Shoot orientation affects vessel size, shoot hydraulic conductivity and shoot growth rate in Vitis vinifera L.

Single-shooted grapevines were grown in containers and their shoot was oriented to the vertical upward, or the 45° downward position. Part of the plants were trained first upward, then downward, then again upward (N-shaped plants). Shoot length and total leaf area were lower in the N-shaped shoots than in the upward shoots, and in the downward shoots in comparison to the N-shaped ones. The decrease in shoot growth rate required the shoot apex to be growing downwards for at least 10 internodes. The apical leaves of downward shoots had lower stomatal conductance and lower water potential. Shoot hydraulic conductivity kh, measured after elimination of xylem embolisms, was lower in downward than in upward shoots. In N-shaped shoots kh was high in upward growing shoot portions, and low in downward growing portions. The reduction of kh in downward shoot portions could be explained by a decrease in xylem cross area and by a decrease in shoot specific conductivity ks. Xylem structure (number of wedges, number of vessels per wedge) showed no differences among the different shoot orientations. On the contrary, in agreement with the conductivity data, average vessel transectional area and the summation of vessel cross areas (total conducting area) were significantly lower in downward shoots and in the downward growing portion of N-shaped shoots. Shoot bending was not necessarily associated with decreases in conductivity and in vessel cross areas. These data are consistent with a model where downward shoot orientation negatively affects xylem development immediately below the apex without directly affecting shoot growth rate, while the lower conductivity of downward shoot portions, accumulated for several internodes, restrains transport of water and solutes to the growing apex, eventually lowering its growth rate.