Alterations in transpiration and stem vascular tissues of two maize cultivars under conditions of water stress and late wilt disease

Abstract

Two cultivars of maize (Zea mays), Giza 2 (G2) and Population 45 (P45), were grown under natural conditions in pots containing clay loam soil, either infested or not infested with inoculum of Cephalosporium maydis, the causal fungus of late wilt disease. When the plants were 21 days old, they were subjected to two irrigation treatments: irrigation to field capacity, and irrigation after 60% depletion of the available soil moisture at the root zone, i.e. soil moisture stress. Under nonstressed conditions plants of cultivar G2 showed a lower percentage of infection than those of P45. In stressed plants, the percentage of infection increased significantly. Compared with controls (nonstressed, healthy plants), higher percentages of plant dry mass (leaves + stem + cobs) were achieved by all treatments with G2 than with P45, which did not produce cobs under stress conditions. Nonstressed healthy plants had higher transpiration rates and relative water contents than the water‐stressed diseased plants. The leaves of diseased plants had a higher content of the amino acid proline than those of nonstressed, healthy plants, probably associated with water stress due to the disease.Infection resulted in a reduction of the number of the vascular bundles in the cross‐section of the internode. The values of the phloem area per unit leaf area were greatly reduced in water‐stressed and infected plants. In addition, many xylem vessels were occluded in diseased plants but much more in the susceptible P45 than in the moderately resistant G2. Occlusion may be the most important factor causing the symptoms of the disease.The hydraulic conductance (Kh) of the xylem tissue supplying the transpiring areas was calculated from the conduit lumen diameters using the Hagen–Poiseuille relation. The highest Kh values were obtained in the nonstressed, healthy plants, particularly those of P45; plants with both water stress and disease showed the lowest Kh values. The efficiency of xylem conductance was further examined by calculations of the leaf specific conductance and the predicted maximum pressure gradients. Conductance was strongly reduced by infection in both cultivars and there was a corresponding increase in pressure gradients. These observations were consistent with observed reduction in xylem tissues and occlusion of xylem vessels by the infection.