HESTA HATTING, SEED TESTING LABORATORY, ARC-SMALL GRAIN INSTITUTE
Plant seed act as both reproductive structures as well as a vital nutritional resource for human consumption, worldwide. Bread flour is derived directly from wheat, Triticum aesitivum. The seed of wheat is botanically referred to as a caryopsis.
Unlike many other plant species where vegetative reproduction is possible, reproduction in wheat is restricted to seed, following a process of self-pollination between the male anthers and female stigma. The resulting seed is then harvested for processing into nutritional substrate or used for propagation purposes.
This article explores the most important factors affecting germination which is the first and most critical process during propagation by means of seed. From a producer’s perspective, successful seed germination and subsequent seedling establishment is seen as a first step towards economically feasible wheat production.
Although initial input costs are linked to the acquisition of genetically sound seed, several other factors may also affect the process of germination. This process is complex and can be affected at different stages by many interacting factors such as temperature, water availability, oxygen, light, substrate, maturity of the seed and physiological age of the seed.
Environmental factors affecting seed in the soil
The most important external factors include water, oxygen, suitable temperature, and sometimes light or darkness. Various plants require different variables for successful seed germination. Often this depends on the individual seed variety and is closely linked to the ecological conditions of a plant's natural habitat.
Water constitutes a basic requirement for germination. Mature seed are often extremely dry and need to absorb, through a process of imbibition, a significant quantity of water, relative to the dry weight of the seed. Generally, the minimum water content required in the grain for wheat germination is 35% to 45% by weight.
However, seed may germinate in soil with low moisture content and the initial stages may even proceed, but such conditions are usually not conducive in allowing the seed to perform at its full genetic potential. Conversely, germination is generally impeded by excess moisture mainly due to a restriction of oxygen availability. When seed imbibes water, enzymes are activated which break down stored food reserves in the seed into metabolically useful chemicals.
Shortly after seedling emergence the seedling's food reserves are typically exhausted and photosynthesis provides the energy needed for continued growth. At this point the seedling requires continuous supply of water, nutrients and light.
Air is composed of around 20% oxygen, 0,03% carbon dioxide and 80% nitrogen, and the seed of most plant species germinate well in an environment providing this mixture of gases. Oxygen is required by the germinating seed for aerobic respiration, the main source of the seedling's energy until it grows leaves, which will enable photosynthesis.
Seeds planted in an oxygen-deprived environment, such as a waterlogged or tightly compacted soil, may germinate very poorly or fail to germinate altogether. A delay in germination due to unfavourable environmental conditions is termed seed quiescence and is not to be confused with seed dormancy, briefly discussed below.
Soil temperature plays a significant role in the rate at which germination proceeds. Although germination may occur between 4°C and 37°C, optimal temperatures range from 12°C to 25°C. The rate of water absorption or imbibition, the diffusion of respiratory gases and the rate of chemical reactions involved in the metabolism of the seed are all affected by temperature.
Species-specific seed often have a temperature range within which it will germinate, and it will not do so above or below this range. Suboptimal temperatures lead to lower success rates and longer germination periods. Higher temperatures will, up to certain limits, increase the rate of germination. Once the limit is reached, further increases in temperature will reduce or prevent germination. High temperatures reduce enzyme efficiency and eventually a temperature is reached at which cellular protein is denatured and the seed is killed.
Winter wheat requires exposure to cold temperatures to enable flowering. This process is termed vernalisation (from the Latin: vernus, of the spring). This is the acquisition of a plant’s ability to flower in the spring by exposure to the prolonged cold of winter. Winter annuals are responsive to vernalising temperatures at all stages of development, including imbibed seeds (i.e. seed vernalisation).
Factors affecting the seed quality of pre- and post-harvest seed
Weather experienced during seed development
Precipitation directly prior harvesting can cause preharvest sprouting of the seed (Photo 1) and may also favour the development and spread of microflora, which can discolour the seed, affect normal germination or even kill the seed. Common organisms include fungi in the genera Cladosporium and Alternaria. Also, damage to embryonic structures can be caused by alternating wet and dry events and this can lead to seedlings of the Poaceae developing split coleoptiles (Photo 2).
Harvesting and storage of immature seeds with high moisture content may promote the growth and development of storage microflora within the seed lot. The subsequent microbial activity then increases the temperature, thus affecting both mature (normal) and immature seed. As an indirect result, problems with germination can be expected when such seed is planted.
Mechanical damage can be problematic, especially in dry years, when the embryo is exposed and thus vulnerable to physical damage. Mechanical damage due to harvest, handling and other processes, is an important factor affecting the general quality of seed.
Damage usually occurs at harvesting and processing following a dry season when low moisture content renders seed brittle and susceptible to cracking/breaking during combining. Although broken seed is easily removed, damage to the embryo is not detected until the seed has germinated. Mechanical damage can also be caused during cleaning, dressing with chemical seed treatments, bagging and transportation of seed.
