Proc. Indian Acad. Sci. (Plant Sci.), Vol. 99, No. 6, December 1989, pp. 517-522. (~ Printed in India. Comparison of physiological responses of pearl millet and sorghum to water stress V BALA SUBRAMANIAN and M MAHESWARI Division of Crop Sciences, Central Research Institute for Dryland Agriculture, Hyderabad 500 659, India MS received 27 May 1989; revised 11 September 1989 Abstraet. Physiological responses of pearl millet and sorghum to water stress were compared for better understanding of crop adaptation in drylands. Pearl millet (MBH-110) and sorghum (CSH-6) were raised in pots andwere subjected to short term water stress at anthesis stage. Water stress decreased leal water potential, rates of stomatal conduct photosynthesis and transpiration and transpiration efficiency in flag leaves relatively more in sorghum than in pearl millet. Stomatal conductance and transpiration rates recovered quicker in pearl millet than in sorghum in 24 h after rewatering. accentuated the water stress effects. Transpiration efficiency of pearl millet was more responsive to increase in atmospheric moisture. The physiologicat responses explain the better adaptation of pearl millet to drier regions than of sorghum. Keywords. Water stress; millet; sorghum; transpiration efliciency. 1. Introduction Pearl millet (Pennisetum americanum (L.) Leeke) and sorghum (Sorghum bicolor (L.) Moench) are major dryland cereal food crops and so their productivity is limited by period(s) of water stress of varying intensity during growth. In both the crops, flowering time is the most sensitive stage to water stress in terms of maximum grain yield reduction (Lewis et al 1974; Mahalakshmi et al 1987). Seetharama et al (1984) reported variation between the two crops in responses to water stress environments in phenology, leaf area development, root growth and water use efficiency. Most area of cultivation of pearl millet receives relatively less rainfall than that of sorghum. Peari millet has a lower water requirement over growth period (Frere 1984). It is likely that responses in specific physiological processes under water limited conditions vary between the two crops. It is also of interest to. know the influence of wet or dry condition of growing seasons on crop responses to water stress for better understanding of crop adaptation and productivity in drylands. This paper reports responses in the instantaneous rates of stomatal conductance, photosynthesis and transpiration under water stress and after the stress was relieved at anthesis stage of pearl millet and sorghum in wet and dry seasons. 2. Materials and methods Pearl millet cv. MBH-110 and sorghum cv. CSH-6 were raised in plastic pots inside a net house during wet (rainy) season (June-October) of 1986 and dry season (January-April) of 1987. The seeds were sown on 4 July 1986 and 15 January 1987. Three plants were maintained per pot containing about 20 kg of red soil (Alfisol). They were adequately fertilised for their N and P requirements and were free from 517
518 V Bala Subramanian and M Maheswari pests and diseases. The experiments were conducted in a randomized block design and each pot constituted a replication. At anthesis stage (21 August 1986 and 7 March 1987 in pearl millet, and 6 September 1986 and 20 March 1987 in sorghum), watering was withheld to impose water stress treatment. After two days, when the plants showed signs of wilting in the morning, water stress was relieved by rewatering. Non-stressed plants served as control. The environmental conditions were similar for both the crops during the anthesis stage in the wet (rainy) season. Photosynthetically active radiation (PAR! ranged from 600-1300 pmol m -2 s -t, relative humidity (RH) 60-70% at 1415 h, maximum temperature 29-31~ and vapour pressure deficit (VPD) 1.1-1.4 KPa. In the dry season, the intensity of PAR was more for sorghum (1330-1650) than for pearl millet (1240-1450). Although in the optimum range, the maximum air temperature was slightly higher for sorghum (33-34~ than for pearl millet (32-33~ However, RH (25-40% at 1415 h) and VPD (2'1-2"6 KPa) were more or less similar. Leaf water potential was determined using Pressure Chamber (PMS, Oregon). Rates of stomatal conductance, photosynthesis and transpiration were measured using Li-6000 Portable Photosynthesis System. Instantaneous transpiration efticiency was calculated from the rates of photosynthesis and transpiration. Observations were made in flag leaves between 1000 and 1100 h at the end of water stress (just before rewatering) and on the next day (a day after rewatering) from at least 4 replications and data were analysed statistically. 3. Results Water stress decreased water potential (~kl) (table 1), stomatal conductance (table 2), photosynthesis (table 3), transpiration (table 4) and instantaneous transpiration efficiency (table 5) in flag leaves, relatively more in sorghum than in pearl millet. Differences in ~'L, stomatal conductance and transpiration between non-stressed and water stressed plants were more in the dry than in the wet season. After water stress was relieved by rewatering, ~b L (table I) and rates of the physiological processes (tables 2-5) increased. Pearl millet recovered relatively faster than sorghum in stomatal conductance and transpiration rates. In fact, peari millet Table 1. Effect of water stress and subsequent rewatering on flag leal water potential in pearl rnillet and sorghum at anthesis stage. Leaf water potential (MPal Control - 1-52 - 1-56 - 1" 17-1-07 Stressed ~ - 2.09-2.57-3.06-3.10 Control - 1-47 - 1-49 - 1-12 -0-97 Stress relieved b - 1.45-1-65-1.21 - t.16 LSD at 5% -0-134 -0-134 -0-072 -0-146 ~Observation at the end of water stress (just belore rewatering). bobservation a day after rewatering.
