A few visits and emails have highlighted differences in leaf drop patterns observed across varieties within an orchard. Full rows of trees will have yellow leaves that drop in time, characteristic of water stress while other varieties appear unaffected. These “in-field” observations suggest that almond varieties respond differently to the amount of water applied. For example, the variety ‘Monterey’ and ‘Aldrich’ have been observed to show signs of stress before ‘Nonpareil’ trees even though crop load is similar. In these cases, these trees may undergo severe leaf drop while ‘Nonpareil’ appears unaffected.
Research conducted by Shackel and Doll in 2011-2012 (and funded by the Almond Board of CA) found that although varieties may be exhibit differing stress levels, the impact of this stress on the physiological processes was the same. Using a pressure chamber and a steady-state porometer, they performed diurnal studies that compared stem water potential (SWP) to stomatal conductance. They found that even though different varieties may have different SWP values at the same point of the day, the trees responded to increasing stress levels similarly. In other words, regardless of variety, once stress levels hit a specified level of stress, transpiration was reduced to a similar rate across all varieties.
These findings indicate that the differences in the field are not due to a tree being more sensitive to stress (i.e. ‘Monterey’ wilts at a -18 SWP while ‘Nonpareil’ wilts at -22 SWP), but rather that in-field differences observed are due to factors that may limit water uptake (i.e. root system depth, lateral length, or architecture), increase consumption (canopy architecture, or leaf count), or an interaction of the two. This highlights the importance of properly delivering water to all of the trees within the orchard to prevent the onset of severe water stress.