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II. Constraints in Mediterranean Environments

II. Constraints in Mediterranean Environments

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MEDITERRANEAN WHEAT YIELD INCREASES



233



-E

E



2,



E



Ec



f



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

(1972).



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

growth.



B. SOLARRADIATION

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



234



S. P. LOSS AND K. H. M. SIDDIQUE



Month



Figure 3 Mean monthly climatic data for Aleppo, Syria (-) and Merredin, Australia (---); (a)

solar radiation, (b) maximum and minimum temperatures, and (c) pan evaporation. Months are

January-December for Merredin and July-June for Aleppo.



solar radiation may limit photosynthesis of the lower leaves. In midsummer, the

elevation of the sun is high, there is a low incidence of cloud cover, and mean

midday solar radiation is about 25-30 MJ m-* day-'.



C.TEMPERATURE

As well as lack of rainfall, cereal growth can also be constrained by both high

and low temperatures in mediterranean environments. Temperatures follow trends

in solar radiation (Fig. 3). Summer maximum temperatures range between 25

and 40°C along western coasts and between 30 and 45°C inland and in the more

easterly regions. Even if water was available in summer, as temperatures increase

cereal development and respiration increase, while assimilation rates reach a

plateaux, and thus growth at high temperatures (>30"C) is suboptimal.

In most mediterranean environments, at least 1 month has an average temperature below 15°C (Fig. 2) and less than 3% of the year experiences minimum

temperatures below 0°C (Aschmann, 1973). Mean monthly minimum tempera-



MEDITERRA"



WHEAT YIELD INCREASES



235



tures in midwinter range from about 0 to 7°C and in some regions, especially

inland areas, temperatures fall below 0°C during individual nights. Vegetative

growth rates of cereals are restricted by low temperatures in midwinter and minimum temperatures are generally not low enough to cause long-term freezing

damage to cereals during their vegetative stages of development. In northern

Syria, severe frosts occasionally reduce the leaf area of cereal crops; however,

the effect of frosts on grain yield during the vegetative stages is usually small

because of compensation in later growth (Harris et al., 1989).

Frost damage to the wheat stem and ear during early spring is an important

constraint to wheat yields in mediterranean environments (Harris et al., 1989;

Loss, 1989). Wheat becomes more susceptible to freezing damage as it enters its

%productive stages of development, and the period from ear emergence to

2 weeks after anthesis is the most susceptible stage. Minimum temperatures may

fall below - 2°C in early spring, killing the ear and/or restricting the movement

of assimilates in the stem. Frosts can cause devastating yield losses to wheat

crops that reach anthesis early in individual years. At these late stages of the life

cycle, there is little opportunity for recovery from frost damage, although there

is some compensation in the growth of unaffected grains and if spring conditions

remain mild and moist, plants may be able to produce late tillers.



D. GROWING

SMON

In mediterranean environments, the period of crop growth is usually restricted

by lack of rainfall, water deficits, and high temperatures at the start and end of

the season. Potential evaporation (Epan) exceeds rainfall for a large proportion

of the year. The timing of the first autumn rains can vary considerably, and

sowing times may vary from year to year over a period of 8- 10 weeks. Developments in machinery and weed control have enabled farmers to sow soon after

the first autumn rains and maximize autumn water use (Perry et al., 1989; Anderson and Smith, 1990; Kerr et al., 1991); however, because of the limitations

of rainfall and evaporation, there is little scope for improving yield by extending

further the period for crop growth. With the adoption of early sowing, there is

also the risk of an extended period of dry weather after the initial autumn rain,

and under such conditions, early sown crops can be subjected to water stress

soon after emergence (Ken and Abrecht, 1992).

Winter can change abruptly into spring and the termination of the growing

season varies considerably depending on rainfall, temperatures, and soil type.

Soil type is an important factor affecting the moisture status of a crop and soils

can vary considerably within a small area. For example, there is often a variety

of soils in Western Australia within a transect of 100-200 m, ranging from deep

infertile leached coarse-textured soils on the higher parts of the landscape to



236



S. P. LOSS AND K. H. M. SIDDIQUE



poorly drained clay loams in the valley floors, including transitional and duplex

soils. Most crops in Western Australia depend largely on current rainfall because

of the poor water-holding capacity of many soils, particularly in low-rainfall

regions. Crops grown on fine-textured soils, such as those common in northern

Syria and South Australia, tend to rely more heavily on stored soil moisture.

