Physiological and agronomic responses of Sudangrass to water stress

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PHYSIOLOGICAL AND AGRONOMIC RESPONSES OF SUDANGRASS TO WATER STRESS

A. Bibi, H. A. Sadaqat*, H. M. Akram**, T. M. Khan*

and B. F. Usman***

ABSTRACT

A study was conducted in Plant Physiology Section, Agronomic Research Institute, AARI, Faisalabad, Pakistan during 2007 to evaluate ten accessions of Sudangrass (Sorghum sudanense) (337, 8614, HOK, 9580, TV-3368, 958, 39501, Succro, 4158 and Line 7) for drought tolerance on the basis of physiological and agronomic attributes. Data were recorded for stomatal conductance (gs), water potential (Ψw), osmotic potential (Ψת), turgor pressure (Ψp), shoot length (SL) and root length (RL) under control as well as water stress conditions. Analysis of variance indicated significant differences among all accessions for all characters. Average reduction in gs, Ψw, Ψת, Ψp, SL and RL was 32.30, 3.32, 1.53, 13.02, 7.28 and 3.47 percent, respectively. Accession HOK comparatively maintained its leaf water relations under water stress conditions. Based upon principle component analysis, proportional contribution of osmotic potential, water potential, turgor pressure, root length, shoot length and stomatal conductance was 38.3, 24.3, 14.7, 7.4, 5.3 and 10 percent for drought tolerance. So proportional contribution of osmotic potential was the highest towards drought tolerance. Osmotic potential could be used as selection trait for drought tolerance as it contributed maximum to all drought parameters. Line 7, 337 and 958 had the highest multivariate scores under water stress, whereas, 9580, 39501 and 8614 had the lowest scores. The most promising and drought tolerant accessions (three accessions with highest scores and three with lowest scores) were screened through multivariate scoring index. KEYWORDS: Sorghum arundinaceum; genotypes; stress; osmotic pressure;

agronomic characters; Pakistan.

INTRODUCTION

Sudangrass (Sorghum sudanense), an important annual summer grass, is planted for pasture, silage, green chop and hay. Importance of sudangrass is expected to increase with sorghum-sudangrass hybrids yielding more forage than either parent. It is originated from the African Center for Diversity. Sudangrass or CVS thereof, is reported to tolerate anthracnose, disease,

*Plant Breeding and Genetics, University of Agriculture, Faisalabad, **Agronomic Research Station, Karore ***Institute of Horticultural Sciences, University of Agriculture, Faisalabad, Pakistan.

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drought, fungus, grazing, high pH, heavy soil, heat, virus and weeds (8). It grows upto 3 meter (10ft) high. The thick and erect stems usually arise in groups from a single clump. The leaves are long and narrow and are arranged at the ends of stems of loose-bending branches. Because of its resistance against long, hot, dry periods of weather, Sudangrass is well adapted to the Great Plains of North America and in more humid areas it is valuable as emergency forage. In regions where the moisture content of soil is low, the grass is usually the principal crop (2). Water deficit is the most important of abiotic stresses often encountered by the plants. Its effects are severe in semi-arid environments where droughts are unpredictable and cause instability in crop production. A period of time without measurable rainfall is termed as drought. It may also be defined as deficiency or dearth of water severe enough to check the plant growth (20). The induction of turgor pressure below the maximal potential pressure is also termed as water stress (21). Drought affects almost all physiological and biochemical processes including water relations, nutrient acquisition, CO2 fixation, etc. The drought induced malfunctions ultimately result in reduced germination, poor seedling establishment and hampered growth and development of plants (11). Drought is a natural calamity and has devastating effects on crop yields. Crop plants are frequently subjected to water stress during the course of their life times. However, certain stages, such as germination, seedling growth, crown root development and flowering are the most critical for water stress damages (1). Drought is a critical issue of the day in Pakistan, as silting of dams has curtailed their storage capacity to an alarmingly low level. The options available to combat the situation could be: the usage of agricultural practices that conserve water, precision agriculture for efficient use of water in agriculture and development of crop varieties capable of growing under water deficit conditions with very little or without any loss in crop productivity and quality. The last option sounds cheaper and durable for its impacts in agriculture of Pakistan. Identification and testing of drought resistant germplasm is arduous and slow process. Drought tolerance is a complex trait; the genetic and physiological mechanisms that condition its expression are poorly understood (9). It is one of the most difficult and seemingly intractable agronomic trait to characterize and study (2). Further, it is difficult to conduct drought stress treatments in a quantitative and reproducible way. These difficulties have significantly impeded research on plant drought tolerance. Consequently, the biological basis for drought tolerance is still largely unknown and few drought tolerance determinants have been identified

