The effect of puccinia striiformis on some aspects of photosynthesis of wheat leaves

The rates of net photosynthesis of leaves of the wheat (Triticum aestivum), variety Joss Cambier, were measured by two methods during the development of yellow rust infection. The rate was determined initially by measuring the uptake of radioactive carbon dioxide by leaf discs. In later work, the rates of photosynthesis of attached leaves were measured with an infra-red gas analysis system. The rate of photosynthesis of infected leaves decreased significantly when infection of the leaf was well advanced and the fungus was sporulating. A number of physiological processes involved in photosynthesis were investigated in an attempt to elucidate the mechanism by which the fungus affects the photosynthetic system. The rate of dark respiration remained constant in healthy leaves throughout the experimental period. The rate increased, however, in infected leaves 10 days after inoculation, when the leaves were showing signs of flecking, reaching a peak 12 days after inoculation, coinciding with sporulation. The rate of respiration of infected leaves subsequently declined, but was still higher than that of healthy. In contrast, other factors investigated were similar in healthy and infected leaves until advanced stages of infection when there was a general decline. There was a decrease in diffusive resistance 16 days after inoculation, with presumably an associated decreased resistance to CO2 movement into, the leaf and an increased water movement out of the leaf, as the decrease in diffusive resistance was produced by enhanced stomatal aperture and rupturing of the cuticle, as well as by water loss from the fungal mycelium. The photochemical system was unaffected by yellow rust infection; chloroplasts isolated from infected leaves 18 days after inoculation were able to perform both the Hill reaction and photophosphorylation at the same rate as chloroplasts isolated from healthy material of the same age. There was also no difference between thee number of chloroplasts per cell cross-section in healthy and infected leaves, although the amount off chlorophyll per chloroplast was lower in infected leaves than in healthy leaves at a late stage of infection. The activity of ribulose 1,5-diphosphate carboxylase, the first enzyme in the Calvin cycle, was lower in infected leaves from the time of inoculation. However, the importance of this decrease in affecting the rate of photosynthesis is difficult to ascertain as enzyme activity decreased in both healthy and infected leaves before there was any corresponding decrease in photosynthetic rate. The general decline in leaf metabolism in the later stages of infection was very similar to that occurring during senescence, therefore ultrastructural changes in infected and healthy senescing leaves were compared. Degradative changes in infected leaves occurred earlier and progressed more rapidly than those in healthy, senescing leaves. Chloroplast breakdown was rapid in infected cells with the outer envelope of the chloroplast rupturing, releasing the thylakoidsystem into the cell cytoplasm. In contrast, the thylakoid system of healthy, senescing cells disappeared before the chloroplast envelope broke down and the chloroplasts were typically full of large osmiophilic globules, which were probably composed of lipid breakdown products of the thylakoid membranes. It was concluded that yellow rust infection caused enhanced senescence of the infected tissue, with an associated decrease in the rate of photosynthesis. The type of senescence, however, differed from that occurring in healthy, senescing leaves, since it was more rapid and the chloroplast breakdown patterns were different.