Blackleaf is a leaf disorder which is believed to be associated with low leaf and/or petiole potassium. Correlative evidence for potassium deficiency has been variable and increasing potassium fertilization has not eliminated the disorder. Additionally, blackleaf symptom development differs from classic potassium deficiency in several critical aspects, which include location within the canopy and individual leaves. We tested the hypothesis that blackleaf influences the photosynthetic process such that potential vine productivity is reduced. Coincident with veraison and appearance of visual symptoms in Concord grapevines, carbon exchange rate (CER) declined in blackleaf-affected source leaves (9.46 µmol CO2 · m -2 · s-l) compared to unaffected source leaves (12.46 µmol CO2 · m -2 · s-l). By September the decline in CER was > 50% in affected leaves (6.18 µmol CO2 · m -2 · s-1). These data suggest blackleaf results in a 22% to 55% reduction in carbon on a daily basis. The level of chlorophyll fluorescence (Chl-F) decreased in affected leaves, which is opposite of the expected increase in Chl-F with a reduction in CER. Loss in CER did not appear to be associated with photoinhibition because the maximum intrinsic efficiency of Photosystem II (PSII), as measured by F/Fm, was unaffected. When CER, Chl-F and chlorophyll a and b concentrations for green and black portions of affected leaves were compared, there was no significant difference in CER. CER was reduced in green and black portions of affected leaves by ca. 55% compared to unaffected leaves. Green portions of affected leaves had the highest Chl-F values, thus less light energy was utilized in photosynthesis, suggesting the potential efficiency of the photosynthetic apparatus had been reduced. Measurement of chlorophyll concentrations confirmed that there was a loss in chlorophyll green and black portions of affected leaves. Collectively, the data suggest a possible transition in photosynthetic system damage between unaffected leaves, green tissue of affected leaves, and black tissue of affected leaves.
- Copyright 1998 by the American Society for Enology and Viticulture