Physiological and agronomic responses of Sudangrass to water stress

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(3,6,7,14). The slow pace in revealing drought tolerance mechanisms has hampered both traditional breeding efforts and use of modern genetics approaches to improve drought tolerance of crop plants. The findings of present research will be helpful in further breeding programmes; inter-specific hybridization progamme for production of sorghum-sudangrass hybrids capable of forages throughout the year and in return the health of livestock will be better to provide more milk and meat.

MATERIALS AND METHODS

Ten accessions of Sudangrass (337, 8614, HOK, 9580, TV-3368, 958, 39501, Succro, 4158 and Line 7) were collected from National Institute of Plant Genetic Resource (NIPGR), National Agricultural Research Centre. Islamabad, Pakistan. Ten seeds of each accession were sown during May 2007 at the depth of 1cm in earthen pots filled with sand (10 kg each) and supplemented with Hoagland solution (20 ml/pot) as nutritive media in laboratory of Plant Physiology, Agronomic Research Institute, AARI, Faisalabad, Pakistan. The experiment was laid out in a two factor factorial completely randomized design with three replications. Two factors included irrigated conditions (control) and the water stress conditions. In control block plants were irrigated daily (total 7 irrigations) with 50ml of distilled water per pot. In stress block, stress was simulated by withholding irrigation after one irrigation after sowing, three plants of each accession from each replication in each block were taken ten days after sowing to record the data on stomatal conductance, leaf water potential, osmotic potential, turgor pressure/ pressure potential, seedling shoot length and root length. Counts of stomatal conductance for three randomly selected flag leaves of intact plants from each genotype were measured between 10-11 hours with the help of a porometer. Water potential (Ψw) was measured using pressure chamber; three fully expanded flag leaves were sampled per replicate from each treatment between 11.00 and 13.00 hours. This pressure is balance pressure and is equal in magnitude but opposite in sign, to the negative pressure that existed in the xylem column before the plant tissue was excised.

Plant leaves from each replication were washed in distilled water, blot dried with tissue paper and transferred to eppendorf tubes in deep freezer. The frozen samples were thawed, crushed with glass rod and the sap was centrifuged out at 1100rpm. Osmotic pressure was measured with micro osmometer by calibrating the equipment in m.osmol/kg of water. The pressure was converted into potential by putting a negative sign as prefix to

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the values. The concentration units (m.osmol/kg of water) were converted into pressure units, MPa using Vent Hoff relationship at 200 C (19).

Ψת = - miRT where m is the concentration in molarity of the solute, I is the Van’t Hoff factor, the ionization constant of the solute (1 for glucose, 2 for NaCl, etc.) R is the ideal gas constant, and T is the absolute temperature. Turgor pressure /turgor potential or pressure potential of flag leaves was calculated putting the determined values of water potential and osmotic potential in the equation as (13) follows:

Ψw = (Ψת) + Ψp

Where Ψw = Water potential, Ψת = Osmotic potential and Ψp = Turgor pressure/turgor potential Randomly selected three plants from each genotype per replicate were uprooted carefully after ten days of sowing to measure the seedling shoot length with a ruler in centimeters. The data for root length were recorded from same seedlings uprooted for shoot length again with a ruler in centimeters under normal and water stress conditions.

Statistical analysis /biometrical approach

The recorded data were subjected to analysis of variance (25). Then multivariate scoring index was applied to find out the best accessions performing well under water stress for all six physiological parameters.

