Adaptation of barley to harsh Mediterranean environments = Aanpassing van gerst aan ongunstige Mediterrane milieu's

Research Objectives

Barley is in Syria the dominant crop in areas receiving less than 300 mm annual precipitation. Grain yield is often below 1 ton ha ^-1, and is reduced by low temperatures in winter and terminal drought stress in spring. Variation in the timing and intensity of the stresses, however, can cause considerable fluctuations in yield between both locations and seasons. For environments where low yields are predictable, but not the stresses causing these low yields, selection for a stable yield across years and a reduced risk of no grain yield, is more important than selection for yield potential. Breeding targeted at these environments, however, is hampered by genotypex environment interactions.

This thesis had several aims. The first was the identification of a combination of morphological and physiological traits, or a plant ideotype, related to adaptation of barley to environments where both low temperatures early in the season and high temperatures and drought during grain filling (terminal drought) are likely. A next step was assessing the effect of plant ideotype on yield in contrasting environments and the identification of the most appropriate environment to select for plant ideotype and yield under stress. These results together culminated in the development of a selection procedure for breeding programs targeted at harsh Mediterranean environments.

Results

Adaptation of barley to Mediterranean environments depends on the development pattern of the apex. Within a group of 36 cultivars, four contrasting development patterns were distinguished, of which two were adapted. The first pattern constituted early heading spring types: they did not have a vernalization requirement, were cold sensitive, but avoided terminal drought stress. This pattern is especially adapted to Mediterranean environments with mild winters and terminal drought, like those in Jordan and North Africa. The second pattern represented medium early heading winter types. They had a mild vernalization requirement, a very rapid development in spring and are especially adapted to Mediterranean environments where both cold winters and terminal drought are likely, e.g., those in northern Syria. Two other development patterns, late heading spring types and late heading winter types, are unadapted to Mediterranean environments, because of an inadequate avoidance of terminal drought stress: the former group had a too slow development in spring, whereas in the second group development was delayed too much by a too high vernalization requirement. The development pattern of the apex thus depended on the vernalization requirement in winter and the response to photoperiod in spring.

To select indirectly for development pattern, morphological traits were identified, which are related to the rate of development in winter or spring. In winter, a slow development was strongly associated with a plant ideotype, which could be described by a prostrate growth habit, dark plant colour, and cold tolerance. This ideotype was characteristic of winter types. In spring, a rapid development resulted in an early heading. Selection for these traits together thus enabled selection for an appropriate development pattern. The results indicate that it is the combination of traits, rather than an individual trait, which determines adaptation of barley to Mediterranean environments.

Plant ideotype in winter and heading date both influence yield. Cold tolerance and a prostrate growth habit and dark plant colour in winter had a positive effect on yield in low-yielding (LY) environments, but a weakly negative effect in high-yielding (HY) environments. Early heading was very important in LY environments, but of minor importance in HY environments, apparently because terminal drought stress was less important in those environments. Early heading winter types thus had the highest yield in LY environments. This was caused by a fast crop growth rate in early spring, combined with a long green leaf area duration. In addition, the yield of early heading winter types was little affected by frost, what improved yield stability. The differences between HY and LY environments, concerning the effect of plant ideotype and heading date on yield, show that HY environments are not representative of LY environments. Selection for yield in HY environments has the risk of selection for a plant ideotype which is not adapted to LY environments.

Implications for Plant Breeding

Yield selection in early generations is difficult, especially under harsh conditions. Based on the results presented in this thesis, a selection procedure can be proposed, where ideotype breeding and empirical yield selection complement each other. In early generations (F ₃ - F ₄ ), selection must focus on the identification of ideotypes which are adapted to the LY target environment. This can be done in HY environments by selection for plant ideotype in winter and heading date in spring. In later generations, the adapted material can be tested for yield in the LY target environment; in addition, selection for other desirable traits can be done. Since empirical selection for yield in LY Mediterranean environments is most efficient if selection is carried out in representative LY environments, using adapted germplasm, the proposed combination of ideotype breeding and empirical yield selection seems to be efficient: it combines a low risk of losses of adapted germplasm in early generations with a relatively efficient empirical selection in later generations.

The proposed selection procedure is easily applicable and can be used for many crops and types of stress environments. Essential is, before yield testing, the identification of a plant ideotype which is adapted to the dominant stresses in the LY target environment. Because landraces are often adapted to the local environment, landraces may be very useful in this identification.