| The objectives of this dissertation were to characterize the adaptation of dwarfing apple (Malus domestica Borkh.) rootstocks (B.9, M.9, and Mark) to soil water deficit conditions. Three experiments were performed during the period 1997--1999. A first trial was designed to determine whether partial soil drying could influence transpiration, photosynthesis, and chlorophyll fluorescence in B.9, M.9, and Mark rootstocks, and, if so, to provide information on the mechanism of root-to-shoot-signaling. In particular, to determine whether a dry portion of the root system could improve the ability of other parts of the plant to withstand stress, roots subjected to uniform watering were compared with those receiving non-uniform watering. Results seemed to suggest an increase in tolerance to water deficit in plants with half of the root system growing in non-irrigated soil. Leaf water potential measured at predawn and midday indicated that B.9 experienced the biggest change in the water status, especially when equal water distribution was applied. Gas exchange analysis indicated that Mark had the greatest reduction in assimilation rate when only part of the roots were irrigated. Water use efficiency increased with time in stressed B.9, which also showed high stomatal conductance and transpiration rate. In Mark, assimilation rate and water use efficiency were frequently lower when half of the root system was not watered. The second experiment was conducted to test the use of infrared thermometry as a rapid, non-invasive tool to detect drought stress in potted plants of 'TRECO Red Gala #42' grafted on three different dwarfing apple rootstocks, B.9, M.9 and Mark. The variation in leaf temperature induced by water deficit was studied together with the variation in assimilation rate, leaf expansion rate, and sap flow. Differences in canopy temperature could be detected as early as 3--4 days after initiating the stress. Gas exchange appeared to be more affected by water deficit in 'TRECO Red Gala #42' grafted on B.9 than on the other two rootstocks. Leaf expansion rate was reduced in 'TRECO Red Gala #42'/Mark within the first 2 days of stress. Heat-pulse sapflow sensors installed on 'TRECO Red Gala #42'/Mark indicated that the rate of the xylem sap flow decreased after 4 days of water deficit. In the third study, the stable isotope 13C was used to evaluate the influence of soil water content on carbon partitioning among the various tree organs and to determine a possible correlation between carbon partitioning and root respiration. Canopies of 1-year-old rooted cuttings of B.9 were pulsed with 13CO2 and isotopic emissions were monitored for 16 days. The lower soil moisture had no effect on predawn leaf water potential or single leaf net assimilation rate but reduced the evolution rate of labeled 13CO2 from the root. 13Carbon partitioning within the tree was not affected by water supply over the 16-day duration of the experiment, and evolution rate of 13C from the roots was not related to the amount of labeled C allocated to the roots themselves. |
