Water Stress and Varietal Differences

Leaf drop observed in the almond variety 'Monterey.' Senescence and drop is most likely due to water stress. Photo courtesy of James Nichols.

Leaf drop observed in the almond variety ‘Monterey.’ Senescence and drop is most likely due to water stress. Photo courtesy of James Nichols.

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.


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4 thoughts on “Water Stress and Varietal Differences

  1. David;

    Could some of these observed symptoms be related to rootstock, their aggressiveness as scavengers of water, or possible genetic differences in susceptibility to phytopthora or pythium, or any impeding circumstances such as genetic nematode resistance.

    Mark Brady
    Plant Food Systems, Inc.

    1. Mark, I agree – these symptoms of early water stress could occur on any tree that has a compromised root system. Within our experiments, however, I dont think this was a limiting factor – our design included five different trees from three varieties from three different orchards and we found consistency across the locations. I would suspect that if it was something biological that was influencing the result, the variability would have shown up within a plot.


    1. Hey Jeremiah,
      If leaves are falling from over-watering, the trees are losing roots and are showing signs of water stress because of it. Generally, the initial symptoms will show up as a light yellow color, yellow leaves with green veins, sometimes shortened internodes at the new growth (essentially observed micronutrient deficiency), or general poor growth. Usually, trees can be spotted due to the off color.


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