ENVIRONMENTAL FACTORS AFFECTING PLANT PROPAGATION MUSHAYABASA T.

INTRODUCTION In order to enhance propagation of plants there is need to manipulate or modify the environment of the propagules (cuttings or seeds) by managing the following: Microclimatic conditions: any environmental factor in the immediate vicinity of the propagule during propagation e.g relative humidity, T oC, light, gases (O2 and CO2). Edaphic/abiotic factors -mineral nutrients, propagation media and water. Biotic factors - Biological organisms that interfere with plant production e.g. bacteria, viruses, fungi, insects/mites, nematodes, weeds, birds/mammals

MICROCLIMATIC factors Water-Humidity control Water management and humidity control are critical for any propagation procedure. Water management can be used in regulation of plant growth Evaporative cooling when using intermittent misting can help control the propagation house microenvironment and reduce the heat load on propagules. This also permits increase in light intensity for higher rates of photosynthesis.

WATER-HUMIDITY control While leaf water potential (leaf) is an important parameter for measuring water status of seedlings and cuttings, and influences the rooting of cuttings, turgor pressure (p) is physiologically more important as it tells much on the turgidity of plant cells. The water status of a seedling or cutting is a balance between transpirational losses and the uptake of water.

TEMPERATURE Affects propagation in several ways. Seed dormancy is broken in some woody species by cool-moist stratification conditions that allow germination to occur. In grafting heating devices are sometimes placed at the grafting union area to speed up the healing process or union formation, while the rest of the rootstock is kept dormant.

LIGHT It is important for photosynthesis as a source of radiant energy and it also generates a heat load. All light is made up of energy. It has 3 major components which are: -light intensity (light given off by a point source) -light duration or photoperiod (response to duration and timing of day and night, a mechanism evolved by organism for measuring seasonal time). - Light quality (radiant energy capable of causing photosynthesis and measured as photosynthetic photon flux (PPF))

LIGHT Light can be manipulated by controlling irradiance (the relative amount of light as measured by radiant energy per unit area) as well as light duration (day length or photoperiod) and light quality (wavelength) Light management is critical for the: rooting of cuttings, germinating seeds, growing of seedlings, shoot multiplication of explants during tissue culture propagation.

IRRADIANCE The relative amount of light as measured by radiant energy per unit area (energy content) Plant propagators are still measuring light intensity, while determining the photon flux is more accurate. This is due to the fact that photosynthesis depends on the number of photons accepted (photosynthetic photon flux) and not just the light given off by a point source (intensity).

DAY LENGTH Plants are classified as long-day, short-day or day-neutral, according to the effect of photoperiod on initiation of reproductive growth. Long-day plants: these are plants whose physiological response to flowering or reproductive growth is evoked or triggered when critical photoperiod is equalled or exceeded. For example spinach.

DAY LENGTH Short-day plants: these are plants whose physiological response to flowering or reproductive growth is evoked when the critical photoperiod is not exceeded e.g. chrysanthemums. Day-neutral (self inductive): these are plants that do not respond to variation in photoperiod e.g. roses.

DAY LENGTH Reproductive growth is not triggered by photoperiod. NB: Garner and Allard who discovered photoperiodism demonstrated that the dark period and not the light period is most critical to initiation of reproductive growth even light cycles are used to denote a plant’s photoperiod.

DAY LENGTH Photoperiod is normally extended under short day conditions. Supplemental lighting or ways of extending short days (winter) include lighting incandescent lights or high intensity discharge lights (HID). Under long photoperiods (summer) photoperiod can be shortened by shading i.e.: covering the stock plants and cuttings with black cloth or plastic which eliminates light.

Light quality (Wavelength) Perceived by the eye as colour but usually tallies to a certain range of wavelength (cm).   10-14 10-12 10-10 10-8 10-6 10-4 10-2 100 106 --X-rays------- ---Infra-red--------- Cosmic –Gamma---- --UV-- --solar rays- ----------Radio Waves---- Red light induces germination in lettuce cultivars while far red light inhibits germination.

Light quality (Wavelength) – cont. Far red light can promote bulb formation in long day plants such as Allium cepa Greenhouse cladding materials used can have different spectral quality characteristics hence affecting propagation

GASES AND GAS EXCHANGE For adventitious root formation, germination of seeds and plug development, oxygen must be available as high respiration rates occur. These aerobic respiration processes requires oxygen and releases carbon dioxide by the propagules. Gas exchange must be promoted at the site of root initiation/seed germination.

GASES AND GAS EXCHANGE Impervious seed coats reduce germination in seeds due to failure of gases to penetrate into the seed. During propagation in enclosed propagation structures eg. Greenhouses, ambient carbon dioxide levels can drop to below optimal levels thereby reducing the photosynthetic rates. Increasing CO2 can result in a 200% increase in photosynthesis and therefore increases dry weight.

GASES AND GAS EXCHANGE (cont.) CO2 + H20 + light energy = CHO + O2 + H2O Ethylene gas (C2H4) can build up in the greenhouse, or in storage of propagules thereby resulting in deleterious conditions.  

MINERAL NUTRITION Optimal nutrition is required for stock plants to avoid stress. Nutrients are generally applied to seedlings and plugs by fertigation (fertilisers added to irrigation water). Cuttings are normally fertilised with slow-release fertilisers like composts and manure. Important nutrients include P for good root establishment.

BIOTIC FACTORS One of the goals of plant propagation is to keep stock plants and propagules as clean and pest-free as possible and to suppress pathogenic fungi, viruses, nematodes and weed seed from the propagation media.