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 Essay - mar/apr 2006
Seed ecology II crop challenges
James C. Delouche
Professor Emeritus Mississippi State University
Crop ecosystems were essentially and necessarily established by the transformation or replacement of parts of natural ecosystems and this process also continues to the present time. The historical and ecological evidence is that complex natural ecosystems were manipulated in gradual, phased and discrete steps to create simpler, artificial production systems as crop agriculture evolved. Two contrasting crop production systems emerged from these transformations and manipulations. A seed-culture system, based on seed-reproduced plant species, developed in the drier temperate and subtropical areas of the Old and New Worlds, while a vegeculture system, based on vegetatively-propagated species developed in the humid tropical areas of the Americas, Southeast Asia and West Africa. Although indigenous vegeculture crop systems are usually much more varied and ecologically stable and conserving than indigenous seed-culture systems, they have long been in decline through intrusion of seed reproduced crops and the simplification of vegecultures into monocultures, e.g., sugarcane and cassava plantations.
Seasonal Adaption of Crops
In the context of crop agriculture, seed ecology is mostly concerned with changes in the composition, interactions and relationships in seed-culture ecosystems during their transformation from complex natural systems to simpler artificial systems that, in turn, can change every few years or even every season. The manipulation of farming ecosystems to make them simpler, more effective, efficient and convenient for the production of food, fibers and other materials and, especially, to adapt the systems to other environments and seasons, have presented farmers and researchers with many challenges and problems and this also continues to the present. In the 1930s Russian agricultural scientists were presented with the ecological challenge of adapting a winter season crop system to the spring season, i.e., to convert winter wheat into spring wheat. The problem was that in the semi-arid region of the Ukrainian steppes spring wheat started off well but was then frequently seriously damaged by high summer temperatures at the beginning of grain formation. The Russian plant scientist T. D. Lyssenko, who had done some pioneering work on the phasic development of plants thought that the problem could be resolved by subjecting seeds of winter wheat to simulated winter conditions.
The simulated winter conditions or treatment, termed vernalization, would (theoretically) shorten the vegetative period and accelerate flowering and grain formation of spring sown winter wheat under the long days of late spring and early summer. Seeds of the winter wheat were vernalized by adjusting moisture content to 50% and holding them at winter-like temperatures just above freezing, i.e., 1 - 2°C, for periods up to 60 days. The treatment appeared to work according to theory and for the next 20 years or so was strongly promoted in Russia and taken up by researchers in other countries for converting winter cereal crops to spring crops, e.g., wheat, barley, rye, inducing flowering and seed formation of some temperate climate forage crops, e.g., alfalfa, ryegrass, under unfavorable conditions and hastening flowering and seed formation in vegetable crops such as lettuce and onion. The results produced by the treatment, however, were rather inconsistent and in many cases only marginally better than the controls. While vernalization was still an interesting and popular seminar topic during my graduate studies in the early 1950s, research and practical applications of the treatment had sharply declined even in Russia.
Several other approaches to changing the planting season for crops have been tried with limited success. Planting spring crops in the more northern and colder regions such as in Canada is frequently delayed because of poor conditions for cultivation and sowing. Some workers, therefore, begin to think of ways they could be sown during a suitable period in autumn but not germinate and emerge until conditions became favorable the following spring. One early approach, 1960s, was genetic and physiological. Enough seed dormancy was breed into Avena sativa so that it could be sown in the autumn, lose dormancy over the winter and germinate early the following spring. The speciality varieties developed were named dormoats. Another somewhat later approach, 1970s, was chemical and mechanical. Wheat seeds were coated with a thermo sensitive material for autumn sowing that prevented absorption of water until the soil warmed to a favorable level the following spring.
These were very imaginative approaches to solving ecological problems at sowing time but the ideas were more advanced than the available technology for successful application. Interestingly, rather recent advances in polymer chemistry and coating technology have renewed interest in the chemical/mechanical and similar approaches while genetic engineering just might be the appropriate technology for the genetic/physiological approach for changing the seasonal adaptation of crops.
Environmental Impediments
Many cool season vegetable crops are produced in the western deserts of the USA during the winter season. Some of the crops are sown in September when the soil temperature can be quite warm. The seeds of lettuce, one of the most important crops, are light and warm temperature sensitive so it is usually difficult to achieve a commercial population of seedlings planted in September. Several treatments have been developed to overcome the warm temperature and darkness environmental constraint on the production of lettuce in the otherwise ideal irrigated desert location. Treatment of the seeds with gibberellin and/or seed priming extends the temperature range for germination and seedling establishment into the warmer temperatures that are common in September. Priming or osmoconditioning is also effective in promoting the germination of other kinds of seeds under conditions that are normally unfavorable.
Changes in the time of sowing and the crop cycle, i.e., maturity, can also change the environmental conditions, the ecology, of the crop at the critical flowering, seed filling and maturation periods. The considerable increase in cropping intensity, i.e., multiple cropping, in many countries has been achieved by improvements in irrigation, fertilization, crop protection and, especially, in the maturity period of crops. Crops that had maturity periods of 200 or more days in the past now mature in 120 - 140 days or earlier. In regions with distinctive wet and dry seasons, shortening the crop cycle frequently resulted in maturation of the crop during the last of the rainy season rather than the first of the dry season that required important changes in crop management, harvesting, and handling of the production to minimize post-harvest losses. Multiple cropping is also usually accompanied by different fertilization requirements, irrigation regimes, insect problems and weed populations as a result of changes in the crop ecosystems that have to be addressed to gain the expected benefits.
Difficulties in the production of quality seeds and grains of early maturity soybean varieties in the Southeast U.S. was a long term constraint to use of such early types in a mix of varieties to achieve the advantages of spreading risks, labor, machinery, and irrigation over a longer timeframe. The early varieties were usually planted after the cotton crop from about May to mid-June which subjected them to frequent prolonged droughts in late July and August during the critical flowering and grain filling period and frequent rains and high humidity during the late maturation and harvest period in September. All too often the results were low yields, poor quality seeds and badly weathered grain. Some persons, however, decided to go against the usual practice and plant the early maturing varieties early. They did and the crop matured early, escaped the drought and was harvested before the late September rains. The transition to early planting for soybeans, of course, involved much more than someone trying it and succeeding. There were challenges and problems in just about every aspect of planting and stand establishment that required adjustments in the crop ecosystem. The adjustments were made and early planting of early soybean varieties is now a widely accepted practice.
Rice culture is one of the most interesting and not-so-simple crop ecosystems. It and the ecology of weedy plants will be examined in the next segment of this essay on seed ecology.
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