ROOT GROWTH DYNAMICS AND CONSTRAINTS ON ABOVE GROUND GROWTH AND YIELD OF WALNUTS

There are three components to this project. The first is a minirhizotron root growth study that was installed to monitor seasonal root growth patterns in an existing Chandler pruning and irrigation trial. The peak in root growth occurred in late June in both the fourth and fifth growing seasons. In the fourth growing season, initiation of roots was greatest in the minimal pruned treatment and in the fifth growing season root initiation was greatest in the heavily pruned treatments. The peak of root growth in the minimally and heavily pruned treatments tended to be at a shallower depth compared to the peak in both the deficit and untrained/unpruned treatments. The root growth patterns observed were quite different from those in the Walnut Production Manual. Our results showed a single peak occurring right between the two peaks in the manual. The second component is a clonal rootstock replant trial that was initiated in a Howard orchard which had lost hundreds of trees to the yellow Howard problem. The third component is a a new experiment that was planted in the winter of 2013 which is looking at different rootstock and scion interactions as influenced by irrigation with the goal of understanding the yellow Howard problem. These experiments are providing some of the first field level observations of root growth patterns in walnut orchards over the season as well as with depth. PROBLEM AND ITS SIGNIFICANCE Optimization of walnut management under current and future climates requires better knowledge of root growth dynamics and root growth interactions with aboveground growth. Several current problems in walnut cultivation, such as canopy yellowing in years with wet springs and increased pathogenesis of soil borne pathogens with soil moisture (Lampinen et al., 1994), may be due to constraints on fine root growth and functioning. In Yuba and Sutter Counties where the yellowing problem was acute in 2011, yellow-leafed trees comprised 30% of many orchards. Observations collected during the 2006 and 2011 season suggest that yellow-leafed trees either had overall stunted root systems or lacked ephemeral fine roots that function in nutrient uptake and appeared unable to resume growth within the same growing season after wet spring conditions ceased. There is a need to understand if root growth (or subsequent re-growth if spring roots are lost to wet conditions) is genetically controlled by rootstocks, scions, or interactions between rootstocks and scions to insure that proper recommendations are made for new plantings of current and new clonal rootstocks and scions. Better information on root dynamics could open possibilities of making a broad range of advancements to orchard management as well as provide solutions for current problems such as the yellowing Howard problem.