| Corn stalk rot, caused by Fusarium graminearum, gets worse and worse and becomes one of the main factors to limit corn production in China nowadays. As a soil-borne disease, it can be protected effectively by planting resistant varieties. So it is very important to breed varieties resistant to corn stalk rot.In order to understand the relationship between sugar contents, Phenylalanine ammonia-lyase (PAL) activity, peroxidase (POD) activity and resistance of corn to stalk rot, and to explore the resistant mechanism of corn to stalk rot, which might be used to assess the resistance to stalk rot in corn breeding, changes of sugar contents in internode pith tissue of corn stalk, changes of PAL activity, Changes of POD activity and isoenzymes, changes of Chlorogenic acid and lignin related with phenylpropanoid metabolism in corn plants inoculated with F. graminearum were mainly studied.A new method was first designed to indicate the dependence of resistance of corn upon sugar contents in stalk through artificially reducing sugar contents in resistant hybrid and increasing sugar contents in susceptible one.Four corn hybrids, Shanda931, Shanda9, Xinongll and H4vb x 9372, were selected for studies. In three-year previous tests these hybrids had been assessed widely with their resistance or susceptibility to stalk rot and represented varieties of high resistance, medium resistance, medium susceptibility and high susceptibility. The primary results from our studies were as follows:(1) It was first affirmed that sucrose contents in the pith tissue of corn stalk toke a key role in the resistance to stalk rot. High level of sucrose in stalk can keep corn plant under physiological vigor and producing some substances that have certain functions to stop the growth of the pathogen and prevent it from damaging host cell. Then corn plant keeps symptom-free. From the results of our studies we deduced that sucrose itself in the pith tissue of corn stalk could rule as a represser in someway to suppress the pathogen producing enzyme or toxin which can destroy host cells, or it could destroyor inactivate the enzyme produced by the pathogen, or it was not suitable for toxin surviving in it. Those may be key mechanism of corn resistance to stalk rot.(2) It was considered firstly that in the pith tissue of the second internode above ground, the reducing sugar content (X1) or the sucrose content (X2), or the total sugar content (X3, the sum of reducing sugar and sucrose) could be used as indicators of the resistance of corn to stalk rot. However reducing sugar must be measured at grain filling stage, and sucrose and total sugar must be measured at pre-waxy maturity stage. Then by applying the regression equations Y=252.5739-6.8369X1 or Y=68.2385-1.5702 X2 or Y=72.9206-1.2231X3 (where Y= expected incidence of corn variety expressed as a percentage) resistance to stalk rot can be assessed.(3) It was clarified that the lignin produced originally in corn plant had no relation to corn resistance to stalk rot. It was the reduced lignin produced by the corn plant after the infection of F. graminearum that showed contribution to the resistance only. It was affirmed that lignin deposit in or between cells of corn plant after pathogens' invitation was controlled by POD directly. Lignin deposit process seemed no relationship to PAL.(4) It was proved that corn plant could produce some substances that could suppress the growth of F. gramineanim. One of the substances was chlorogenic acid. What the substances were still need further study in the future. It seemed that the accumulation of chlorogenic acid in corn stalk after the infection of F. gramineanim paralleled with the activity of PAL.(5) It was postulated that PAL activity, or POD activity and the increased number of POD isoenzymes after the infection of F. gramineanim could be used as indicators of resistance of corn to stalk rot.
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