Responses of millet and sorghum to water stress Table 2. Effect of water stress and subsequent rewatering on conductance in pearl millet and sorghum at anthesis stage. 519 flag leaf stomatal Stomatal conductance (cm s- ~) Control 1-0 1-3 2+7 1-9 Stresse& 0-7 0.6 1"4 0'7 Control 0-9 1-5 2-7 1"8 Stress relieved b 0-8 0-9 2.0 1"0 LSD at 5% 0.15 0-15 0-27 0-17 Same notations as in table 1. Table 3. Effect of water stress and subsequent rewatering on flag leaf photosynthesis in pearl millet and sorghum at anthesis stage. Photosynthesis rate (mgco2 m -2 s- 1) Control 1.4 1-7 1.5 1.3 Stressed* 0-3 0-2 0-2 0-1 Control 1-6 1-8 1.5 1-3 Stress relieve& 1.3 1.3 1-0 0-8 LSD at 5% 0-23 0-23 0.15 0-18 Same notations as in rabie 1. Table 4. Effect of water stress and subsequent rewatering on flag leaf transpiration in pearl millet and sorghum at anthesis stage. Transpiration tate (mghzo m -2 s-1) Control 173 259 373 297 Stressed* 148 187 286 177 Control 178 238 427 328 Stress relieve& 186 206 385 259 LSD at 5~ 19.4 19-4 20.9 23-7 Same notations as in table 1. recovered completely in a day after rewatering in @L, stomatal conductance and transpiration in the wet season, whereas sorghum did not fully recover in both the seasons during this period of recovery. In both the crops, however, recovery in stomatal conductance and transpiration rates were relatively more in the wet than
520 V Bala Subramanian and M Maheswari Table 5. Effect of water stress and subsequent rewatering on instantaneous transpiration efficiency in flag leaf of pearl millet and sorghum at anthesis stage. Transpiration.efficiency (mgco2 g- ~ H20) Treatment Pearl rnillet Sorghum Pearl millet Sorghum Control 8.1-6.6 4.0 4.4 Stressed ~ 2.0 1.1 0-7 0-6 Control 9.6 7.6 3-5 4-5 Stress relieved b 7.0 6-3 2.6 3.0 Same notations as in table 1. in the dry season and photosynthesis rate, though increased, was still significantly less than in the non-stressed plants by 24 h after rewatering. 4. Discussion Stomatal conductance and transpiration in pearl millet were lesss affected under water stress and recovered faster upon relief of the stress than in sorghum. Leaf area difference was a likely cause for sorghum to be relatively more affected by water stress. Leaf area at anthesis stage was larger in sorghum than in pearl millet in the same environment (Seetharama et al 1984). However, besides less leaf area which is advantageous during water stress, quicker recovery in stomatal conductance and transpiration rates when water becomes available after a period of stress is probably a physiological basis of better adaptation of pearl millet to drier regions than of sorghum. Ir should be pointed out that the results from pot studies may or may not truly reflect the plant adaptation to stress under field conditions because water stress develops slower under fieid than under pot conditions. Stomatal closure response to decreasing ~k L depends on the rate of development of leal water deficit in sorghum (Jones and Rawson 1979). In pearl millet, rates of rehydration in plants under pot conditions were similar to those found in field (Henson et al 1984). The aim of the present study is limited to comparing the responses of the two crops under similar stress and environmental conditions and the results do bring out the differences in their physiologicai responses to water stress. Under water stress, instantaneous transpiration efficiency (TE) decreased because photosynthesis was more sensitive to the stress than transpiration. Baldocchi et al (1985) observed that TE measured as CO2-water flux rate decreased with increasing stomatal resistance in soybean. Moreover, the internal resistance to CO2 uptake further increased with decrease in stomatal conductance during water stress (Fischer 1981). On the other hand, decrease in transpiration rate under water stress tends to be relatively less because increase in leaf temperature results in steeper humidity gradient between leaf and atmosphere, thus favouring transpiration (Sinclair et al 1975). In spite of regaining ~L on rewatering, there was lag in the recovery of photosynthesis rate. As Boyer (1970) explained, it was likely due to disruption of metabolic components of the photosynthetic process.