RESULTS AND DISCUSSION

Analysis of variance (Table) indicated significant differences among the accessions for all characters indicating the greater genetic variability among the breeding material (10,30). Significant differences were also found for treatments and interaction for stomatal conductance, shoot length and root length which resulted in varying performance under both treatments (control and water stress). It is clear from the graph (Fig.1) that stomatal conductance decreased under water stress conditions. Average reduction in stomatal conductance was 32.30 percent. The highest and the lowest reduction in stomatal conductance was 50.45 percent (accession 8614) and 12.25 percent (accession 337). The results are in accordance with earlier findings (16,27,29) where a decrease in stomatal conductance was reported under Table. Mean squares of analysis of variance for seedling traits of Sudangrass.

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Characters Accessions(A) Treatment (T) Interaction (A x T) Error D. F. 9 1 9 40 Stomatal conductance (gs) 146.32** 527.06** 33.70** 0.36 Water potential (ψw) 3.48** 0.147NS 0.06NS 0.01 Osmotic potential (ψn) 1.71** 0.067NS 0.01NS 0.02 Turgor pressure (ψp) 3.87** 0.016NS 0.07NS 0.07 Shoot length 294.59** 8.52** 20.25** 30.17 Root length 101.15** 5.7** 10.65** 2.29

**Significant at 0.01% probability level.

water stress conditions in sorghum. As stomata tend to close under water deficit conditions to avoid water loss it results in lower photosynthesis and yield (15). Reduction was also observed in water potential under stress conditions (Fig 2). Average reduction in water potential was 0.27 percent (accession HOK). Similar findings were reported earlier (17,24,28,31). This fall in water potential was associated with reduced water supply from the soil to roots and ultimately to the leaves.

The average decrease in osmotic potential was 1.53 percent under water stress conditions. The highest reduction (2.78 %) in osmotic potential was noted in accession 337 and lowest (0.53%) in HOK (Fig 3). Remarkable reduction in osmotic potential has also been observed earlier (24,31). This reduction was mainly due to dehydration and partly related to active accumulation of solutes.

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Turgor pressure was too adversely affected by water stress irrespective of accessions (Fig. 4). Average, the highest and lowest reduction in turgor pressure under water stress 13.02, 43.48 (accession 39501) and 0.90 percent (accession Line 7), respectively. such notable fall in turgor pressure in plants subjected to water stress was also reported previously (24,31).

Physiological and agronomic responses of Sudangrass to water stress

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Accession HOK maintained turgor pressure under water stress conditions by virtue of osmoregulation thus expressing relatively more drought tolerance.

Overall reduction was observed in shoot and root length (Fig. 5-6). But few accessions showed rise in root length under water stress. Increase in root length was observed as a result of water stress in accessions 337, 8614,

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9580 and TV-3368. These findings are in harmony with those of other scientists (4,15,18,24). Soil moisture stress reduced root length by 30 percent (23). Drought stress affected more shoot growth than root growth and in certain cases root growth increased (29). Similarly in seedling root growth was less affected by drought compared to seedling shoot growth and also reduction in coleoptile, shoot and root growth and an increase in root shoot ratio due to water stress was noted. The increase in root length may be due to deeper penetration in search of water. Multivariate scoring index Multivariate scoring was carried out through principle components analysis (Fig.7-8) using replicated data of stomatal conductance, water potential, osmotic potential, turgor pressure, shoot length and root length. Based upon principle component analysis proportional contribution of osmotic potential, water potential, turgor pressure, root length, shoot length and stomatal conductance was 38.3, 24.3, 14.7, 7.4, 5.3, and 10 percent for drought tolerance, respectively (Fig. 7). Line 7, 337, 958 and Succro had the highest multivariate scores under water stress. On the other hand, 9580, 39501 and 8614 had the lowest scores. So, three accessions with highest scores and three with lowest scores were selected.

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CONCLUSION

Generally reduction was observed in all traits in most of the accessions under water stress. An increase in root length was observed in accessions 337, 8614, 9580 and TV-3368. Osmotic potential could be used as selection trait for drought

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tolerance as it contributed maximum towards water stress among all drought parameters.

ACKNOWLEDGMENT Financial assistance provided by Higher Education Commission, Pakistan for this research work is gratefully acknowledged.

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