INVERTASE - UNDERSTANDING CHANGES IN THE PHOTOSYNTHETIC AND CARBOHYDRATE-METABOLISM OF BARLEY LEAVES INFECTED WITH POWDERY MILDEW

Infection of barley leaves with powdery mildew results in an increase in the activity of acid invertase, concomitant with an accumulation of glucose, fructose and sucrose in the infected leaf; this increase is confined to the mesophyll cells. The rate of photosynthesis is control led by different factors depending upon the experimental conditions un der which it is measured. In saturating light and ambient CO2, photosy nthesis is determined to a large extent by Rubisco whereas, in saturat ing light and saturating CO2, it is mainly determined by the rate of e nd-product synthesis (Pi-limitation). The rate of photosynthesis was m easured under these conditions to reveal which of the partial processe s was most affected in mildewed leaves. Under conditions of saturating light and ambient CO2, the rate of photosynthesis declined in mildewe d leaves from 3 d after inoculation, suggesting that carboxylation had been affected. However, the maximum capacity for photosynthesis, meas ured at saturating CO2 and irradiance, increased in mildewed leaves fo r the first 3 d after infection and then decreased to below control va lues on days 5 and 7, suggesting that Pi was not limiting photosynthes is. This hypothesis was investigated by measuring changes in photosynt hetic intermediates and in the activity and amount of key enzymes of t he Calvin cycle as infection progressed. There was a decline in the ac tivity of the stromal fru-1, 6-bisPase, Rubisco and NADP-GAPDH in mild ewed leaves. These decreases were consistent with changes in the ratio of metabolites. As infection progressed, there was an increase in the ratio of Rul,5P(2):PGA and triose-P:Rul,5P(2), the first indicating a restriction in the carboxylation of CO2 and the second a restriction in the regeneration of Rul,5P(2). The PGA:triose-P ratio was similar i n control and mildewed leaves until day 7 when it decreased, suggestin g that the reduction of PGA to triose-P was not affected by the diseas e. There was little evidence of Pi-limitation in the mildewed leaves; the amount of Pi in infected leaves was similar to controls, infected leaves showed no secondary oscillations during a transition from dark to light and there was a reduction in the amount of PGA in infected le aves. We suggest that the high concentration of carbohydrates, resulti ng from the increase in invertase activity, causes directly or indirec tly a gradual down-regulation of the Calvin cycle leading to an inhibi tion of photosynthesis.