GAS-EXCHANGE AND WATER RELATIONS OF FIELD-GROWN CONCORD (VITIS-LABRUSCANA BAILEY) GRAPEVINES

Gas exchange and water relations of irrigated and non-irrigated vines and the effect of the duration of leaf exposure to direct sunlight on gas exchange parameters of irrigated vines were studied on field-grown Concord (Vitis labrusca) grapevines. Leaf water potential of the non-irrigated vines decreased from -1.3 MPa (900 h) to -1.6 MPa (1600 h), while that of the irrigated vines decreased from -1.0 MPa to -1.6 MPa during the same time period. Differences in xylem water potential between the two treatments changed from -0.45 MPa (900 h) to -0.37 MPa (1600h). Xylem water potential was found to be more indicative of vine water status than that of leaf water potential. Shoot hydraulic resistance of irrigated vines remained unchanged in spite of low afternoon leaf water potential. Assimilation rate and stomatal conductance of non-irrigated vines were significantly reduced as leaf water potential decreased, where only a slight reduction was measured in the irrigated vines. These results may indicate the existence of a root signal controlling stomatal aperture in Concord. Lower afternoon assimilation rates and stomatal conductances were found in continuously exposed leaves compared to leaves exposed to direct sunlight only in the afternoon. However, the relationship between assimilation rate and stomatal conductance remained the same regardless of the timing or duration of exposure of the leaves.