INRA, Thiverval-Grignon, France

Background and objectives
Leaf rust (Puccinia recondita f. sp. tritici) modifies leaf gas exchange. Photosynthesis is reduced owing to carbon metabolism alterations and transpiration is modified by at least two opposite mechanisms: stomatal closure and epidermal rupture. It is well known that a water stress first reduces plant growth and second gas exchange. Water stress is also a factor predisposing the plant to the influence of fungal stresses [1]. This study aims to (1) assess the impact of leaf rust on both leaf area growth and plant water status, and (2) quantify the leaf gas exchange response to interacting fungal stress and water constraint.

Materials and methods
Wheat seedlings (Triticum aestivum, cv. Michigan Amber - very susceptible to leaf rust) were grown in a growth cabinet under controlled conditions. The fifth leaf was inoculated 20 days after sowing with the leaf rust isolate B86-20-3. By submitting half of the plants to water stress (irrigation reduction by a factor of 2), four treatments were obtained: no stress (control, NS), water stress (WS), fungus stress (FS) and water-fungus stress (WSXFS).

Disease severity was assessed by the number of lesions per unit area of leaf. The water stress intensity was characterized by measuring pre-dawn water potential (PWP). Leaf gas exchanges were measured using an open circuit chamber (Li-6400, Li-Cor, USA) for (1) seven radiation levels between 0 and 1500 Ámol/m2.s under external CO2 concentration of 330 Ámol/mol, and (ii) seven external CO2 concentrations between 330 and 0 Ámol/mol under a PAR value of 1500 Ámol/m2.s. Variables calculated were net CO2 assimilation, stomatal conductance for CO2 and CO2 concentration in the air spaces of a leaf.

Results and conclusions
Lesions appeared 8 days after inoculation and their number increased until the day 19. The WSXFS treatment showed twice the number of lesions present on treatment FS. Pre-dawn water potential (PWP) of WS and FS treatments decreased over time in a similar way; on the WSXFS treatment PWP was much lower.

On the fifth leaf, area reduction reached 37% on WSXFS treatment, and 19% and 14% on independent WS and FS treatments. The influence of both stresses was also shown to be cumulative on leaf 7.

In the control treatment (NS), there was no time change in dark respiration, maximal radiation-use efficiency (MRUE) and net assimilation under saturating PAR (Anmax). In the WS treatment, with an increasing water stress, net assimilation and stomatal conductance for CO2 decreased with constant CO2 concentration in the leaf air spaces. On FS and WSXFS treatments MRUE and Anmax decreased over time in relation to growing fungal attack; stomatal conductance for water decreased before sporulation and then increased with concomitant increase of CO2 concentration in the leaf air spaces.

The relationship between Anmax and PWP suggested that plant water loss caused by fungal lesions could explain the net assimilation reduction, which is consistent with the cumulative influence of both water stress and fungal constraint on leaf area expansion.

1. Ayres PG, 1984. Annual Review of Phytopathology 22, 53-75.