81There were several periods in the history of wheat breeding when substantial
changes in the growth habit or genetic background of the plants led to a steep rise in
productivity. Landraces and the fi rst cultivars bred from them had long stems and
poor lodging resistance, making them unsuitable for intensive production and
mechanical harvesting. In the fi rst half of the last century, the Italian breeder
Nazareno Strampelli succeeded in transferring the Rht8 dwarfi ng gene from the
Japanese cultivar Akakomugi into numerous new cultivars, thus creating the fi rst
group of intensive wheats, which had an enormous infl uence not only on wheat
production, but also on further breeding efforts in many parts of the world. A few
decades later Norman Borlaug developed semi-dwarf wheat cultivars well adapted
to intensive production. Cultivars carrying the dwarfi ng genes Rht1 or Rht2 had a
plant height only half or two-thirds of that of traditional cultivars and were so suc-
cessful in fi eld production that by 1963 95 % of the wheat-growing area in Mexico
was sown to semi-dwarf wheats. When cultivated using a satisfactory technology,
these new cultivars resulted in a spectacular rise in yield averages. Large quantities
of seed from dwarf Mexican cultivars (Lerma Rojo, Sonora 64, etc.) were sent to
famine-struck India and Pakistan, and by 1970 wheat production had almost dou-
bled in both countries, putting an end to the famine.
This process, dubbed as the ‘Green Revolution’, also took place in countries
growing winter wheat. Rht1 and Rht2 genes responsible for semi-dwarf plant height
are widespread in the English, French and other West European wheat cultivars giv-
ing them better lodging resistance and higher yield (Worland and Snape 2001 ). In
winter wheat-growing regions of Eastern Europe and Middle East the semi-dwarf
cultivar Bezostaya 1 with dwarfi ng gene Rht8 had the most far-reaching effect.
The positive effect of dwarfi ng genes was manifested not only in the fact that
the better lodging resistance resulting from reduced plant height enabled them to
be grown at a higher nutrient level, but also in the substantial change that occurred
in the distribution of the assimilates between the vegetative and generative organs.
A major reduction in plant height may, however, also have a negative effect on the
biological yield. According to Richards ( 1992 ) each 10 cm decrease in straw
length below a plant height of 100 cm leads to a 4.4 % drop in the aboveground
biomass. This could be observed in India, where the introduction of dwarf cultivars
resistant to lodging was accompanied by a decline in the biomass (Sinha and
Aggarwal 1980 ).
Most experiments designed to compare old and new wheat cultivars either
detected no genetic gain for the biological yield (Siddique et al. 1989 ; Slafer and
Andrade 1989 ; Bodega and Andrade 1996 ) or only a very slight change (Balla et al.
1986 ; Perry and Antuono 1989 ; Nedel 1994 ). Due to the negative correlation
between biomass and lodging resistance, breeders automatically selected for greater
harvest index (HI) (Riggs et al. 1981 ). While in the case of non-intensive wheats
around two-thirds of the aboveground dry matter was made up of stems and leaves,
the harvest index of modern cultivars approached or even exceeded 50 % (Peltonen-
Sainio and Peltonen 1994 ; Hay 1995 ). The main reason for the rise in HI is that
modern cultivars form a larger number of seeds per unit area (Perry and Antuono
3 Wheat Breeding: Current Status and Bottlenecks