1 Institute of Soil and Water Conservation, Chinese Academy of Sciences, Shaanxi 712100, RP.China
2 Northwest Sci-Tech University of Agriculture and Forestry, Shaanxi 712100, RP.China
3 Arid Land Research Center, Tottori University, Hamasaka 1390, Tottori 680, Japan
4 CSIRO Plant Industry, Wembley, WA 6913, Australia
Increasing crop water use efficiency (WUE) and drought tolerance by genetic improvement and physiological regulation may be one means to achieve highly efficient use of water. In this paper we discuss the molecular mechanisms, physiological principles, strategies and future perspectives of plant biological water-saving and highlight some approaches of plant biological water-saving which could contribute not only new water-saving techniques but also scientific base for application of water-saving project.
Biological water-saving, water use efficiency, drought tolerance, physiological regulation,
About 40% of the land in the world is under arid and semiarid climatic conditions (Gamo 1999). The sufficient use of precipitation and optimization of crop use efficiency are important in such conditions. In China, water limited farm land occupies above 70% of total arable land, located mainly in Northern part of China, in particular Loess Plateau and surrounding area, which accounts for about 70% of China’s total dry land. The soils in Loess Plateau region are characterized by relatively high runoff / rainfall ratio and much of the rainwater evaporates through the soil surface between rainstorms. Therefore, soil management to minimize both water runoff and evaporation is the primary objective in rain-fed farming under these regions.
Periods of drought alternating with short periods of wet conditions are common to many semiarid areas of the world. Typical semiarid area in China, for instance, characterized by water shortage and low productivity, has special natural conditions and ecological environment. Annual precipitation in the area is about 350-550 mm and the percentage of its distribution in the spring, summer, autumn and winter are 12～15%, 46～65%, 20～35% and 1～3%, respectively. Rainfall, in the form of storms, occurs mainly in the period from July to September, characterized by irregular distribution and intensity (Shan and Chen 1993). Plant response to this water deficit and variable environments is complex and uncertain, because such conditions can cover different situations including variable frequency of drought and wet periods; variable degrees of drought; speed of onset of drought conditions; and varying patterns of soil water deficit and/or atmospheric water deficit (Deng et al. 2003). A crop’s sensitivity to the dissimilar drought patterns varies during different growth stages of its life cycle. Drought tolerance in terms of yield is a complex trait at the whole plant or crop level, with a range of adaptation pathways and physiological mechanisms in the varied types of ‘drought’ environments that occur.
Reduced cell expansion carries a primary effect on meristematic development of yield components, such as the inflorescence or the tiller initial in the cereals, leading to potentially small reproductive organs and reduced yield. This is an irreversible structural effect that is difficult to be amended by re-watering. It can however be amended to some extent by inter-organ compensation following watering, such as tillering in the cereals. The meristematic tissues are generally positioned within the plant in a relatively protected environment as compared with that of fully expanded leaves and therefore it may take a severe stress for meristems to lose turgor.
Under the water deficit conditions photosynthesis responds variably under different drought patterns and rates of drying. Under gradual soil drying conditions, crops exhibited higher photosynthetic rate than under fast soil drying conditions. In the former, osmotic adjustment increased to a certain extent while under the latter process it remained constant. Osmotic adjustment allows for maintenance of photosynthesis and growth by stomatal adjustment and photosynthetic adjustment (Turner 1986; Shangguan et al. 1999).
The reported evidence showed that under mild and/or moderate soil water deficit conditions, photosynthetic depression was caused by stomatal closure or stomatal limitation, but not by biochemical reactions. However, under severe soil water deficit conditions, non-stomatal factors including some limiting enzymes could have been responsible for the decline in photosynthetic capacity (Du et al. 1998). Midday declines in photosynthesis were mainly induced by severe vapor pressure deficit (VPD), and stomatal limitation was suggested as a major cause (Xu and Shen 1997). Under natural semiarid conditions, however, this decline usually resulted from soil water deficit that induced a decrease in leaf water potential at midday. Deng et al. (2000) reported that both soil water deficit and high VPD simultaneously induced the midday depression in photosynthesis, indicating that both stomatal and non-stomatal limitations were responsible for photosynthetic decline in spring wheat under the semiarid environment.
