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Conclusions: Ideotypes and physiology: Tailoring plants for increased production
Field Crops Research, 26 ( 1991 ) 221-226 Elsevier Science Publishers B.V., Amsterdam
221
Conclusions: Ideotypes and physiology: Tailoring plants for increased production R.K. Belforda and R.H. Sedgleyb aWestern Australian Department of Agriculture, Baron Hay Court, South Perth, WA 6151, Australia hCrop and Pasture Sciences Group, School of Agriculture, University of Western Australia, Nedlands, WA 6009, Australia
INTRODUCTION
The workshop on 'Ideotypes and Physiology' had three aims: (1) to review the principles behind Donald's original concept of the ideotype; (2) to compare crop improvement strategies, particularly in relation to cropping environments and crop production in Australia; and (3) to identify areas and opportunities where physiological approaches could complement breeding techniques towards improvement of the major crops grown in Australia. This paper draws together the main points to emerge from the workshop and the final session; it considers both the philosophical approaches to physiology and breeding in the future, and reviews the group discussions for each crop/environment combination. THEVALUEOFTHEIDEOTYPEAPPROACH
The original ideotype concept as proposed by Donald has been extremely useful in the three main areas: ( 1 ) generating hypotheses for further research, particularly in identifying potential ways to enhance yield, and at a more physiological level, in encouraging careful thought about which plant characters could contribute to improved crop performance and improvement; (2) encouraging breeders to adopt innovative ways of choosing parents and making crosses to ensure that there is adequate variability in breeding material for characters considered important for maximising yield; and ( 3 ) giving breeders a useful set of goals to make choices about the structure of their breeding programme, the type of germplasm they introduce into it, and the testing strategies used to evaluate their material. 0378-4290/91/$03.50
© 1991 - - Elsevier Science Publishers B.V.
222
R.K. BELFORD AND R.H. SEDGLEY
THE IDEOTYPE CONCEPT TODAY
The ideotype concept was thought to be more relevant to yield improvement, and of lesser value where quality characters are under consideration. However, the development of ideotype traits in breeding programmes has required a long-term investment of effort, and produced generally small improvements in crop yield. This criticism could equally be levelled at other approaches thought to be promising by some breeders, but which have also shown disappointing results after similar or even longer periods of time (e.g. the use of heterosis in wheat). The ideotype concept, as currently understood, was therefore generally considered to be of limited use to plant breeders. The groups confirmed the several reasons for this discussed in the papers, i.e. ( 1 ) m identifying traits likely to improve yield in any environment is difficult (2) - - there is unlikely to be an optimum ideotype for any one crop type or environment, and the ideotype could change with management factors such as time of planting; and (3) - - for many crops there is a lack of genetic diversity from which to select appropriate characters. An ideotype is also likely to change as there is a better understanding of the physiology of the crop and the environment in which it is being grown. Although the groups were critical of the emphasis to date on static aspects of the ideotype approach, they endorsed the concept of a dynamic ideotype, which retains flexibility to cope with variation in environment, agronomy and other external factors. For example, early-season growth for cereals requires a prostrate canopy to maximize light interception and reduce evaporation from the soil, while later in the season, an upright canopy structure is needed for good penetration of light to maximize photosynthesis. Cereal root systems also need to be aggressive to maximize nutrient uptake early in the season, but at the same time should conserve water to avoid moisture stress in the post-anthesis period. Ideotype breeding encourages the idea of seeking character extremes, but the avoidance of extremes is a useful strategy to develop tolerance of diverse environments. The uniculm character proposed by Donald has not been satisfactorily incorporated into cereal plant material partly because of pleiotropic effects, but modern wheats which have reduced or low-tillering characteristics have proved more reliable in yield terms than uniculms, whilst giving greater safeguards again environmental stress. Agricultural practices and management systems are constantly changing in response to new technologies, and changing markets and prices. Improved plant types will therefore need to be adaptable to a wide range of environments. CROP IMPROVEMENT
IN AUSTRALIA -- PROBLEMS AND POSSIBLE SOLUTIONS
Many of the discussion groups identified similar problems, including the difficulty of identifying physiological traits affecting production and adapta-
CONCLUSIONS: TAILORING PLANTS FOR INCREASED PRODUCTION
223
tion, and the genetic variation available when selecting for these traits. Phenology was also identified as a general problem, because of the wide variation both within and between environments; many breeding centres serve large areas which contain a range of environments, each dictating a specific crop phenology. Grain legumes ~ sub-tropical a n d tropical
