The influence of physical factors on growth and development

Growth conditions Most important environmental conditions in a tissue culture growth room are: Light Temperature Humidity Oxygen

Light For plants, it is measured in PAR (photosynthetically active radiation) - the number of photons per square meter per second for the spectral range from nm.

Light PAR ( 光合作用有效光 ): 在 nm 的光線與光合作用有很密切的關係 ，此光帶的光稱為光合作用有效光 (PAR) 。 作物的生長速率直接受光照射的影響，波長在 nm 的紫外光與 nm 的近紅外線（遠 紅光）會影響作物的生化反應及其外觀。

Light Photosynthetic Photon Flux Density (PPFD): 光合作用光子通量密度。

Light PAR is a more applicable measurement than other types such as lux because it measures the light spectrum that plant can use for photosynthesis. The amount of light needed for tissue culture explants is much lower compared to plants in grown in the field. Generally, in tissue culture growth rooms,  mol.m -2 s -1 PAR is used.

Light High levels of light will damage the explants. Photosynthesis does not usually take place in tissue culture plants. In cases where there is photosynthesis, it is very limited mainly due to the physiology and anatomy of explants.

Light requirements of plants The growth and development of plants is dependent on light for: 1. Photosynthesis The process whereby light energy is converted to chemical energy in the biosynthesis of chemicals from carbon dioxide and water.

Light requirements of plants 2. Photomorphogenesis The light-induced development of structure or form. It does not necessarily involve the absorption of light energy, it uses receptor which act as switches to set in motion the morphogenetic processes of plants.

The influence of light on tissue culture 3 qualities of light which most clearly influence in vitro growth and morphogenesis. Wavelength Photoperiod Light intensity

Wavelength Plants absorb blue and red lights, which have the greatest effect on plant growth. Red light: Photosynthesis Seed germination, seedling growth Flowering, fruit development Blue light: Photosynthesis Vegetative growth (leaf) growth

Wavelength Growth rooms are mostly equipped with cool white fluorescent tubes. Gro-lux tubes are specifically made to plants, but are more expensive. Traditional incandescent light bulbs are not appropriate, since most of the light emitted is heat.

Light Limited reports on the effect of light on explant growth is available due to the complex relationship between light and plant. The numerous factors that will affect plant response to light are: Plant species Type of explant (leaf, stem, root, etc) Type of development of the explant (embryo, callus, meristem, etc.) Often conflicting results are reported with different plants exposed to similar light conditions.

Blue light Blue light (420nm) at a lower intensity stimulates callus and shoot development in tobacco explants. While at higher intensities, it inhibits callus growth and cell division. The intensity of blue light has the biggest effect on stimulation and inhibition of callus growth.

Red light In general, red light (660 nm) promotes adventitious shoot formation in most plants. In addition, red light stimulates adventitious root growth in sunflowers and tobacco more than blue light. While red light inhibits organogenesis in tobacco.

Photoperiod Photoperiod is the length of time a plant receives light in 24 hours. Photoperiod influences plants in 2 ways: 1.Growth of plant is proportional to the length of time that they are exposed to light. High light (summer) = more growth Low light (winter) = less growth 1.Plants are able to sense changes in the photoperiod and respond accordingly. In nature, photoperiod affects flowering and morphogenesis.

Photoperiod Photoperiod of tissue culture growth rooms is dependent on the type of explant cultured. When uncertain, the photoperiod of plants in nature are used in vitro. Most explants grow well with hours of light. For specific purposes, complete darkness is used (eg., seed germination).

Temperature In vivo photosynthesis: Increases with temperature up to a point. Although photosynthesis is low in tissue cultured explants, optimum temperature is still required for growth. In vivo respiration: Rapidly increases with temperature Sugar, starch and oxygen is converted to CO 2 and energy.

Temperature The temperature of a growth room is usually kept constant at  C. Sometimes in experiments, depending on the origins of the explants, lower temperature (18  C) for bulbous species, or higher temperature (28  C) for tropical species is chosen. The temperature within the test tubes is 3-4  C higher than the growth room due to irradiance.

Temperature Sometimes alternating temperature conditions may be needed. This is particularly evident in seed germination. Common alternating temperature regimes include a 26  C daytime temperature, and 15  C night temperature. For example, callus tissue of carrots grows best under a day temperature of 26  C and night temperature of 20  C.

Humidity Little is known about the influence of growth room humidity and in vitro growth. However, a growth room with high humidity increases the chances of contamination. Humidity is usually very high in the test tubes ( %). This causes stomatal malfunction and hyperhydricity. (This will be discussed in another section)

Oxygen Oxygen availability is important for in vitro root formation, as is the case in vivo. For woody plants, it is extremely difficult to regenerate roots when they are in solid medium. Root formation is much better in a liquid medium. In this case, the explant is supported on a paper- bridge or on rafts.

Growth room Most commonly used conditions: Temperature: 24ºC - 28ºC Photoperiod: 16 – 24 hours Lighting: 60 – 70 μmol m -2 s -1