. Fluoride, present in high levels in tap water, can accumulate in the composts of vivaria causing marginal and tip scorching in susceptible species such as Dracaena.
Neil Sharp Abbots Langley, Herts