Morphological and Physiological Basis for Rice Drought Tolerance
Authors: Kuldeep Jangid¹, Pukh Raj², Arvind Kumar2 and Ravi Kumar^2
1Research Scholar, Department of Plant Breeding and Genetics, CCSHAU, Hisar
²Research Scholar, Department of Plant Breeding and Genetics, SKN College of Agriculture, Jobner

Rice is an important staple food crop in world. Plant growth and productivity of rice is adversely affected by various biotic and abiotic stress factors. Water deficit is one of the major abiotic stresses, which adversely affects rice growth and yield due to that rice is targeted for improving drought tolerance. Rice cultivation requires large quantities of water for cultivation. At present we require those varieties or cultivar of rice which are drought tolerant due to the drought problem increasing day by day. Drought tolerant rice plants should be able to grow in small amounts of water and can produce equals to wetland rice. Water stress is a limiting factor in rice production by preventing the crop from reaching the genetically determined theoretical maximum yield. In rice, a better understanding of the morphological and physiological basis of changes in water stress resistance could be used to select or create new varieties of rice to obtain a better performance under water stress conditions. Drought stress in rice affects the crop in different ways. Drought stress is considered to be a loss of water, which leads to stomatal closure and limitation of gas exchange. Drought stress is characterized by reduction of water content, diminished leaf water potential, turgor pressure, stomatal activity and reduction in cell enlargement and growth. Severe water stress may result in the arrest of photosynthesis, disturbance in metabolism and finally the death of plant (Jaleel, et al., 2008c).

The reduction in soil moisture may have led to lower water content in the leaves causing guard cells to loose turgor pressure and hence the size of stomatal pores are reduced (Tezera, et al., 2002) and/ or causing stomatal closure. In addition, increased stomatal resistance may have led to reduced water transport in the leaves further causing a decrease in stomatal conductance. Reduction in stomatal conductance decreases transpiration by closing of the stomata, resulting prolong the plant survival by extending the period of availability of essential soil water reserves in the root zone. Stomatal closure also helps to maintain high leaf water content and thereby a higher leaf water potential, which leads to a reduction in photosynthetic activity (Hsiao, 1973). Higher photosynthetic rates could increase biomass and crop yield. The presence of cuticular wax is also important for water stress and is more in dry land adopted rice as compare to irrigated rice, resulting the leaves are thick and leathery which prevent the water loss from the surface of the rice plant.

In rice, leaf rolling character and death of leaves are good criteria found useful in assessing levels of drought tolerance in a large scale screening. Leaf of any crop plant frequently rolls when plants is suffering from water stress condition. When leaf temperature is increases, the stomata become close and transpiration rate decreased sharply with leaf rolling. Leaf rolling scored visually in rice either in the morning or mid day. Delay leaf rolling is used as an important selection criterion for drought tolerance in rice, which could be improved by incorporating the gene(s) into those lines/ varieties that perform better under irrigated condition but not well under water stress condition. A plant having the characteristics of delay leaf rolling under water stress and faster recovery rate after removing the water stress in rice (Singh and Mackill, 1991) was considered as good trait because flag leaf in rice crops plays the important role in grain filling and development. Therefore, the selection proceeds to identify genotypes which had almost erect flag leaf enabling photosynthesis for longer duration.

Type of root system is also a good selection criterion for selecting the drought tolerance line or varieties. Deep root system has been identified as the targeted for drought tolerance improvement (Boyer, 1996). Production of root system under drought is very important and had good correlation with yield under moisture stress (Darofeev and Tyselano, 1982). Cultivars having deep and thick roots are good for drought stress condition and are positively correlated with xylem vessel area, which are vital to the conductance of water from soil to the upper parts of the plants to meet the evaporative demand. Large and vigorous root system and the continued production of new root hairs are required for maximum response to nutrients supply and optimum environmental conditions and that this positively correlates with the dry matter accumulation within the shoot (Willumsen, 1993). Drought affected plants generally exhibit small root system configuration and in many causes the reduction in size of root system is directly proportional to the magnitude of water storage.

High dry weight under water stress conditions is a desirable characteristic for survivability of the plant under water stress condition.


Mitigation strategies :

Development of early maturing rice varieties to escape the drought. Development of drought tolerance varieties that perform better under drought stress condition. Improvement and incorporation of those characteristics which are essential for survivability of rice plants under water stress condition like deep root system, leaf rolling, cuticle wax, position of stomata and rapid recovering ability etc. Root traits are key components of plant for adaptation under drought condition. Root depth and extension into deep soil is important for crop performance under limited water supply if there is moisture available at deep soil level. Leaf rolling is used as an important selection criterion for drought tolerance in rice. Lines with good drought tolerance have leaf rolling character in stress condition to prevent the water loss. Therefore, the selection proceeds to identify genotypes which had almost erect flag leaf enabling photosynthesis for longer duration. Drought stress affects the growth, dry mater, yield and its associated traits in rice plant but the lines/ genotypes of rice having genetic potential for tolerance to water stress this menace varies remarkably. A morphological traits viz., deep root system, cuticular wax, stomatal activity, leaf rolling character, high tissue water potential, membrane stability, rapid recovering ability after water stress etc. has been implicated in the drought tolerance and high biomass production primarily due to its ability to extract more water from soil and its transport to aboveground parts for photosynthesis. In addition to these factors, changes in photosynthetic pigments, production of biochemichals are also importance to drought tolerance. Thus, these morphological and biochemical characters may be cosidered during the development of drought tolerance varieties.

References:

1. Boyer, J.S. 1996. Advances in drought tolerence in plants. Adv. Agron., 56: 187-218.
2. Darofeev, V.F. and Tyselano, A.M. 1982. Number of seminal roots in spring wheat in the course of selecting pair for hybridization. Vestnik Selokokhozysist., 8:50-56.
3. Jaleel, C.A., Manivannan, P., Lakshmanan, G.M.A., Gomathinayagam, M. and Panneerselvam, R. 2008c. Alterations in morphological parameters and photosynthetic pigment responses of Catharanthus roseus under soil water deficits. Colloids Surf. B: Biointerfaces, 61: 298-303.
4. Singh, H. and Mackill, K.T. 1991. Senstivity of rice to water deficit at different growth stages. Phil. Crop Sci. 16(suppl. No. 1): S11.
5. Tezara, W., Mitchel, V., Driscul, S.P. and Lawlor, D.W. 2002. Effects of water deficit and its interaction with CO2 supply on the biochemistry and physiology of photosynthesis in sunflower. Exp. Bot., 53: 1781-1791.
6. Willumsen, J. 1993. Assessment of fluctuations in water and air content of pot substrates during plant growth. Acta. Hort., 295: 249-259.




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