Water Deficit and Crop Level Influences on Photosynthetic Strain and Blackleaf Symptom Development in Concord Grapevines

Abstract

Field-grown Concord grapevines subjected to water deficit stress by withholding irrigation before veraison (IN), after veraison (NI), or throughout the growing season (NN) were compared with vines irrigated for the entire season (II) during 1996 and 1997. The influence of water deficit stress on the photosynthetic system and blackleaf development was assessed throughout both seasons. Blackleaf symptoms were associated with a light dependent, sequential discoloration of the mesophyll leaf layer and the adaxial epidermal cells. Blackleaf symptoms were most severe in the NN treatment and least severe in the II treatment. Blackleaf symptom development was associated with a loss of chlorophyll and photosynthetic ability. Water deficit stress, expressed as xylem water potential below -1.1 MPa, reduced the photosynthetic rate 24 to 41% before veraison and 31 to 66% during fruit ripening. Comparison of morning and mid-day photosynthetic efficiency by measuring chlorophyll fluorescence indicated photoinhibition occurred during the day and was intensified by water deficit. Moreover, photosynthetic rate in the NI treatment was 25 to 37% lower than in the II treatment after veraison while stomatal conductance increased, suggesting possible damage to the photosynthetic system. Photosynthetic dysfunction during stress may be accompanied by damage to cell components, further reducing photosynthesis and increasing blackleaf development. Symptom development occurred before veraison in vines with and without fruit the entire season and in vines with sufficient potassium levels, suggesting high crop levels and leaf potassium deficiency were not primary causes of blackleaf. Water deficit before veraison reduced yields and sugar produced per vine. Reducing photosynthetic strain by minimizing water deficit early in the season reduced blackleaf and resulted in a greater photosynthetically functional leaf area during fruit ripening.