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Chapter 5. Morphological and Physiological Traits Associated With Wheat Yield Increases in Mediterranean Environments
S. P. LOSS AND K. H. M. SIDDIQUE
provements in crop management and plant selection, wheat spread from the
mediterranean climate of west Asia to most other parts of the globe and it is now
one of the most widely adapted plants in the world. Modern wheats perform best
in the temperate regions of Europe and North America, where yields are higher
and less variable than in the region where wheat originated and in other similar
parts of the world that experience a mediterranean-type climate-west Asia,
north Africa, South Africa, southern Australia, and in southwest North and
South America (Fig. 1). It is somewhat ironic that plant scientists are currently
trying to improve the adaptation of modern wheats to the environment where
they originated. Nevertheless, about 10- 15% of the world’s wheat is produced
in mediterranean-type environments.
An important factor contributing to the widespread adoption of wheat was the
recognition of the significance of the environment to adaptation. For example,
after European settlement of Australia in 1788, the first two wheat crops failed
miserably partly because the European wheats did not cope with the long, hot,
and dry Australian summer (Macindoe, 1975). Australian settlers then began
introducing from South Africa, India, and the mediterranean region earlymaturing wheats, which were better adapted to warm climates.
With the development of plant breeding techniques during the twentieth century, breeders all over the world began to modify crops to increase yields, using
Figure 1 The distribution of mediterranean environments in the world
MEDITERRANEAN WHEAT YIELD INCREASES
mainly an empirical approach, that is, by trial and error. The progress of breeders in mediterranean environments has been slower than in other regions (Slafer
et al., 1993), probably because of the limitations that the mediterranean environment places on plant growth, in particular, water stress. Yield increases associated with the genetic improvement of wheat have been demonstrated by comparisons of old and modern cultivars grown under the same conditions, and the
mean yield increase attributed to wheat breeding in Western Australia is 6 kg
ha-’ year-’ (Perry and D’Antuono, 1989), about one-eighth of that measured in
Europe (Austin e t a ! . , 1980) and North America (Dalrymple, 1980).
Breeders have successfully combined many desirable traits in cultivars, primarily by selecting for yield; however, except for two traits, time to anthesis and
plant height, breeders have not been convinced of the value of selecting for other
morphological and physiological traits recognized by physiologists as important
in determining grain yield (Whan et a l . , 1993). Many studies have identified
traits that have contributed to increased wheat yields in the past (Austin et a l . ,
1980; Cox et a l . , 1988; Perry and D’Antuono, 1989; Kirby et a l . , 1989, Siddique et al., 1989a,b; Loss et al., 1989; Slafer et al., 1990; Siddique et a l . ,
1990a,b; Slafer and Andrade, 1993), and the authors of these studies proposed
that further improvements in many of these traits may lead to future yield increases. In addition, on the basis of physiological research, plant scientists have
identified new unexploited traits that may increase wheat yields, for example,
narrow xylem vessels (Richards and Passioura, 1981) and osmoregulation (Morgan, 1983).
In the past, most physiologists and breeders operated independently, but recently a new level of cooperation has arisen. Future yield improvements may be
hastened by a better understanding of factors that control growth, development,
and yield of cereals (Shorter et a l . , 1991), and physiologists are helping breeders
develop the most appropriate plants for particular environments. Richards ( 1 982)
termed this breeding approach “analytical,” rather than empirical.
This article reviews the wheat physiology/breeding work relative to the constraints of dryland cropping in mediterranean environments and explores opportunities for additional yield improvement associated with morphological and
physiological traits. Other reviews have dealt with related aspects (Simmons,
1987; Ludlow and Muchow, 1990; Bidinger and Witcombe, 1989); however,
these discussed a number of crops in a number of environments. By addressing
wheat in mediterranean environments, we specifically review the progress of
breeders and physiologists working in these regions and develop more concrete
conclusions. Relatively few physiological studies have been conducted in mediterranean environments, therefore we occasionally draw on data from other
Improvements in disease and pest resistance, and increased tolerances to salinity, waterlogging, acidity, and mineral toxicities, are important contributions
S. P. LOSS AND K. H. M. SIDDIQUE
made by plant breeding; however, these are mainly localized stresses. Maintaining and improving grain quality is also an important breeding objective. Such
types of breeding subprograms may also involve considerable physiological understanding, but they are not considered in this article.
We begin by discussing the environmental constraints to crop growth in the
parts of the world that experience mediterranean-type climates.
11. CONSTRAINTS IN MEDITERRANEAN
Mediterranean climates are basically characterized by long, hot, dry summers
and short, mild, wet winters. Cereals are mainly grown under dryland conditions
in these areas, and although dryland implies unirrigated, water-limiting situations, the growth of cereals is not always limited by lack of water in mediterranean environments. Cereals are planted soon after the first autumn rains and they
undergo vegetative growth in winter. They switch to reproductive growth as temperatures and photoperiods increase in spring, and they mature in early summer.
The constraints to cereal growth vary in mediterranean environments, but inadequate rainfall is usually the most limiting factor (Nix, 1975; Fischer, 1979).
Cornish (1950) reported that 70-80% of the variation of yield in South Australia
was due to variation in annual rainfall, and similar relationships exist in North
Africa, west Asia and Western Australia (Srivastava, 1987; Blum and Pnuel,
1990; Karimi and Siddique, 1991a).
According to Aschmann (1973), mediterranean environments receive between
275 and 900 mm annual rainfall, with the majority (>65%) in winter. Figure 2
illustrates the winter-dominated rainfall pattern at six sites that experience mediterranean climates, three in the Northern Hemisphere and three in the Southern
Hemisphere. In general, winter rainfall exceeds crop demand because of mild
temperatures, low evaporation, slow growth rates, and the high reliability of
these rains. The coefficient of variation of the midwinter rainfall is about 6% at
Merredin, Western Australia, whereas summer rainfall has a coefficient of variation of about 15%. Hence, intermittent drought during winter is rare, and, on
the contrary, waterlogging can be a problem on some soil types in wet years.
During spring, rainfall becomes less frequent, temperatures and vapor pressure deficits (VPDs) increase, and soil moisture is usually exhausted by the time
MEDITERRANEAN WHEAT YIELD INCREASES
Figure 2 Mean monthly rainfall and temperature at six locations with mediterranean climates;
(a) Merredin, Australia (31"20'S 118"17'E); (b) Cape Town, South Africa (3396's 19"29'E);
(c) Rancagna, Chile (34"IO'S 7Oo45'W); (d) Aleppo, Syria (36"Il'N 37'13'E); (e) Rabat, Morocco
(34"OO'N 6"50'E); and (f) Davis, California (38"32'N 121'45'W). Sites in the Southern Hemisphere
are for January-December and those in the Northern Hemisphere are for July-June. Numbers in
parentheses are the number of years of records used to calculate the means. Data from Wernstedt
the crop reaches maturity. This is often referred to as terminal drought and its
timing varies according to the last spring rains, temperatures, soil type, and crop
Solar radiation has a large influence on temperature and evaporation regimes,
and hence crop growth. In most mediterranean environments, midday solar radiation is about 6- 10 MJ m-* day-' in midwinter (Fig. 3), and it is unlikely that
radiation limits crop growth, especially because temperatures and crop leaf areas
are low at this time. In spring, however, when the leaf area index (LAI) is about
3, the lower canopy of the crop becomes shaded by the upper leaves and ear, and