G. Stanley Howell

The effect of leaf removal and canopy height on whole-vine gas exchange and fruit development of Vitis vinifera L. Sauvignon Blanc

Canopy topping and leaf removal are management practices commonly used in New Zealand vineyards to increase light and pesticide penetration to the fruit zone, thus, reducing disease incidence. Previous research has suggested that an increase in photosynthesis occurs when leaves are removed, and this may compensate for the reduced leaf area. However, it is difficult to extrapolate single-leaf photosynthesis measurements to a whole-plant scale. Therefore the extent of the compensation is unknown. To evaluate the impact of leaf removal and canopy height on whole-vine photosynthesis, treatments were imposed during the lag phase of berry growth. Leaves were removed from the lower quarter of the canopy, or vines were topped to three quarters of the height of control plants, in a two-by-two-factorial design. Both topping and leaf removal caused a decrease in whole-vine photosynthesis immediately after the treatments were imposed. Leaf removal, but not topping height, reduced photosynthesis on a per unit leaf area basis. This suggests that the lower portion of the canopy contributes more than the upper portion of the canopy to whole-vine photosynthesis. When measurements were made again approximately two months later, tall vines without leaf removal had a higher photosynthesis rate than the other treatments. Fruit yield, sugar content, vine carbohydrate reserves and pruning weights followed trends similar to those observed for photosynthesis, suggesting that although some photosynthetic compensation occurred, the defoliation treatments had a negative effect on vine growth.

Postveraison application of antitranspirant di-1-p-menthene to control sugar accumulation in sangiovese grapevines

The effectiveness of a postveraison application of the film-forming antitranspirant Vapor Gard (VG, a.i. di-1-p-menthene) was investigated as a technique to delay grape ripening and reduce sugar accumulation in the berry. The study was carried out over the 2010–2011 seasons in a nonirrigated vineyard of cv. Sangiovese in central Italy. Vapor Gard was applied at 2% concentration to the upper two-thirds of the canopy (most functional leaves) and it significantly lowered leaf assimilation and transpiration rates and increased intrinsic water use efficiency. The Fv/Fm ratio was not modified, emphasizing that photoinhibition did not occur at the photosystem II complex, whereas the reduction of pool size of plastoquinone matched well with reduced CO2 fixation found in VG-treated vines. In both years VG treatment reduced the pace of sugar accumulation in the berry as compared to control vines, scoring a -1.2 Brix at harvest and wine alcohol content at −1% without compromising the recovery of concentrations of carbohydrates and total nitrogen in canes and roots. Concurrently, organic acids, pH, and phenolic richness of grapes and wines were unaffected, whereas there was a decrease in anthocyanin content in the berry (−19% compared to control vines) and in the wine (−15% compared to control vines). The application of VG at postveraison above the cluster zone is an effective, simple, and viable technique to hinder berry sugaring and obtain less alcoholic wines. To be effective the spraying should be performed at ~14 to 15 Brix, making sure that the lower leaf epidermis is fully wetted by the chemical.

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 I, 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 (Gs) and directly proportional to internal CO2 concentration (Ci). There was a positive correlation between A and Gs, while A and Ci 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, Gs, and Ci 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 Gs and increases in Ci, 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.