These present a range of new and diverse species to Australian agriculture, but as yet with relatively small economic significance - - thus at present there are limited breeding opportunities based on a relatively small genetic base. Historically, yield advances have been related to greater determinacy, synchrony of development and maturity, monocarpic senescence, and a higher harvest index. Constraints to improvements by breeding and selection include poor phenological adaptation, and the requirement for mechanical harvesting. A dynamic ideotype would thus be less photoperiod-sensitive, more synchronous in flowering, shorter in height, have a higher harvest index, and be capable of growing at a greater plant density. The penalties for such a strategy may include loss of morphological and developmental plasticity, and the requirement for more rigorous agronomy. Grain legumes ~ temperate a n d mediterranean
Grain legumes grown in southern Australia (particularly lupins and peas) have gained prominence only in the past 20 years, so knowledge of their crop physiology is still sketchy. Thus, firm definition of an ideotype is difficult, and there is a limited genetic background for the development of traits to be exploited. Again, greater determinacy is attractive, not least because it should result in more favourable partitioning between vegetative and reproductive growth, and possibly a better harvest index. However, it would remove some of the flexibility of current cultivars to respond to variable seasons, which food legumes do largely by expanding or contracting their flowering period. Greater determinacy may be associated with characters such as reduced branching in lupins. Pleiotropic effects will probably hinder any major changes to plant type, and will need to be removed before such changes make their full contribution to improved productivity. Pastures
Pastures provide the resource base for one of Australia's largest sources of export income, and there is considerable diversity in pasture types across the range of environments in Australia. Characters selected for pasture improvement will therefore depend on local requirements and the availability of significant variation. Some characters are linked. Unlike many crop species, total yields per se are not always as important as are factors such as seasonal distribution of production, persistence, hard-seededness, or maintaining a
224
R.K. BELFORD AND R.H. SEDGLEY
desirable composition of grasses and legumes. A constraint on selection is that some compatibility between grass and legume is required, whilst a competitive ability of the pasture against weeds is retained. Many pasture species are cross-pollinated, and this has an effect on the degree of selection that is possible. A reasonable degree of variation within varieties is often considered desirable, and the ability of cultivars to adjust to local microclimatic variation is necessary. In Australia there has been limited research, apart from that on maturity times, that can be used to define ideotypes. Both experimental and theoretical studies are needed to define plant types which can be used by breeders in setting their objectives. Donalds concept of the 'isolation', as opposed to the 'crop' ideotype, might be useful here.
Cereals By comparison with old varieties, many m o d e m cereals include a number of Donald's ideotype characters, such as shorter straw, awns, fewer tillers, and a greater harvest index. Published research indicates that, while such varieties produce similar total biomass as did their predecessors, grain-yield is significantly higher because of the changed partitioning between vegetative and reproductive structures. In the mediterranean environment of southern Australia, it was felt that the requirement for yield stability was at least as important as the adoption of an ideotype approach for long-term yield improvement. Manipulating major genes which control disease resistance and tolerance to mineral toxicities and deficiencies could be as valuable as developing an ideotype based on physiological characters. Appropriate phenology remains an important issue: for example, the move towards short-season cultivars in drier environments could mean a lower capability to respond to more favourable rainfall seasons. In the subtropical~summer-rainfall areas, it was felt that the key issue was climatic variability, as an enormous range of environments is possible at any one location. A modelling approach can be used to give an estimate of the probability distribution of yields in an environment, based on current understanding of genotypes, and genotype × management interactions. In a waterlimited environment, a yield model based on transpiration, transpiration efficiency and harvest index is appropriate, whilst in wetter environments, radiation, radiation interception and harvest index are more relevant parameters to use. A modelling approach allows identification of traits for breeding strategies, and the effect of changes in these traits can be simulated when run for historical records of climate at a site. Physiologists can then interact with breeders to insert a trait into an appropriate background and assess it in the target environment, i.e. model characters before incorporating them into adapted genetic material.