Drought stress during cereal grain development reduces the duration of grain filling. If the rate of grain filling is not adjusted upward, final grain weight is reduced. Increased grain growth rate under drought stress depends on the supply of assimilates. This supply is becoming short due to the inhibition of current photosynthesis during stress. An alternative source of assimilates are pre-anthesis stem reserves in the form of sugars, starch or fructans, depending on the species. These reserves are readily utilized for grain filling and their availability may become a critical factor in sustaining grain filling and grain yield under drought stress.
According to the available data, it is suggested that the order in which crop physiological processes are serially affected by drought seems to be growth, stomatal movement, transpiration, photosynthesis and translocation (Shan and Chen 1998; Deng et al. 2000).
Many studies have looked at the yield losses associated with drought at different stages of plant development. Villarreal et al. (1999) showed that crown root initiation and anthesis are the two stages at which yield losses from drought stress can be most critical to wheat.
Deng et al. (1995) showed that, in the Guyan County of the Ningxia Uh Autonomous Region in China, where the annual precipitation was 450mm and the annual mean temperature was 6.5℃, the optimum time for limited irrigation in spring wheat was the jointing stage. With a single irrigation of 600 m3/ha applied at the jointing, booting and grain filling stages, respectively, yields up to 75% of the highest yield were recorded only at the jointing stage. The water deficit critical period and the optimum irrigation time in wheat, however, are not at the same growth stage. It seems essential to make a distinction between the critical growth stage at which yield is greatly reduced by drought from that one at which supplemental irrigation results in the highest yield improvement. Crop’s response to drought stress and the degree to which yield is reduced vary across the different growth stages of the crop, especially those closely related to yield formation.
In recent years, crop breeders are looking forward for achieving positive outcomes from research on drought tolerance. Despite crop’s poor performance in some fields, it is undergoing changes in the molecular structure of plants as they respond to extremely dry conditions and any variation offers potential keys to fortify drought tolerance. Drought conditions severely reduce yields, but such extremes are vital for studying the genetic differences of crop species and varieties in yields under water stress.
With the methods of gas exchange and carbon-13 stable isotope, Zhang and Shan (1998) demonstrated that sequence of WUE in modern wheat cultivars is irrigated varieties → varieties of both irrigated and dry land → dry land varieties. Zhang et al. (2002) showed that in wheat evolution from 2n→6n, WUE at whole plant level increases with the increase of ploidy chromosomes, root system size and root/shoot ratio of wheat decrease with the increase of ploidy chromosomes under drought and irrigated conditions. Root system growth has an adverse redundancy for WUE, and the root redundancy reduces with the increase of ploidy chromosomes, which result in the increase of wheat WUE at whole plant level. These results suggested that use genetic breeding to uncover water-saving potential of wheat is possible.
Loomis and Connor (1992) suggested that there are three strategies available to improve the water use of crops in dry areas. The first is to maximize crop evapotranspiration (ET), the second is to maximize crop transpiration, as a fraction of total evapotranspiration while the third is to maximize crop WUE. Deng et al. (2002) demonstrated that suitable supplementary irrigation water to satisfy the highest crop WUE was 100 mm. The limited supplementary irrigation water to meet the optimal irrigated WUE is 60 mm. In the case of 60 mm supplementary irrigation, crop water consumption, water use efficiency and irrigation efficiency were much improved synchronously. Limited irrigation induced a significant compensatory effect on wheat WUE. Liang et al. (2002) demonstrated that the drying-rewatering alternation had a significant compensatory effect that could reduce transpiration and keep wheat growing and WUE significantly increasing under drought conditions. According to our recent year research results, crop compensatory effect contributed to the biological water-saving is summarized in Fig. 1.
Figure 1. Compensatory effects of crop plants adapted to moderate water deficit.
