The Crush September 2022 - 2

for multiple benefits. A 2003 research paper by
Walker, now Professor Emeritus Larry Williams, and
Marilark Padgett-Johnson (now Viticulture Program
Director at Santa Rosa Junior College) evaluating
drought tolerance factors of 17 Vitis species in ownrooted
field trials identified six species as highly
drought tolerant: V. champinii, V. doaniana, V. longii,
V. girdiana, V. arizonica and V. californica.
Walker and his lab conducted multiple grape
collection trips to collect wild native American Vitis
plant material from the Southwest U.S. and northern
Mexico, resulting in hundreds of new accessions
for the UCD germplasm collection. Many of these
accessions carry genetic backgrounds and traits for
drought, salt and lime tolerance and the ability to
adapt to water stress, and some have been bred
and evaluated. In developing new rootstocks for
drought tolerance, a focus has been on rooting traits
that include: root architecture with shallow to deep
rooting angles, root density, and fine root recovery
after drought. Species and rootstocks vary in their
ability to produce/regenerate feeder roots, their
ability to produce abundant fine roots, and some
produce more structural roots.
In his final report to the California Grape Rootstock
Research Foundation (CGRRF) in June 2021,
" Development of Next Generation Rootstocks for
California Vineyards, " Walker provided an update
on rootstock breeding for drought tolerance and
avoidance. " One of our long-term goals has been
to produce rootstocks with a deep plunging root
phenotype that can exploit the entire soil profile
to scavenge for water in seasons and times of year
when water is limiting, " Walker stated.
Recent breeding work has looked at potentially
drought tolerant selections tested for rootability and
downward growth of roots with a focus on selections
with either Dog Ridge or Ramsey as the female
parents. These have been crossed with male parents
that include Southwest U.S. grape accessions from
Walker's collection trips that include: V. girdiana
selections collected in southern Nevada; V. girdiana
collected near St. George, Utah; V. arizonica from
Overton, Nevada; and a V. berlandieri selection.
Promising selections may be used for future
rootstock trials. Copies of Walker's annual reports
from 2013 to 2021 can be found at the CGRRF
website at
UCD Viticulture and Enology Assistant Professor
Megan Bartlett is conducting research to identify
leaf and root cell traits that improve drought
tolerance and WUE in grape cultivars and
Page 2 | September 2022
rootstocks. Identifying drought tolerant traits is
intended to provide targets for breeding improved
rootstocks. Bartlett explained: " Root cellular traits
impact a plant's sensitivity to drought. The living
cells in roots are a major bottleneck for water uptake
and transport, especially under water stress and in
dry soil. Roots shrink as they lose water, leaving a
disconnect from the soil. Cell collapse and death
under water stress reduces the ability of the root to
transport water, leaving fewer pathways for water to
move from the soil to the xylem. "
Bartlett has been evaluating drought tolerance traits
measured under water-stressed conditions related to
turgor loss point (TLP)
and root capacitance
(CAP). Plant cell walls
are held up by turgor
pressure, the pressure
produced by water
pushing against the
inside of the cell walls.
TLP is the water deficit
at which the cells no
longer have enough
turgor pressure to
support the cell walls,
causing the cells to
collapse and severely
limiting water
movement through the roots and leaves. TLP is
measured as a water potential, so a plant with more
negative TLP value can experience more negative
water potential, and thus, more severe drought
before cell collapse. Further, grapevines during
drought can change their cellular biochemistry
to make TLP more negative to reduce their
susceptibility to cell collapse. CAP measures the
volume of water lost from the root as the root
dehydrates to TLP. A lower CAP indicates the
roots retained more water as root water potentials
Bartlett performed greenhouse studies to evaluate
these two traits using eight commercial rootstocks
grafted to Chardonnay. Four are considered to have
high drought tolerance: Ramsey, 140Ru, 1103P, and
110R; and four are considered to have less drought
tolerance: 420A, 5C, Riparia Gloire, and 101-14.
CAP and TLP were significantly different across the
rootstocks and shifted to significantly lower values in
water-stressed plants. These initial findings suggest
that root TLP and CAP are promising traits for
breeding to improve rootstock drought tolerance,
but more work is needed to understand these traits
in deep and varied soil rooting environments under
field conditions.
Dr. Megan Bartlett, UCD Professor

The Crush September 2022

Table of Contents for the Digital Edition of The Crush September 2022

The Crush September 2022 - 1
The Crush September 2022 - 2
The Crush September 2022 - 3
The Crush September 2022 - 4
The Crush September 2022 - 5
The Crush September 2022 - 6
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