Photosynthetic Performance of Pinot gris (Vitis vinifera L.) Grapevine Leaves in Response to Potato Leafhopper (Empoasca fabae Harris) Infestation

Potato leafhopper (Empoasca fabae Harris) is a polyphagous insect pest that feeds on Vitis vinifera L. grapevines in North America. In sensitive grape cultivars such as Pinot gris, feeding symptoms include leaf yellowing, leaf cupping, and stunted vine growth. Two greenhouse experiments were conducted to determine how photosynthesis and other physiological processes are affected by E. fabae infestation. In Experiment 1, Pinot gris leaves at four different positions along shoots were infested with either 0, 1, 2, 4, or 8 E. fabae nymphs for 43 hr to determine how the relationship between infestation level and leaf position affects leaf photosynthesis and whether or not damage thresholds exist for the photosynthetic response. Empoasca fabae infestation was inversely proportional to carbon assimilation (A), transpiration (E), and stomatal conductance (G(s)) and directly proportional to internal CO2 concentration (C-i). There was a positive correlation between A and G(s), while A and C-i were negatively correlated, indicating that reductions in A were due to both stomatal and nonstomatal limitations. Damage thresholds, defined as the number of insects necessary to cause damage to the plant, were calculated for A, E, G(s), and C-i at most leaf positions. In Experiment II, response curves were generated for infested and uninfested regions of the same leaves to determine how light and CO2 utilization were affected by E. fabae infestation. Declining A in response to E. fabae infestation was due to a decreased capacity of leaf tissues to utilize light and CO2. Reductions in A were correlated with decreases in G(s) and increases in C-i, indicating that stomatal and nonstomatal limitations were relevant, with evidence of photosynthetic compensation in the postinfestation period. These results indicate that E. fabae infestation causes injury through rapid effects on the capacity of leaves to produce photosynthate through effects on internal tissues and on stomata. These effects might be transient at low infestation levels, but leaf tissue can be compromised at higher infestation levels, leading to irreversible damage.