Friday, September 5, 2008

Drought tolerance in wheat

Drought tolerance refers to the degree to which a plant is adapted to arid or drought conditions. Desiccation tolerance is an extreme degree of drought tolerance. Drought is one of the most important environmental challenges growers have to face around the world. Droughts are the cause for large grain losses every year, especially in developing countries, and the current trend in global climate change will likely lead to further losses.

At least 60 million ha of wheat is grown in marginal rainfed environments in developing countries. National average yields range from 0.8 to 1.5 t/ha, approximately 10 to 50% of their
theoretical irrigated potential. Rainfall distribution patterns vary considerably among locations and years, and additional stresses may include heat and cold stress, soil micro-element deficiency or toxicity, and a range of biotic stresses. Physiological assessment of drought tolerance characteristics in the field is therefore a complex task.

Traits that improve drought tolerance

1) Large seed size. Helps emergence, early ground cover, and initial biomass.
2) Long coleoptiles. For emergence from deep sowing.This is practiced to help seedlings reach the receding moisture profile, and to avoid high soil surface temperatures which inhibit germination.
3) Early ground cover. Thinner, wider leaves (i.e., with a relatively low specific leaf weight) and a more prostrate growth habit help to increase ground cover, thus conserving soil moisture and
potentially increasing radiation use efficiency.

4) High preanthesis biomass. Potential for vigorous growth prior to heading provides the
opportunity to take advantage of relatively good growing temperatures and moisture availability earlier in the cycle.
5) Good capacity for stem reserves and remobilization. Stored fructans can contribute substantially to grain filling, especially when canopy photosynthesis is inhibited by drought. Traits that may contribute include long and thick stem internodes, with extra storage tissue perhaps in the form of solid stems. In studies where crosses where made between lines contrasting in the solid stem trait, the solid-stem progeny contained more soluble carbohydrate per unit of stem length, though total stem carbohydrate was unaffected due to narrower and shorter stems.
6) High spike photosynthetic capacity. Spikes have higher WUE than leaves and have been shown to contribute up to 40% of total carbon fixation under moisture stress. Awns contribute substantially to spike photosynthesis and longer awns are a possible selection criterion.

7) High RLWC/Gs/CTD during grain filling to indicate ability to extract water. A root system that can extract whatever water is available in the soil profile is clearly drought adaptive, but difficult to measure. Traits affected by the water relations of the plant, such as relative leaf water content (RLWC) measured pre-dawn, stomatal conductance (Gs), or canopy temperature depression (CTD), during the day, and C13 discrimination or ash content of grain or other tissues, can give indications of water extraction patterns.

8) Osmotic adjustment. Adjustment will help maintain leaf metabolism and root growth at relatively low leaf water potentials by maintaining turgor pressure in the cells. Some research suggests that the trait can be assayed relatively easily by measuring coleoptile growth rate of seedlings in polyethylene glycol (PEG) solution.

9) Accumulation of ABA. The benefit of ABA accumulation under drought has been demonstrated. It appears to pre-adapt plants to stress by reducing stomatal conductance, rates of cell division, organ size, and increasing development rate. However, high ABA can also result in sterility problems since high ABA levels may abort developing florets.
10) Heat Tolerance. The contribution of heat tolerance to performance under moisture stress needs to be quantified, but it is relatively easy to screen for.

11) Leaf anatomy: waxiness, pubescence, rolling, thickness, posture. These traits decrease radiation load to the leaf surface. Benefits include a lower evapotranspiration rate and reduced risk of irreversible photo-inhibition. However, they may also be associated with reduce radiation use efficiency, which would reduce yield under more favorable conditions.
12) High tiller survival. Comparison of old and new varieties have shown that under drought older varieties over-produce tillers many of which fail to set grain while modern drought tolerant
lines produce fewer tillers most of which survive

13) Stay-green. The trait may indicate the presence of drought avoidance mechanisms, but probably does not contribute to yield per se if there is no water left in the soil profile by the end of the cycle to support leaf gas exchange. It may be detrimental if it indicates lack of ability to
remobilize stem reserves. However, research in sorghum has indicated that staygreen is associated with higher leaf chlorophyll content at all stages of development and both were associated with improved yield and transpiration efficiency under drought.

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