THE EFFECT OF THE INTERACTION OF TEMPERATURE AND NITROGEN NUTRITION ON GIBBERELLA ZEAE SEEDLING BLIGHT OF MAIZE
Abstract
Root temperature and nitrogen nutrition influence the growth of maize. However little is known of how these factors interact to influence the development of Gibberella zeae seedling blight. An understanding of the altered metabolism of G. zeae infected plants, induced by temperature and nitrogen nutrition, may help to understand the host-parasite interaction of this disease. Twelve inbreds were tested for resistance to G. zeae. All inoculated inbreds energed at 19 C but all were blighted. Oh43 was the most susceptible inbred to seedling blight at 19 C and nitrate (NO(,3)) and a combination of nitrate and ammonium (N + N) induced the most blight. Isolates of G. zeae differed in pathogenicity to maize at 19 and 22.5 C. Nitrogenous fractions of maize varied in content at different treatments. Free ammonium content was low in both susceptivle (Oh43) and resistant (W64A) inbreds, probably due to rapid assimilation of NH(,4). Ammonium accumulated most in NH(,4) treatments and at 22.5 C. Levels of NO(,3) were generally higher than NH(,4) in all treatments except NH(,4) treatments and increased with age, probably due to an inactive nitrate reductase in older plants. W64A accumulated more free nitrate than Oh43. Total nitrogen accumulated most in both inbreds in NH(,4) and N + N treatments and at the 2-leaf stage. Shoots contained more total nitrogen than roots and total nitrogen was generally higher at 22.5 C. Qualitative and quantitative differences in hydrolysis of amino acid-beta-naphthylamides were detected in both G. zeae and maize. Isolates of G. zeae differ mainly quantitatively in aminopeptidase activity but their aminopeptidase profiles can be used to identify virulent isolates. Repeated subculturing reduces virulence and aminopeptidase activity of G. zeae. Qualitative differences in hydrolysis of amino acid-beta-naphthylamides in maize were detected mainly at 6 hour incubation rather than 24 hour incubation. Total non-structural carbohydrates increased in inoculated Oh43 and W64A seedlings. However, no pattern exists in resistant or susceptible tissues to suggest association of either with total non-structural carbohydrate concentration. Nitrate promoted an increase in free carbohydrates, whereas, the interaction between 28 C and inoculation with G. zeae caused lower percentages of free carbohydrates. Perhaps, nitrate enhancement of photosynthesis was responsible for the increase in free carbohydrates. Ammonium promoted production of higher free amino acid content than NO(,3) or N + N, possibly through enhanced assimilation of NH(,4). The resistant inbred (W64A) responded to inoculation with G. zeae by lowering its pool of free amino acids, unlike the susceptible inbred (Oh43), which generally increased its free amino acid pool. Nitrate reductase activity was localized primarily in shoots, induced by NO(,3) and N + N, decreased in inoculated plants, and increased at higher temperatures. Dry weight values were generally higher in both inbreds at the higher temperatures and in older plants. N + N promoted higher dry weight values at 22.5 C and 28 C, whereas, NO(,3) was the best medium at the 4-leaf stage. Defined nutritional media with different sources of nitrogen and at different temperatures cause changes in the metabolism of nitrogen by the plant which affect the general physiological condition of the plant. These changes are enhanced or reduced in inoculated tissues. However, it is difficult to determine which class of metabolic constituents are basic to disease expression. Carbohydrate metabolism, which is regulated by nitrogen form and pathogenesis, is conjectured as an excellent starting point for answering basic questions about this disease.
Degree
Ph.D.
Subject Area
Plant pathology
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