Water stress and drought

Water Scarcity, Water Stress and Drought, Expressions of Change in River Ecosystems. .. 16... 

The results have shown that under the given drought conditions. In addition to the altered water capacity, both maize lines suffered the changed photosynthetic processes. It was further shown that the fluorescence may be a very sensitive parameter of water stress and thus be used successfully for the determination of plant drought susceptibility. Further Investigations Into relationships of fluorescence parameters and photosynthetic activity are necessary, so as to acquire a quantitative dimension of the photosynthesis by means of a fast and undestructive measurements of chlorophyll fluorescence in vivo. 

Consistent with conductance values, transpiration rates were much higher in Seaton Park under irrigation and closer to Clare ones under drought. Good agreement was presented between water economy indexes and photosynthetic parameters so transpiration efficiency and 1-Ce/Ca index values were always higher in Clare, both in water stressed and nonstressed plants at irradiation levels above and bellow 1000 pE m s (table 4). ... 

A positive feedback between vegetation and atmospheric CO2 will occur if biomass declines. This will happen to the extent that climatic warming causes increased water stress, either through decreased precipitation or increased evap-otransporation, particularly on soils of low water-holding capacity. Decreases in soil nutrient availability, either directly caused by drought or indirectly caused by replacement with taxa with more recalcitrant litter, may further decrease the net release of carbon from the biosphere to the atmosphere. Positive feedback will also arise if the current standing biomass of trees is replaced by small trees, shrubs, and herbs that store less carbon. 

Kramer, P.J. (1980). Drought stress and adaptation. In Adaptation of Plants to Water and High Temperature Stress, ed. N.C. Turner P.J. Kramer, pp. 6-20. New York Wiley. 

Drought is perhaps one of the most complex examples to choose but it illustrates well the possibilities of, and pitfalls to, progress. Drought affects almost every facet of plant function and we are faced with the paradox that yield and evapotranspiration are intimately linked. In general, increases in yield when water supply is limiting are likely to result from characteristics which increase the available water supply, increase water use efficiency or increase biomass allocation to the economically useful plant parts (Pass-ioura, 1986). Additionally, features which maintain cell viability and protect metabolism in water-stressed tissue and allow rapid recovery after dry periods will contribute yield under some circumstances. 

Drought also has a profound effect on protein synthesis. In many plant tissues, a reduced water potential causes a reduction of total protein synthesis and a rapid dissociation of polyribosomes. The latter has been shown not to be the consequence of increase in ribonuclease activity (Hsiao, 1973 Dhindsa Bewley, 1976). For a specific protein, Jacobsen, Hanson Chandler (1986) have shown in barley leaves that water stress enhances the synthesis of one of the a-amylase isozymes. Using a cDNA probe they found that water-stressed leaves contained much more a-amylase mRNA than unstressed plants. 

Therefore drought resistance (in terms of plant production) is positively associated with T or Ea, m or WUE and HI under drought stress. A simple simulation using (1) would indicate that a given increase in Tor m is most effective towards plant production when water stress is not severe. 

Surprisingly, very little physiological work has been done to understand the nature and processes of plant recovery from extreme drought stress, especially in relation to plant production (Chapter 7). In order for the plant to recover properly from severe water stress, its various meristems must survive. The association between severe plant stress and the factors that affect meristem survival and function upon rehydration are unclear though osmoregulation may have a possible protective role and as a potential source of carbon for recovery. Active plant apices generally excel in osmoregulation and do not lose much water upon plant dehydration (Barlow, Munns Brady, 1980). 

Agarwal, P.K. Sinha, S.K. (1984). Effect of water stress on grain growth and assimilate partitioning in two cultivars of wheat contrasting in their yield stability in a drought-environment. Annals of Botany, 53, 329-40. 

Vaadia, Y. (1987). Salt and drought tolerance in plants regulation of water use efficiency in sensitive and tolerant species. In NATO Conference on Biochemical and Physiological Mechanisms Associated with Environmental Stress Tolerance in Plants, University of East Anglia, Norwich, 2-7 August 1987. 

Hanson, A.D., Nelson, C.E., Pedersen, A.R. Everson, E.M. (1979). Capacity for proline accumulation during water stress in barley and its implications for breeding for drought resistance. Crop Science, 19, 489-93. 

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