Responses of millet and sorghum to water stress 521 Growing seasons influenced both intensity of water stress and recovery after relief of stress. Higher atmospheric humidity in the wet season probably helped stressed plants to maintain relatively higher ~b L than in the dry season. Monteith (1986) attributes part of the response of crops in dry matter production in monsoon climates to an increase in the amount of water vapour in the atmosphere. The TE of pearl millet was more responsive to the increase in atmospheric moisture. The nonstressed plants of both pearl millet and sorghum maintained lower ~bl, stomatal conductance rate and transpiration rate and higher TE in the wet than in the dry season. Increase in transpiration rate in the dry season without any change in the photosynthesis rate results in lower TE in the dry season. In different annual and tree crops, increase in vapour pressure deficit of the atmosphere decreased TE by increase in transpiration (Turner 1986). The decrease in photosynthesis rate relative to that in transpiration tate under water stress was slightly more in the dry season. It indicates that, with increasing atmospheric dryness, photosynthetic process is more susceptible to water stress than transpiration. Within a season, there were differences between the two crops in the rates of the physiological processes of the non-stressed plants. In the wet season, stomatal conductance and transpiration rates were higher and TE was lower in sorghum than in pearl millet. Kanemasu et al (1984) also observed higher stomatal conductance rate in sorghum than in pearl millet under favourable conditions. However, in the drier atmosphere, stomatal conductance and transpiration rates were higher in pearl millet than i sorghum in our study. Coupled with quicker recovery upon availability of soil moisture, these characteristics probably help pearl millet to adapt better to low rainfall regions. References Baldocchi D D, Verma S Band Rosenberg N J 1985 Water use efficiency in a soybean field: Influence of plant water stress; Agric. For. MeteoroL 34 53-65 Boyer J S 1970 Differing sensitivity of photosynthesis to low leal water potentials in com and soybean; Pfant Physiol. 46 236-239 Fischer R A 1981 Optimising the use of water and nitrogen through breeding of crops; Plant Soil. 58 249-278 Frere M 1984 Ecological zones and production of sorghum and millet; Proceedings of the lnternational Symposium on Agrometeorolo~y of sorghum and raillet in the semi-arid tropics, ICRISAT, Patancheru, pp 33-39 Henson I E, Mahalakshmi V, Alagarswamy G and Bidinger F R 1984 Leal abscisic acid content and recovery from water stress in pearl millet (Pennisetum americanum (L.) Leeke); J. Exp. Bor 35 99-109 Jones M M and Rawson H M 1979 Influence of rate of development of leaf water deficits upon photosynthesis, leaf conductance, water use efficiency, and osmotic potential in sorghum; Physiol. Plant. 45 103-111 Kanemasu E T, Singh P and Chaudhu U N 1984 Water use and water use efficiency of pearl millet and sorghum; Proceedings of the lnternational Symposium on Agrometeorology of sorghura and pearl millet in the semi arid tropics, ICRISAT, Patancheru, pp 175-181 Lewis R B, Hiler E A and Jordan W R 1974 Susceptibility of grain sorghum to water deficit at three growth stages; Agron. J. 66 589-591 Mahalakshmi V, Bidinger F R and Raju D A 1987 Effect of timing of water deficit on pearl millet (Pennisetum americanum); Field Crops Res. 115 327-339 Monteith J L 1986 Significance of the coupling between saturation vapour pressure deficit and rainfall in monsoon climate; Exp. Agric. 22 329-338
522 V Bala Subramanian and M Maheswari Seetharama N, Mahalakshmi V, Bidinger F R and Singh S 1984 Response of sorghum and pearl millet to drought stress in semi-arid India; Proceedings of the International Symposium on Agrometeorolo y of sorghum and millets in the semi-arid tropics, ICRISAT, Patancheru, pp 159-173 Sinclair T R, Bingham G E, Lemon E R and Allen L H Jr 1975 Water use effieiency of field grown maize du moisture stress; Plant Physiol. 56 245-249 Turner N C 1986 Crop water deficits: A decade of progress; Adv. Agron. 39 1-51