Biological Sciences, University of Wales Bangor, Gwynedd LL57 2UW, UK

Background and objectives
The total weight of fungi which infect a host plant is usually small, yet plant growth can be appreciably reduced and the plant form modified. I will consider the manner in which biotrophic fungi alter fluxes of photosynthetically fixed carbon in the host. In the future, the climate will be different: it will be warmer, perhaps drier, and the CO2 concentration will be higher. The latter may be of critical importance to fungally infected plants, since it will alter both the flux into host plants, and the concentration of carbohydrates in source leaves. It may also alter host N status and respiration rates. I ask, how will climate change modify the effect of fungi on host plants?

Results and conclusions
Biotrophic fungi substantially alter photosynthate fluxes, but will themselves acquire only part of the photosynthate. The extent to which each of the three potential sources of assimilate - stored carbohydrate, current photosynthate, and imported photosynthate - are adequate and are used will be evaluated, and the implications for flows of water, N and P addressed. It will be concluded that biotrophs rely to a considerable extent on the continued functioning of the cells invaded by their haustoria, a view consistent with what we know of the metabolic state of those cells [1]. I will address the consequences for the host of a diminished supply of photosynthate and altered flows of other resources.

Finally I will address the question of the probable effects of climate change. The preliminary answer will be illustrated by data from mildewed barley [2] and oats infected with crown rust, grown at both current and twice current atmospheric CO2 concentrations.

1. Farrar JF 1995. Aspects of Applied Biology 42, 81-108.
2. Hibberd JM, Whitbread R, Farrar JF 1996. New Phytologist 134, 309-315.