DB LOPES¹, TA DAVOLI¹, A BERGAMIN FILHO² and RD BERGER¹

¹Plant Pathology Department, University of Florida, Gainesville, FL 32611-0680, USA; ²Plant Pathology Department, ESALQ/USP, 13418-900, Brazil

Background and objectives
Crop yield is largely determined by the amount of solar radiation intercepted by the canopy and how efficiently this radiation is used. Plant diseases can affect one or both of these processes. Pathogens that cause necrotic leaf lesions or defoliation are obviously affecting radiation interception, but the impact of these and other pathogens and stresses on radiation use efficiency is not as easy to assess. One way of learning if the radiation use efficiency of diseased leaf is being affected is to monitor its photosynthetic competence in vivo. This information would be useful to improve assessments of healthy leaf area absorption (HAA) and its relation to crop yield [1].

The objectives of this work were to observe how bean rust affects the photosynthesis of plants with different nutritional status and to verify if leaf colour can be used to make inferences about the photosynthetic competence of such plants.

Materials and methods
Bean plants (cv. Rosinha) growing under greenhouse conditions (25-35øC) received the following fertilization treatments: (A) no fertilizer, (B) 50% of the recommended dosage of Peter's fertilizer (20-20-20), and (C) 100% of the recommended dosage of the fertilizer. The three groups of plants were inoculated with Uromyces appendiculatus by spraying spore suspensions of different concentrations to obtain a range of rust symptoms. Photosynthetic rate at light saturation, chlorophyll fluorescence parameters (electron transport rate (ETR) and effective quantum yield (Fm'-F/Fm']), estimated chlorophyll and leaf colour were determined in all plants when symptoms were fully developed. These parameters were obtained by using a LI-COR 6200 Portable Photosynthesis System, a Walz fluorometer PAM-2000, a Minolta Chlorophyll Meter SPAD-502 and a Minolta Colour Reader CR-10, respectively.

The equation Px=Po(1-x)^b was applied to relate relative photosynthetic rate (Px/Po) to disease severity (x), where Px is the photosynthetic rate of a diseased leaf, and Po is the average photosynthetic rate of healthy control leaves; b represents the ratio between virtual and visual lesion size and characterizes the effect of a pathogen on leaf photosynthesis for the range of measured disease severities [2].

Results and conclusions
All measured parameters were significantly reduced in healthy plants with the lower levels of fertilizer. The nutritional status of the plants affected the size of the rust lesions (pustule+halo), but not the number of lesions per unit area. Plants that received no fertilizer had larger lesions and consequently, higher severities than well-fertilized plants.

Bastiaan's equation was used to obtain the b parameter for each fertilization level. The values of b for treatments A, B and C, which were 1.6610.356 (R²=0.81), 2.1510.295 (R²=0.80) and 2.4210.447 (R²=0.68), respectively, were not significantly different. Thus, the fertilization level did not affect significantly the physiological impact of rust on affected leaves. These values, however, were different from one, indicating an impact of this disease beyond the visual lesion. Regardless of the fertilization treatment, the values of ETR, Fm'-F/Fm' and chlorophyll were reduced in plants with greater severities.

A negative exponential relationship (R²=0.75) between absolute values of photosynthetic rate and colour differences (difference between the colour of each plant and the average colour of the control plants in the experiment, in this case, asymptomatic plants with 100% of the recommended dosage of fertilizer) was obtained which included plants with different nutritional status and rust severity levels.