CONCLUSIONS: TAILORING PLANTS FOR INCREASED PRODUCTION
225
Oil-seed brassicas There is wide genetic variability within the major species of oilseeds, and many of these characters that may improve yield are controlled by major genes. These include: early flowering, which is an advantage in almost every environment as it provides a long period for seed growth; rapid emergence and growth at low temperatures to improve the competitive ability of the crop against weeds; short stems to prevent lodging; an apetalous character (see Thurling, pp. 201 ) to increase light penetration into the canopy; and long pods and reasonable ovule number to increase seed survival and reduce podshattering.
Sunflowers Sunflowers are a relatively new crop in terms of improvement. There is a shortage of physiological knowledge, which makes it difficult to define ideotypes. The poor conversion of florets to seed in hybrid lines is a long-standing problem that requires more physiological research and understanding before much progress through breeding is likely. Simulation models may allow the implication of changing canopy structure on light penetration and photosynthetic efficiency to be assessed. In dryland environments, greater droughttolerance is desirable, but this again requires more research into the optimum balance between a deep and vigorous root system and appropriate partitioning of dry-matter between root and shoot. THE IDEOTYPEANDALTERNATIVEBREEDINGTECHNIQUES The workshop felt that an ideotype approach in which mainly morphological characters are emphasised is limited, and is unlikely to replace traditional plant-breeding approaches for crop improvement in Australia. However, the interest in greater use ofphenological, stress and disease-related traits implies widespread acceptance of a broader, more dynamic concept of the ideotype, which is consistent with the original given by Donald. Nonetheless, measuring yield per se in conventional programmes can be an efficient way of selecting for yield, and makes no assumptions about the value of an ideotype character to yield improvement, or its interaction with other characters; it has probably generated most improvement in crop yield. Continued emphasis on 'defect elimination' for yield improvement will remain highly relevant in Australian environments, where achievement of high yield is often limited by environmental and management constraints rather than genetic yield potential. Germplasm discovery, creation and evaluation will be serviced by wide crosses, the use of exotic germplasm, and genetic engineering. This is important for developing a range of genetic material with variability that is relevant
226
R.K. BELFORD AND R.H. SEDGLEY
to improving the adaptation and productivity of plants in the environments in which they will be grown. THE FUTURE There is still a strong need to understand agricultural environments and the ways in which plants are adapted to them. Our lack of knowledge about the interaction between these factors is one of the major limitations to improving real yields. The other problem lies in harnessing the skills of breeders, physiologists and agronomists to work together for crop improvement. Here, the use of the newer techniques of isozymes or restriction fragment length polymorphisms (RFLPs) offer opportunities for interaction between breeders and physiologists. A suitable model for collaboration might therefore be to identify appropriate traits, and screen the available genetic resources for variability of one or more desirable traits; this would be followed by examining the genetics and physiology of diverse genotypes, and then devising a breeding strategy which may include a description of a dynamic ideotype. ACKNOWLEDGEMENTS The Workshop organisers gratefully acknowledge financial support from the Wheat Research Council of Australia for Professor Rasmusson. The University of Western Australia provided facilities during the conference and workshop, Dr. J. Hamblin and Dr. E.J.M. Kirby reviewed the papers; and many colleagues in Western Australia, particularly Professor W.R. Stern, Mr. M.W. Perry, Mr. P.A. Portmann and Dr. W.J.R. Boyd, gave advice and commented during the planning of this workshop.
