Greening and development in wheat seedlings

Abstract

The greening of etiolated first leaf blades of wheat (Triticium aestivum Mercia) seedlings (referred to in the text as leaf tissue) was studied in relation to tissue age and water stress. Use was made of whole seedlings, excised leaf blades and leaf blade segments. Responses to photoperiodic illumination were measured as changes in the levels of chlorophyll, total soluble protein and nucleic acids (both total and specific fractions). The pattern of greening in the whole seedlings, excised leaf blades and leaf blade segments was essentially the same in 6 and 10 day-old dark-grown tissues, where chlorophyll accumulation followed the age sequence along the leaf. Least chlorophyll accumulated in the tip of leaves of both ages but the older leaves contained less overall pigment than the younger leaves. Patterns of total soluble protein and total nucleic acid accumulation did not reflect the pattern shown by the chlorophyll. Protein accumulated most in the tip region, with nucleic acids being highest in the middle region. Water stress treatment reduced chlorophyll accumulation in leaf blade tissue, particularly in the intact seedlings. Protein levels, however, were more variable and appeared to reflect the ability of the younger tissue to accumulate this compound as a stress metabolite. Total nucleic acid levels were also elevated under water stress. Again, these effects were most marked on the intact seedlings, implying that an effect on the roots was also involved. The data from polyacrylamide gel electrophoresis of RNA fractions showed that the level of chloroplast RNA components was maintained up to 17 days for tissue incubated in the dark as well as in the light. Severe water stress treatments applied to the roots of whole seedlings resulted in the loss of ribosomal fraction in the leaves. However, this effect was not seen with mild water stress. Kinetin treatments during water stress did not appear to alter the pattern of cell component accumulation although in unstressed material, treatment with this compound enhanced chlorophyll accumulation slightly, especially in the young tissue. This indicates that, at the concentration used, the growth regulator was not able to alleviate the stress condition. The leaf blades of intact seedlings responded in a specific way which was much more pronounced than for excised leaf blades or leaf blade segments. It was concluded that some signalling was involved between the root and shoot tissues during water stress treatments.