Changes in physiological attributes of the dry matter economy of bread wheat (Triticum aestivum) through genetic improvement of grain yield potential at different regions of the world

Abstract

The understanding of the changes produced by plant breeding on wheat grain yield and on its determinants, through the study of the behaviour of cultivars released at different times during the breeding process, could be a useful tool to define future selection criteria. The aim of this paper was to examine the genetic improvement effects on bread wheat grain yield potential and on its major physiological determinants in different countries.Generally, it seems that the genetic improvement produced important increases in harvest index, but it did not produce important changes in total biomass. The genetic gain in grain yield ranged from 0.58 g m 2 year 1 (India) to 5.84 g m 2 year 1 (Mexico); reflecting important differences among genetic improvement programmes. However, when the comparisons were made in relative values, the difference was much lower, ranging from 0.16% year -1 to 0.90% year ~, for the previously mentioned countries, respectively. Most of the regions showed relative genetic gains of around 0.45% year 1. Similar patterns of relative genetic gains in harvest index were found.Half of the total gain in grain yield in the past was due to genetic gain in grain yield potential. However, it is expected for the future, that further increases in grain yield will rely more on genetic gain than on technologic gain. Thus, a discussion about possible strategies in future breeding programmes of bread wheat is attempted throughout the paper.The changes in grain yield were mostly associated with changes in number of grains per m 2 rather than with changes in individual grain weight. It appears that the source:sink ratio during the grain-filling period was very high in old cultivars, whereas the source:sink ratio in modern cultivars is more balanced. Further increases in number of grains m 2 could be compensated by a reduction in individual grain weight, if simultaneous improvement in source strength during grain-filling is not achieved. 38The crop attributes which have been changed together with the mentioned traits are discussed and alternative selection criteria are suggested.

Fifty years of wheat breeding in Southern Brazil: yield improvement and associated changes

“…Compared to results obtained elsewere, the rate of progress in the yield of spring bread wheat in Southern Brazil is higher than those obtained in Argentina, Australia, Canada, England and the United State of America (Slafer & Andrade, 1991), and lower than those obtained in Mexico in the same period of time. However, these estimates in genetic progress in countries like Australia, England and USA are representative of periods of about a century or more.…”

“…Such gain was higher than the one obtained by Nedel (1994) in similar trials. Slafer & Andrade (1991) summarized studies of genetic progress in the yield of bread wheat in many countries, and reported annual rates of progress from 5.8 kg ha -1 (India) to 58.4 kg ha -1 (Mexico). Countries like Argentina, Canada, Australia, England, and USA produced intermediate values of genetic gain, suggesting a similar efficiency among their genetic breeding programs (Slafer & Andrade, 1991).…”

“…Slafer & Andrade (1991) summarized studies of genetic progress in the yield of bread wheat in many countries, and reported annual rates of progress from 5.8 kg ha -1 (India) to 58.4 kg ha -1 (Mexico). Countries like Argentina, Canada, Australia, England, and USA produced intermediate values of genetic gain, suggesting a similar efficiency among their genetic breeding programs (Slafer & Andrade, 1991). According to these authors, the genetic gains observed (1) Means followed by the equal letters, within a column, were not significantly different (p<0.05) as tested by Tukey's multiple range test.…”

“…According to Slafer & Satorre (1994), the advance in yield potential may result from genetic gain in grain yield potential; and also in resistance to diseases, reduced lodging, tolerance to environmental constraints (from other genetic improvement); and in development of machinery and equipment, better agrochemical usage, more appropriate decisions as to seeding date and rate (non-genetic or agronomic). In order to isolate these components and avoid confusions, studies should consider management practices, soil conditions, lodging control, disease control and other technological advances, all of which may have experienced changes throughout the period of the study (Slafer & Andrade, 1991).…”

Source limitations due to leaf rust (caused by Puccinia triticina) during grain filling in wheat

“…In some cases, source or sink limitations may operate during different developmental stages, as found in various crops. For wheat, research shows that the crop is mainly sink-limited (Jenner 1979;Savin and Slafer 1991;Slafer and Andrade 1991;Slafer and Savin 1994;Miralles and Slafer 1995;Kruk et al 1997;Borrás et al 2004), as little or no grain weight response has been observed when increases in resource availability per grain (i.e. detached treatments) or reductions in assimilate supply (i.e.…”