221
Conclusions: Ideotypes and physiology: Tailoring plants for increased production R.K. Belforda and R.H. Sedgleyb aWestern Australian Department of Agriculture, Baron Hay Court, South Perth, WA 6151, Australia hCrop and Pasture Sciences Group, School of Agriculture, University of Western Australia, Nedlands, WA 6009, Australia
INTRODUCTION
The workshop on 'Ideotypes and Physiology' had three aims: (1) to review the principles behind Donald's original concept of the ideotype; (2) to compare crop improvement strategies, particularly in relation to cropping environments and crop production in Australia; and (3) to identify areas and opportunities where physiological approaches could complement breeding techniques towards improvement of the major crops grown in Australia. This paper draws together the main points to emerge from the workshop and the final session; it considers both the philosophical approaches to physiology and breeding in the future, and reviews the group discussions for each crop/environment combination. THEVALUEOFTHEIDEOTYPEAPPROACH
The original ideotype concept as proposed by Donald has been extremely useful in the three main areas: ( 1 ) generating hypotheses for further research, particularly in identifying potential ways to enhance yield, and at a more physiological level, in encouraging careful thought about which plant characters could contribute to improved crop performance and improvement; (2) encouraging breeders to adopt innovative ways of choosing parents and making crosses to ensure that there is adequate variability in breeding material for characters considered important for maximising yield; and ( 3 ) giving breeders a useful set of goals to make choices about the structure of their breeding programme, the type of germplasm they introduce into it, and the testing strategies used to evaluate their material. 0378-4290/91/$03.50
© 1991 - - Elsevier Science Publishers B.V.
222
R.K. BELFORD AND R.H. SEDGLEY
THE IDEOTYPE CONCEPT TODAY
The ideotype concept was thought to be more relevant to yield improvement, and of lesser value where quality characters are under consideration. However, the development of ideotype traits in breeding programmes has required a long-term investment of effort, and produced generally small improvements in crop yield. This criticism could equally be levelled at other approaches thought to be promising by some breeders, but which have also shown disappointing results after similar or even longer periods of time (e.g. the use of heterosis in wheat). The ideotype concept, as currently understood, was therefore generally considered to be of limited use to plant breeders. The groups confirmed the several reasons for this discussed in the papers, i.e. ( 1 ) m identifying traits likely to improve yield in any environment is difficult (2) - - there is unlikely to be an optimum ideotype for any one crop type or environment, and the ideotype could change with management factors such as time of planting; and (3) - - for many crops there is a lack of genetic diversity from which to select appropriate characters. An ideotype is also likely to change as there is a better understanding of the physiology of the crop and the environment in which it is being grown. Although the groups were critical of the emphasis to date on static aspects of the ideotype approach, they endorsed the concept of a dynamic ideotype, which retains flexibility to cope with variation in environment, agronomy and other external factors. For example, early-season growth for cereals requires a prostrate canopy to maximize light interception and reduce evaporation from the soil, while later in the season, an upright canopy structure is needed for good penetration of light to maximize photosynthesis. Cereal root systems also need to be aggressive to maximize nutrient uptake early in the season, but at the same time should conserve water to avoid moisture stress in the post-anthesis period. Ideotype breeding encourages the idea of seeking character extremes, but the avoidance of extremes is a useful strategy to develop tolerance of diverse environments. The uniculm character proposed by Donald has not been satisfactorily incorporated into cereal plant material partly because of pleiotropic effects, but modern wheats which have reduced or low-tillering characteristics have proved more reliable in yield terms than uniculms, whilst giving greater safeguards again environmental stress. Agricultural practices and management systems are constantly changing in response to new technologies, and changing markets and prices. Improved plant types will therefore need to be adaptable to a wide range of environments. CROP IMPROVEMENT
IN AUSTRALIA -- PROBLEMS AND POSSIBLE SOLUTIONS
Many of the discussion groups identified similar problems, including the difficulty of identifying physiological traits affecting production and adapta-
CONCLUSIONS: TAILORING PLANTS FOR INCREASED PRODUCTION
223
tion, and the genetic variation available when selecting for these traits. Phenology was also identified as a general problem, because of the wide variation both within and between environments; many breeding centres serve large areas which contain a range of environments, each dictating a specific crop phenology. Grain legumes ~ sub-tropical a n d tropical