Artificial drying of harvested wheat seed is necessary if the moisture content is too high. Ideally, this level should be around 12%. The seed must be artificially dried to prevent the spontaneous build-up of microflora, insects and/or mites. However, heat damage or desiccation may occur when excessive heat is used for drying (Photo 3).
Heat damage causes slower germination, delayed emergence of the primary leaf, stunted growth or termination of the germination process. In severe cases, seed death may occur. During bulk storage, areas of excessive moisture can lead to microbial-induced “hot spots” and since moisture moves from hot to cooler areas, further local heating is caused, setting off a chain reaction.
It is common practice to treat seed with fungicides, and in some cases insecticides, for protection of the seed or germinating seedlings against soil-borne fungi and insects. Symptoms of phytotoxicity may develop and germination can be reduced if the seed is treated with the incorrect chemical/s, excessive doses of the chemical, the seed moisture content is above the recommended level, seed has sprouted or the seed is mechanically damaged (Photo 4).
Insects and mites
Germination may be affected following insect or mite damage to the seed prior to harvesting (e.g. bollworm, Helicoverpa armigera) and/or during storage (e.g. grain borers, Rhyzopertha dominica; weevils, Sitophilus spp.; or grain beetles, Oryzaephilus spp.). Apart from direct physical damage, insects also contribute to an increase in temperature and humidity, thus facilitating microbial contamination as mentioned above (Photo 5 and 6).
Apart from microflora causing problems noted above, certain diseases are seed-borne and may have a detrimental effect on germination. These diseases include fungi such as the bunts and smuts (Tilletia spp.), head blight (Fusarium graminearum) and several species of Alternaria and Cladosporium. Seed-borne bacteria such as Xanthomonas spp. may also affect germination (Photo 7 and 8).
Seed has, like all other organisms, a finite longevity. The ageing of seed depends on a number of different factors including storage conditions. Severe conditions during storage, e.g. high temperature and high humidity lead to rapid seed aging thus affecting germination capacity.
Ideally, seed should be stored around 10°C, but the availability of such facilities for bulk storage is limited. Generally, wheat seed with a moisture content of 12%, stored at 20°C for a period of 360 days, will retain about 92% germination. However, seed with a moisture content of only three percentage points higher, will result in a mere 27% germination.
Seed dormancy refers to a condition that prevents germination even though the seed experience optimal environmental conditions suitable for germination. In nature, seed dormancy is an important regulator, enabling/supporting seasonal synchrony, the widening of the range for germination, utilisation of erratic opportunities and exploitation of other organisms to facilitate seed dispersal.
Dormancy is thus a form of environmental adaptation aiding both plant survival and propagation. However, within an agricultural setting, dormancy among planted seeds may lead to staggered germination resulting in an erratic stand and/or plant-age distribution. Seed with low dormancy levels may also be prone to pre-harvest sprouting thus affecting falling number, an important quality parameter of wheat.
The use of certified seed of a registered cultivar will aid the producer in limiting potential problems associated with poor and/or inconsistent germination.
Producers strive to achieve as high a yield as possible from seed sown. One of the most important requirements in achieving this is high quality seed.
As seen above, seed quality can be adversely affected by a large number of factors. The ARCSmall Grain Institute’s seed testing laboratory has been registered with the Department of Agriculture, Forestry and Fisheries (Directorate Plant Production) since 1996 and delivers a service to the local small grain industry. All tests carried out by this laboratory are conducted according to International Seed Testing Association (ISTA) rules to ensure that international quality standards are maintained.
Important information for small grain samples sent to the ARC-SGI for testing:
1 kg seed of each sample (note: only small grains tested, i.e. wheat, oats and barley)
Indicate the year seed was harvested
Full contact information of the client
For further information, please contact Hesta Hatting at the ARC-SGI on (058) 307-3417 or email firstname.lastname@example.org.
Acevedo, E., Silva, P. & Silva, H. 2002. Wheat growth and physiology. In Curtis, B.C., Rajaram, S. & Gómez, H. Macpherson [Eds.], Bread Wheat: Improvement and Production. FAO Plant Production and Protection Series No. 30. Caddick, L., 2002. Early harvest and cool storage maintain seed vigor. Farming ahead no. 130 p. 35 - 36. Don, R. 2009 [Ed.], ISTA Handbook on Seedling Evaluation, 3rd Edition, 2003, with Amendments 2006 - 2009. The International Seed Testing Association, Bassersdorf, Switzerland. Evans, L.T., Wardlaw, I.F. & Fischer, R.A. 1975. Wheat. In L.T. Evans, ed. Crop physiology, p. 101 - 149. Cambridge, UK, Cambridge University Press. Harney, M. 1993. A Guide to the Insects of Stored Grain in South Africa. ARC-Plant Protection Research Institute Handbook No.1, P/Bag X134, Pretoria. Metzger, J.D. 1997. A physiological comparison of vernalization and dormancy chilling requirement, pp 147 - 212. In: Lang, G.A [Ed.], Plant Dormancy: Physiology, Biochemistry and Molecular Biology. CAB International, Wallington, UK.