Response of pearl millet to water stress during vegetable growth
Tfwala, Cinisani Mfan'Fikile
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Pearl millet (Pennisetum glaucum [L.] R. Br.) is a drought tolerant cereal crop planted mainly in arid and semi-arid regions of the world. Water stress still remains one of the challenges facing agriculture. Crops face water stress at various stages due to low and erratic rainfall in arid and semi-arid regions. The response of two pearl millet lines (GCI 17 and Monyaloti) to water stress during vegetative growth was investigated at University of Free State, Department of Soil, Crop and Climate Sciences experimental farm at Kenilworth during the 2009/2010 growing season. The two pearl millet lines were grown under three irrigation treatment levels, namely full (IR3) moderate stress (IR2) and rainfed (IR1). A line source sprinkler system was used to irrigate the experiment. Stressed plants of GCI 17 were about 30% shorter than irrigated plants. For Monyaloti, the stressed plants were 25% shorter than irrigated plants. The highest leaf area index (LAI) of 7.9 was found in IR2 plants of GCI 17 at 7 weeks after planting while the stressed plants of this line attained a highest LAI of 3.6 at 8 weeks after planting. The highest LAI attained by Monyaloti was 9.5 in IR2 plants at 8 weeks after planting and the stressed plants attained a highest LAI of 4.7 during the 9th week after planting thus showing that mild water stress caused a delay in canopy development and limited the size to about half. However, the number of tillers and leaves on the main shoot were not affected by water deficit conditions. The leaf water potential measured by the pressure chamber showed some difference between irrigated and stressed plants after 3 days of withholding rain of 5.6mm from stressed plots. The differences in water potentials of stressed plants and irrigated plants were increasing simultaneously with water stress progression. The water potential of GCI 17 dropped to as low as -1.83 MPa on water stressed plants after 11 days of withholding rain. The leaf water potential for Monyaloti remained significantly higher in the corresponding irrigation treatments. The diurnal changes of leaf water potential showed well watered GCI 17 plants to have water potential of -1.08 MPa around midday while the stressed plants had lower potential of -1.75 MPa. Well-watered plants of Monyaloti had leaf water potential of -0.76 MPa while their stressed counterparts had -1.05 MPa. The seasonal stomatal conductance did not show differences between the pearl millet lines. Stressed plants had lower stomatal conductance values than the irrigated plants, which was also more pronounced as water stress progressed. The stomata of GCI 17 were partly closed for the whole day as revealed by diurnal stomatal conductance. For Monyaloti even the stressed plants had their stomata wide open in the morning and became partly closed by 1300hrs and during the rest of the afternoon. On day 16 after withholding rain (17th February 2010) from water stressed plots, GCI 17 plants had relative water content (RWC) of 72.7% while the well watered plants had 90.3%. Water stressed Monyaloti plants were at 82.8% RWC while the well-watered plants had a RWC of 92.9%. The RWC of stressed plants was continuously decreasing with progress in water stress. The osmotic potential at full turgor was -1.62 MPa for well-watered plants of GCI 17 while -1.83 MPa was measured in the water stressed plants of this line. For Monyaloti, well-watered plants had osmotic potential of -1.11 MPa compared to -1.47 MPa for water stressed plants. At turgor pressure equal to zero, GCI 17 plants from stressed and well-watered plots did not show any adjustments as they were about similar (-2.22 and -2.27 MPa respectively). For Monyaloti water stressed plants had potential of -1.72 MPa and well-watered plants had -1.61 MPa at turgor pressure equal to zero showing an osmotic adjustment of 0.11 MPa. The density of stomata was found to be lower on water stressed plant leaves than on well-watered plants. The abaxial surfaces of pearl millet leaves were found to have lower densities than the adaxial surfaces. The stomata areas calculated from the length and width of the stomata were larger on the adaxial surfaces of GCI 17 plants than those found on the abaxial surfaces. The opposite of this was observed in Monyaloti. The plant height, LAI and biomass accumulation for the two pearl millet lines were found to be lower in water stressed plants when compared with irrigated plants. Monyaloti plants were taller, had higher LAI and accumulated more biomass than GCI 17 plants at corresponding water treatment levels, showing that Monyaloti was less affected by water stress. It was also observed that water stressed plants have lower leaf water potential when compared to irrigated plants. The leaf water potential was maintained higher in Monyaloti plants compared to GCI 17 plants and the same effect was seen with the stomatal conductance which was also lower in water stressed plants than irrigated plants in the pearl millet lines. The highest growth was observed for IR2 plants. Thus from all of growth and physiological field measurements it can be seen that Monyaloti is better adapted to the water stress conditions. It will continue to grow and produce a crop despite the mild water stress due to maintenance of leaf water potential and through osmotic adjustment. Further investigation of the effects of age on the leaf water potential, stomatal conductance, RWC and stomatal characteristics in relation to photosynthesis was recommended.