These present a range of new and diverse species to Australian agriculture, but as yet with relatively small economic significance - - thus at present there are limited breeding opportunities based on a relatively small genetic base. Historically, yield advances have been related to greater determinacy, synchrony of development and maturity, monocarpic senescence, and a higher harvest index. Constraints to improvements by breeding and selection include poor phenological adaptation, and the requirement for mechanical harvesting. A dynamic ideotype would thus be less photoperiod-sensitive, more synchronous in flowering, shorter in height, have a higher harvest index, and be capable of growing at a greater plant density. The penalties for such a strategy may include loss of morphological and developmental plasticity, and the requirement for more rigorous agronomy. Grain legumes ~ temperate a n d mediterranean
Grain legumes grown in southern Australia (particularly lupins and peas) have gained prominence only in the past 20 years, so knowledge of their crop physiology is still sketchy. Thus, firm definition of an ideotype is difficult, and there is a limited genetic background for the development of traits to be exploited. Again, greater determinacy is attractive, not least because it should result in more favourable partitioning between vegetative and reproductive growth, and possibly a better harvest index. However, it would remove some of the flexibility of current cultivars to respond to variable seasons, which food legumes do largely by expanding or contracting their flowering period. Greater determinacy may be associated with characters such as reduced branching in lupins. Pleiotropic effects will probably hinder any major changes to plant type, and will need to be removed before such changes make their full contribution to improved productivity. Pastures
Pastures provide the resource base for one of Australia's largest sources of export income, and there is considerable diversity in pasture types across the range of environments in Australia. Characters selected for pasture improvement will therefore depend on local requirements and the availability of significant variation. Some characters are linked. Unlike many crop species, total yields per se are not always as important as are factors such as seasonal distribution of production, persistence, hard-seededness, or maintaining a
224
R.K. BELFORD AND R.H. SEDGLEY
desirable composition of grasses and legumes. A constraint on selection is that some compatibility between grass and legume is required, whilst a competitive ability of the pasture against weeds is retained. Many pasture species are cross-pollinated, and this has an effect on the degree of selection that is possible. A reasonable degree of variation within varieties is often considered desirable, and the ability of cultivars to adjust to local microclimatic variation is necessary. In Australia there has been limited research, apart from that on maturity times, that can be used to define ideotypes. Both experimental and theoretical studies are needed to define plant types which can be used by breeders in setting their objectives. Donalds concept of the 'isolation', as opposed to the 'crop' ideotype, might be useful here.
Cereals By comparison with old varieties, many m o d e m cereals include a number of Donald's ideotype characters, such as shorter straw, awns, fewer tillers, and a greater harvest index. Published research indicates that, while such varieties produce similar total biomass as did their predecessors, grain-yield is significantly higher because of the changed partitioning between vegetative and reproductive structures. In the mediterranean environment of southern Australia, it was felt that the requirement for yield stability was at least as important as the adoption of an ideotype approach for long-term yield improvement. Manipulating major genes which control disease resistance and tolerance to mineral toxicities and deficiencies could be as valuable as developing an ideotype based on physiological characters. Appropriate phenology remains an important issue: for example, the move towards short-season cultivars in drier environments could mean a lower capability to respond to more favourable rainfall seasons. In the subtropical~summer-rainfall areas, it was felt that the key issue was climatic variability, as an enormous range of environments is possible at any one location. A modelling approach can be used to give an estimate of the probability distribution of yields in an environment, based on current understanding of genotypes, and genotype × management interactions. In a waterlimited environment, a yield model based on transpiration, transpiration efficiency and harvest index is appropriate, whilst in wetter environments, radiation, radiation interception and harvest index are more relevant parameters to use. A modelling approach allows identification of traits for breeding strategies, and the effect of changes in these traits can be simulated when run for historical records of climate at a site. Physiologists can then interact with breeders to insert a trait into an appropriate background and assess it in the target environment, i.e. model characters before incorporating them into adapted genetic material.
