Competition for nutrients between leaf surface microorganisms and spores of plant pathogens

• Main Author: Brodie, Iain D. S.
• Published: University of Aberdeen 1975
• Subjects: 632
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.592166

The work contained in this thesis investigates competition for nutrients between epiphytic micro-organisms and Botrytis cinerea or conidia of other plant pathogens. The hypothesis that epiphytic bacteria inhibit germination of nutrient-independent conidia by acting as a nutrient sink and competitively deplete the endogenous reserves of the conidia during the germination period, was tested. Both the uptake of 14C-label, originally derived from conidia, and the loss of label from conidia on a model leaching system were followed and compared. It was found that conidia were most susceptible to loss of endogenous reserves early during the germination period (up to 5h). A relatively small loss of endogenous reserves, either to bacteria or in the leaching system, was associated with inhibition of germination. However, carbon dioxide evolution from conidia was decreased in the presence of bacteria, whereas leaching did not cause such a decrease when compared with controls. Tests failed to demonstrate the presence of any compounds produced by bacteria which inhibited the germination of conidia. From these studies it appeared likely that selective competition for particular essential nutrients was limiting the germination of -conidia. This was further investigated in vitro , it being shown that amino acids, at levels which were normal on the leaf surface, stimulated germination, whereas glucose, fructose and sucrose had little effect. When amino acids were present with glucose (again at levels normal on the leaf surface) , the uptake of amino acids by bacteria after 5h was Correlated with inhibition of germination of conidia. This relationship held .for a number of different epiphytic micro-organisms, including yeasts. No such correlation was apparent between inhibition of germination and uptake of amino acids, when this was the sole added nutrient. Similar studies were then carried out on the leaf surface. The leaf itself did not reabsorb a significant quantity of nutrients lost to its surface and a relatively large quantity of soluble carbon compounds was detected in drops, as soon as these were placed on dry leaves. The relationship between amino acid uptake by epiphytic microbes and inhibition of germination obtained from studies, using a tracer quantity of 14C-amino acids, was almost identical to that observed in the in vitro studies. On the leaf, approximately ten times fewer micro-organisms were required to deplete the amino acid levels and. bring about inhibition. Leaves, which had been wetted 24h previously, supported sufficient numbers of epiphytes to deplete the tracer amino acids and inhibit the germination of conidia. When three other plant pathogens were tested, two (Phoma betae 8c Cladosporium herbarum) behaved similarly to B. cinerea. However, the germination of Colletotriehum dematium _f spinaciae conidia was not inhibited and in in vitro tests appressoria were formed more rapidly from conidia in the presence of bacteria than from control conidia.