Rainfall largely determines the length of the growing season in mediterranean

environments and the pattern and efficiency of water use has a large effect on

wheat yields. Ludlow (1989) groups strategies of plant adaptation to waterstressed environments into three categories. Reduced life cycle of the plant to

match the average growing season is termed escape. To maximize long-term

yields in mediterranean environments, it would be ideal to have a wheat cultivar

that not only tolerates years when the terminal drought is early, but one that also

takes advantage of years when spring rains and mild temperatures extend the

growing season. Maximizing water uptake and minimizing water loss are termed

avoidance, whereas mechanisms that enable a plant to cope with reduced water

content are termed drought tolerance. These strategies are useful in mediterranean environments and wheat plants are capable of all three.

We will now examine the morphological and physiological traits that have or

are likely to increase wheat growth and yield through reduced water, radiation,

and temperature stresses. We also discuss how these traits have been measured

and possible selection techniques for these traits in breeding programs.



111. BIOMASS PRODUCTION AND PARTITIONING

Donald and Hamblin (1 976) define grain yield (GY) as the product of the

biomass produced and the harvest index (HI; the proportion of the aboveground

biomass that is partitioned to the harvested grain).



GY



=



Biomass x HI



Biomass can be increased by agronomic manipulation (early sowing and increased seed rate and fertilizer) or by genetic means. However, unless the increased biomass is matched to the life cycle of the crop, there is a risk of

exhausting water sources before maturity is reached. In this article, we only

consider the genetic paths to increased biomass.

In the past, genetic increases in wheat yields around the world have largely

been associated with changes in HI, whereas increases in biomass production

have been small or negligible (Deckerd et al., 1985; Cox et al., 1988; Perry and

D’Antuono, 1989; Austin et al., 1989; Siddique er al., 1989a; Safer and Andrade, 1993). Results from Mexico and Canada are exceptions (Evans, 1987;



MEDITERRANEAN WHEAT YIELD INCREASES



237



Hucl and Baker, 1987). Up to the 1970s, biomass production had not increased

for wheats bred by The International Maize and Wheat Improvement Centre

(CIMMYT), but more recent yield increases in cultivars bred under irrigation

have been associated with increased biomass under water-limited situations

(Evans, 1987). Also, Hucl and Baker (1987) found a positive correlation between GY and biomass production when comparing old and modem Canadian

spring wheats.

Figure 4 illustrates changes in GY, biomass, HI, and maturity with the year

of release of Western Australian wheats. This figure is based on the data of Perry

and D’Antuono (1989), but also includes three new cultivars not included in their

study. The inclusion of these latest cultivars indicates that the rate of increase in

GY between 1965 and 1990 is greater than in the preceding 100 years, and the

rate of biomass increase associated with breeding is small (Fig. 4b). Most of the

GY increase can be attributed to increased HI (Fig. 4c), and the duration from

sowing to anthesis has decreased with selection for yield (Fig. 4d). This and

other studies (CIMMYT, 1991; Slafer et al., 1993) also demonstrate that modern

cultivars outperform older cultivars even in dry environments with low yield

potentials.

Several authors (Donald and Hamblin, 1976; Richards, 1987; Turner and

Nicholas, 1987) suggest future wheat yields can be increased by increasing biomass production. Certainly, there is the potential for increased biomass production in mediterranean environments in some circumstances; barley is capable of

producing more biomass and grain than wheat using the same amount of water

(Siddique et al., 1989b; Lopez-Castaneda, 1992; Gregory et a!., 1992; Simpson

and Siddique, 1993). However, as with increased biomass caused by agronomic

manipulation, genetically increased biomass is not always translated into increased GY.

Where lack of water is the major limitation to growth, it appears that it will

be difficult to increase the biomass production of wheat significantly, especially

when soil water is completely exhausted at maturity. Under these circumstances,

higher biomass will be translated into higher yield when rainfall is used more

efficiently for photosynthesis and it may be difficult to improve these fundamental physiological processes with conventional breeding methods. We will discuss

in more detail the potential for increasing water-use and radiation-use efficiencies

later and deal with assimilate partitioning first.

The genetics of HI is probably more easily modified than biomass production,

but given the nature of moisture stress during grain filling, large increases in HI

are unlikely in mediterranean environments (Siddique et al., 1989b; Hadjishristodoulou, 1991). Aspects of the genetic, physiological, and environmental regulation of partitioning of assimilates were reviewed by Snyder and Carlson (1984),

Gifford er al. (1984), and Wardlaw (1990). Germination, ear initiation, terminal



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