CONCLUSIONS: TAILORING PLANTS FOR INCREASED PRODUCTION
225
Oil-seed brassicas There is wide genetic variability within the major species of oilseeds, and many of these characters that may improve yield are controlled by major genes. These include: early flowering, which is an advantage in almost every environment as it provides a long period for seed growth; rapid emergence and growth at low temperatures to improve the competitive ability of the crop against weeds; short stems to prevent lodging; an apetalous character (see Thurling, pp. 201 ) to increase light penetration into the canopy; and long pods and reasonable ovule number to increase seed survival and reduce podshattering.
Sunflowers Sunflowers are a relatively new crop in terms of improvement. There is a shortage of physiological knowledge, which makes it difficult to define ideotypes. The poor conversion of florets to seed in hybrid lines is a long-standing problem that requires more physiological research and understanding before much progress through breeding is likely. Simulation models may allow the implication of changing canopy structure on light penetration and photosynthetic efficiency to be assessed. In dryland environments, greater droughttolerance is desirable, but this again requires more research into the optimum balance between a deep and vigorous root system and appropriate partitioning of dry-matter between root and shoot. THE IDEOTYPEANDALTERNATIVEBREEDINGTECHNIQUES The workshop felt that an ideotype approach in which mainly morphological characters are emphasised is limited, and is unlikely to replace traditional plant-breeding approaches for crop improvement in Australia. However, the interest in greater use ofphenological, stress and disease-related traits implies widespread acceptance of a broader, more dynamic concept of the ideotype, which is consistent with the original given by Donald. Nonetheless, measuring yield per se in conventional programmes can be an efficient way of selecting for yield, and makes no assumptions about the value of an ideotype character to yield improvement, or its interaction with other characters; it has probably generated most improvement in crop yield. Continued emphasis on 'defect elimination' for yield improvement will remain highly relevant in Australian environments, where achievement of high yield is often limited by environmental and management constraints rather than genetic yield potential. Germplasm discovery, creation and evaluation will be serviced by wide crosses, the use of exotic germplasm, and genetic engineering. This is important for developing a range of genetic material with variability that is relevant
226
R.K. BELFORD AND R.H. SEDGLEY
to improving the adaptation and productivity of plants in the environments in which they will be grown. THE FUTURE There is still a strong need to understand agricultural environments and the ways in which plants are adapted to them. Our lack of knowledge about the interaction between these factors is one of the major limitations to improving real yields. The other problem lies in harnessing the skills of breeders, physiologists and agronomists to work together for crop improvement. Here, the use of the newer techniques of isozymes or restriction fragment length polymorphisms (RFLPs) offer opportunities for interaction between breeders and physiologists. A suitable model for collaboration might therefore be to identify appropriate traits, and screen the available genetic resources for variability of one or more desirable traits; this would be followed by examining the genetics and physiology of diverse genotypes, and then devising a breeding strategy which may include a description of a dynamic ideotype. ACKNOWLEDGEMENTS The Workshop organisers gratefully acknowledge financial support from the Wheat Research Council of Australia for Professor Rasmusson. The University of Western Australia provided facilities during the conference and workshop, Dr. J. Hamblin and Dr. E.J.M. Kirby reviewed the papers; and many colleagues in Western Australia, particularly Professor W.R. Stern, Mr. M.W. Perry, Mr. P.A. Portmann and Dr. W.J.R. Boyd, gave advice and commented during the planning of this workshop.