The nutrients that are found to be most limiting in the loess hilly region of China are N and P (Shan and Chen 1993). The deficiency is really a problem of runoff (Wei et al. 2000). The yield and WUE increase from added N were observed in several dryland areas where crops were grown on the same land for several years (Shan and Chen 1993). Liu et al. (1998) indicated that maximum yield and highest WUE were achieved under the optimum fertilizer input of 90 kg N and 135kg P2O5 per ha in the semiarid field conditions of loess hilly area in Ningxia. Increased soil fertility was positively correlated with grain yield and WUE of spring wheat, with correlation coefficients of 0.96 and 0.89. Increasing fertilizer level significantly increased fertile spikelet number, kernels per spike and kernel weight. Fertile spikelet number was sensitive to nutrient supply. Fertilizer applied in spring wheat improved root system extension and especially enhanced root growth in the cultivated soil layer of 0-20 cm. The ameliorated root system was able to improve crop water use and nutrient absorption and hence, crop yield and WUE was increased. Their study highlighted the compensatory effects of raised inorganic nutrition on the high efficient use of limited water in dry land wheat production.
This paper was supported by The Major State Basic Research Development Program of People’s Republic of China (G1999011708).
Deng XP, Shan L, Inanaga S (1995) High efficient use of limited irrigation water by dry land spring wheat. Agricultural Research in the Arid Areas, 13(3): 42-46 (in Chinese with English abstract).
Deng XP, Shan L, Ma Y, Inanaga S (2000) Diurnal oscillation in the intercellular CO2 concentration of Spring Wheat under the Semiarid Conditions. Photosynthetica 38:187-192.
Deng XP, Shan L, Kang SZ, Inanaga S, Ali MEK (2003) Improvement of wheat water use efficiency in semiarid area of China. Agricultural Sciences in China, 2(1): 35-44.
Du YC, Nose A, Wasano K, Uchida Y (1998) Responses to water stress of enzyme activities and metabolite levels in relation to surcrose and starch synthesis, the Calvin cycle and C4 pathway in sugarcane (Saccharum sp.) leaves. Australian Journal of Plant Physiology 25:253-260.
Gamo M (1999) Classification of arid regions by climate and vegetation. Journal of Arid Land Studies, 1: 9-17.
Liang ZS, Zhang FS, Zhang JH (2002) The relations of stomatal conductance, water consumption, growth rate to leaf water potential during soil drying and rewatering cycle of wheat. Botanical Bulletin of Academia Sinica, 43:187-192.
Liu ZM, Shan L, Deng XP, Inanaga S, Sunohara W, Harada J (1998) Effects of fertilizer and plant density on the yields, root system and water use of spring wheat. Research of Soil and Water Conservation, 5(1): 70-75 (in Chinese with English abstract).
Loomis RS, Connor DJ (1992) Crop ecology. Cambridge University Press, Cambridge.
Shan L, Chen GL (1993) The Principle and Practices of Dry Land Farming on the Loess Plateau. Chinese Academic Press, Beijing (in Chinese).
Shan L, Chen PY (1998) Eco-physiological bases of dryland farming. Chinese Academic Press, Beijing (in Chinese).
Shangguan ZP, Shao MA, Dyckmans J (1999) Interaction of osmotic adjustment and photosynthesis in winter wheat under soil drought. Journal of Plant Physiology 154: 753-758.
Turner NC (1986) Crop water deficits: a decade of progress. Advances in Agronomy 39:1-51.
Villarreal RL, Mujeeb-Kazi A (1999) Exploiting synthetic hexaploids for abiotic stress tolerance in wheat. pp. 542-552. In: Regional Wheat Workshop for Eastern, Central and Southern Africa, 10. CIMMYT, University of Stellenbosch, South Africa; Addis Ababa, Ethiopia.
Wei XP, Wang QJ, Wang WY (2000) The Loss of Soil Nutrients on Loess Plateau Affected by Precipitation. pp. 176-184. In: Laflen, J.M., Tian, J. & Huang C. (eds), Soil Erosion & Dryland Farming. CRC Press, New York.
Zhang SQ, Shan L, Deng XP (2002) Change of water use efficiency and its relation with root system growth in wheat evolution. Chinese Science Bulletin, 47(22): 1879-1883.
Zhang ZB, Shan L (1998) Comparison study on water use efficiency of wheat flag leaf. Chinese Science Bulletin, 43(14): 1205-1209