Photosynthesis in stem rust-infected, resistant and susceptible near-isogenic wheat leaves

Secondary leaves of three near-isogenic lines of the wheat (Triticum aestivum) cultivar Prelude, differing in their degree of resistance to the wheat stem rust fungus (Puccinia graminis f.sp. tritici) race 32, were inoculated with uredospores of the fungus 12 days after germination. Photosynthetic O2 evolution, chlorophyll content, 3-phosphoglycerate content, and the gel electrophoretic pattern of thylakoid membrane proteins were investigated in rust-infected and healthy plants of the fully susceptible isoline Prelude, the moderately resistant isoline Prelude-Sr24, and the highly resistant isoline Prelude-Sr5. The rate of net photosynthesis, expressed per unit leaf area, was not significantly changed in any of the three near-isogenic lines during the early stages of infection, then increased slightly in the highly resistant line, while decreasing in the moderately resistant and fully susceptible lines. In the moderately resistant plants, this decrease continued linearly until 8 days after inoculation when no photosynthetic activity was detectable. In the susceptible line, the decrease gradually slowed down around 4–6 days after inoculation and photosynthetic activity remained constant thereafter. The chlorophyll content decreased only slightly during infection in highly resistant plants, but chlorophyll eventually disappeared completely from the infected, moderately resistant plants. The initially similarly strong decrease in chlorophyll content in the susceptible plants gradually came to a halt during the later stages of infection. Expressed per unit chlorophyll, photosynthetic O2 evolution increased slightly in highly resistant plants, remained unchanged in moderately resistant plants, and decreased to a stable level in the susceptible plants. The infection-induced changes in the 3-phosphoglycerate content closely resembled the changes in O2 evolution. No infection-induced changes were observed in the thylakoid membrane proteins from leaves of all three isolines. The results are discussed in relation to previously described changes in the primary metabolism of the three near-isogenic wheat lines after inoculation with the stem rust fungus, namely polyamine content, fructose-2,6-bisphosphate content, respiratory activity, invertase activity, and assimilate contents. Key words: Triticum aestivum, Puccinia graminis f.sp. tritici, O2 evolution, chlorophyll, 3-phosphoglycerate, thylakoid membrane proteins.