Are xylem radial development and hydraulic conductivity in downwardly-growing grapevine shoots influenced by perturbed auxin metabolism?

Downward-growing grapevine shoots have smaller and more frequent vessels and as a consequence a lower hydraulic conductivity. In a first experiment, grapevine shoot growth orientation was manipulated in order to test whether downward shoot orientation negatively affects vessel growth in the apex via a shortage of water and nutrients. The orientation of the central shoot portion in vines was inverted by two consecutive 135° bends, the first one downwards and the second one upwards, resulting in double-bent N-shaped vines; the plant central downward shoot portion was of different length in the experimental treatments, in order to induce increasing reductions of shoot conductivity. These treatments effectively reduced shoot conductivity and water flow, but had no effects on vessel development and frequency in the apex. In a second experiment, auxin concentration was assessed in shoots of upward and downward growing plants. IAA concentration at the apical internodes was higher in downward oriented shoots than in shoots growing upwards. In addition, a higher density and a lower vessel diameter were observed in the lower shoot side of the shoot than in the upper, suggesting an increased accumulation of auxin in the lower shoot side. These results suggest that the downward orientation induces accumulation of auxin in the apex which in turn affects the density and the size of the xylem vessels, causing reduction of hydraulic conductivity.