Resumen de Eco-physiological bases of genetic gains of bread wheat yield in Mediterranean Spain during the 20th century
Wheat breeding has played an important role in increasing yields during the 20th century. In general, the increase in yield was characterised by an exponential trend with the year of release of the cultivars: slow increase during the first half of the century followed by a fast gain in the second half. However, very few studies were conducted under Mediterranean conditions characterized by a dry and hot summer, a mild and wet winter, and by a high variability in the year-to-year rainfall. As the vast majority of wheat in Spain is grown under these particular conditions, the aim of this Thesis was to identify the main eco-physiological attributes that have been modified by bread wheat breeding in Mediterranean Spain during the 20th century. The particularity of this environment allowed the hypothesis that under Mediterranean conditions breeders may have selected for high yields through different physiological avenues to those explored in other temperate regions implying that cultivars successfully performing in this environment might express a different, more conservative, behaviour, privileging reserve accumulation rather than maximising grain number per unit of crop dry matter at anthesis. Field experiments were conducted during five consecutive growing seasons, under different environmental backgrounds mainly characterised by distinct yield potentials. We tested bread wheat genotypes representing important steps in wheat breeding in Spain selected due to their success during at least a decade in farm or experimental crops in the Mediterranean region of Spain, including two advanced breeding lines of IRTA's bread wheat programme performing consistently well in several comparative trials. Descriptive trends in yield, yield components and other physiological attributes were analysed, but also a more complex prospective physiological approach was developed in order to determine key physiological traits of value for future breeding.
Bread wheat breeding in Mediterranean Spain has dramatically increased yields between the 1940s and 1970s with no clear gains in yield during the last decades. The increase of yield in modern genotypes was associated to the increase of harvest index (HI) and the number of grains per m2. The lack of success in improving wheat yields in Mediterranean Spain during the last decades could be associated to the fact that although modern genotypes had higher HI than their older counterparts, the resource partitioning to reproductive organs was lower than those reported under non-Mediterranean conditions, showing that there was an important part of the carbohydrates that were not remobilised to grains. The higher sink size of modern genotypes generated higher resource demand associated to higher post-anthesis radiation use efficiency (RUE). The increase in the number of grains per m2 was associated to the decrease in grain nitrogen content corroborating that the accumulation of nitrogen in the grains is source-limited. The higher grain number of modern genotypes was associated to both the number of grains per unit of spike dry weight at anthesis, or fruiting efficiency, and the spike dry weight at anthesis, revealing the relevance of the grain determination period. This shows that even under the Mediterranean environment the number of grains per m2 is the main determinant of yield and that further increases of the grain number in modern genotypes would be associated to higher yields. In fact, decreasing grain number by post-anthesis spike trimming or pre-anthesis shading resulted in reductions of crop growth, RUE and leaf photosynthetic rate during post-anthesis. These manipulative experiments also showed that breeding tended to increase the degree of source-limitation during post-anthesis from negligible values in the oldest cultivars to a sort of co-limitation in the most modern line, a fact that could generate a negative correlation between the grain number and average grain weight (AGW). However, the results of this work show that the negative relationship between the number of grains per m2 and the AGW is produced by increases in the proportional contribution to the final number of grains per m2 of grains that are constitutively smaller.
Finally, this Thesis shows that yield sensitivity to lodging, beyond differences in lodging itself, has been reduced by bread wheat breeding. In general, the lower yields in lodged plants were associated to the decrease of both the number of grains per m2 and the AGW. The decrease of the number of grains per m2 was associated to a reduction in crop growth, while the AGW loss was apparently simultaneously related to the reduction of available assimilates (crop growth during post-anthesis as well as water soluble carbohydrates at anthesis) and to a direct effect of lodging on grain weight potential (as grain weight responses to source-sink manipulations were similar in lodged and un-lodged canopies). Thus it appears that overall grain filling in un-lodged and in strongly lodged canopies was similarly balanced in terms of source-sink ratios.
