Estilo Burocrático

Tissue was removed from the young leaves of maize plants which had been planted in the presence or absence of Striga seeds for 11 weeks. It was thought that any differences in ultrastructure between leaves of control and infected plants would be apparent by that time. No shoots of the parasite had emerged from the soil of the infected plants at that time, and the leaf tissue used was healthy-looking, with no chlorotic patches or leaf curling. Leaf tissue was processed (2.12). Because the leaf tissue had been fixed before sectioning, it was possible to get high magnification of the cells and their contents, compared with the fresh sections seen in 5.1 and 5.2.

Figure 5.6 shows electron micrographs of vascular tissue surrounded by bundle sheath and mesophyll cells from leaf tissue of control and infected plants. These sections are typical of those seen under the electron microscope. In the centre of the electron micrograph of control tissue are small thick-walled phloem elements. Below these are the large lumenous xylem vessels. Thick-walled bundle sheath cells surround the vascular tissue; the chloroplasts are packed with starch grains and are arranged towards the outside of the bundle sheath cells. Outside the bundle sheath cells are mesophyll cells (out of the field of view for the control tissue section). In the centre of the electron micrograph of tissue from infected plants are thin-walled distorted-looking phloem elements, which do not appear to have expanded to their full size, as the corresponding cells in the control section have. The xylem vessels are smaller in the infected leaf section, and thinner-walled than in the control tissue. The outside walls of the bundle sheath cells in the tissue from infected leaves are convoluted in appearance, and the walls of the bundle sheath cells are apparently thinner than in the control leaf tissue. The bundle sheath cell chloroplasts are packed with starch grains. Beyond the bundle sheath cells are mesophyll cells, containing small granal chloroplasts. There is httle evidence of sclerenchyma fibres within the vascular bundle, which looks distorted in the infected leaf section, compared with control leaf tissue.

Figure 5.7 compares mesophyll tissue in maize leaves between the vascular bundles of infected and control plants. The sections from control and infected

Figure 5.6: Electron micrographs of ultrathin sections of leaves from control (top micrograph) and infected (bottom micrograph) plants. The vascular tissue is surrounded by bundle sheath cells and mesophyll cells. X: xylem P; phloem BS: bundle sheath cell M: mesophyll cell ch: chloroplast n: nucleus

Figure 5.7; Electron micrographs comparing mesophyll cells found between the vascular bundles from leaf tissue of control (top micrograph) and infected (bottom micrograph) plants. M: mesophyll cell w: cell wall ch: chloroplast g: thylakoid membranes arranged into grana pg: plastoglobuli

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plastoglobuli, but no obvious starch grains. The walls of the mesophyll cells are thinner than the bundle sheath cell walls. At the right hand side of the electron micrograph of the infected section are very thick-walled sclerenchyma cells.

Figure 5.8 examines a mesophyll cell chloroplast from a leaf section of a control plant and from a leaf section of an infected plant. Stacking of thylakoids into grana are common to both chloroplasts. There are many plastoglobuli present, but few starch grains. More plastoglobuh are present in the chloroplast from the infected plant, compared with the control leaf chloroplast, and the interthylakoid lamellae appear to be spaced further apart than those of the chloroplast of the control leaf. Differences in the staining of the sections are probably due to slightly different staining times, rather than because of any differences in stain retention.

Figure 5.9 shows a bundle sheath cell chloroplast from each of a control and infected plant. Not all of the chloroplast is in either electron micrograph; comparison to Figure 5.8 thus demonstrates that bundle sheath cell chloroplasts are larger than mesophyll cell chloroplasts. Both electron micrographs show chloroplasts full of starch grains but with few plastoglobuh. The chloroplast of control bundle sheath tissue contains a greater number of larger starch grains than the chloroplast of the bundle sheath from the infected plant. In both cases the pigmented membranes surround the starch grains, and there are clear regions around the starch. There appear to be more thylakoids in the chloroplasts of infected plants than in the chloroplasts of the control, but this may only be because less starch grains are present.

Figure 5.10 shows electron micrographs of the vascular tissue from control (5.10 a,c) and infected (5.10 b,d) maize leaf tissue. Figure 5.10 a,b is a closer view of Figure 5.6. In the electron micrographs of control leaf tissue the xylem vessels are large and have thick walls. The phloem elements appear rounded; they are smaller than the xylem vessels, but also have thick walls. In the electron micrographs of tissue from the infected plant the vascular tissue firstly appears to be smaller than in the control sections. The xylem elements are not completely rounded, indicating that they have not expanded fully, and the phloem elements appear to be contorted in shape. The outer walls are bent inwards, possibly giving a larger surface area over which to load leaf photosynthate. The area

Figure 5.8: Electron micrographs comparing mesophyll cell chloroplasts from leaves of control (top micrograph) and infected (bottom micrograph) plants, c: cytoplasm pg: plastoglobuli s: starch granule g: thylakoids arranged into grana

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Figure 5.9: Electron micrographs comparing bundle sheath cell chloroplasts from leaves of control (top micrograph) and infected (bottom micrograph) maize plants. s: starch granule w: cell wall pg: plastoglobuli

Figure 5.10: Electron micrographs comparing the arrangement of vascular tissue within sections of leaves of control (a, c) and infected (b, d) plants. X: xylem

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inside the phloem elements appears larger than in the control leaf tissue. The walls of the phloem elements are thinner than in the control sections.

From the electron micrographs of leaf tissue from control and infected plants the most apparent difference is in the structure and arrangement of the cells within the vascular bundle and the thickness of the walls of bundle sheath and vascular cells.

5.5

Discussion:

Effects of

Striga hermonthica on Maize Leaf