EFFECTS OF A SHORT-TERM P-HYDROXYBENZOIC ACID APPLICATION ON GRAIN-YIELD AND YIELD COMPONENTS IN DIFFERENT TILLER CATEGORIES OF SPRING BARLEY

The aim of these experiments was to evaluate how thresholds for phytot oxic substances obtained in seedling bioassays relate to yield losses or changes in yield components of mature barley crops after a short-te rm exposure to p-hydroxybenzoic acid. Under laboratory conditions a tr eatment with 1.81 mM p-hydroxybenzoic acid significantly reduced the r adicle length of barley, whereas coleoptile elongation was less sensit ive. The inhibition of the radicle length and coleoptile elongation wa s greater if the pH of the test solution was not buffered at pH 5.5. I n a glasshouse trial the effect of p-hydroxybenzoic acid on the radicl e and coleoptile elongation of spring barley was compared with the yie ld response after a three day exposure either during germination or at the double ridge stage of apex development. Applications of 0.72 mM, 1.44 mM and 3.62 mM p-hydroxybenzoic acid averaged over the treatments during germination or at the double ridge stage of development caused a yield reduction in the single ear weight of 5%, 13% and 19% in comp arison with the control, respectively. The higher tiller categories in general showed a greater sensitivity towards an application of p-hydr oxybenzoic acid and, therefore, could not compensate for the yield dec rease of the main stem tiller. A single application of p-hydroxybenzoi c acid either at germination or at the double ridge stage may cause yi eld losses, as reported from no-till systems or cereal monocultures. T he data have implications for the interpretation of seedlings bioassay s in allelopathic research and their applicability in estimating yield losses caused by phytotoxic substances.