Scion Water Relations and Leaf Gas Exchange
Rootstock affected midday stem water potential. In this irrigated
orchard, mean midday stem water potential (Ψm ) did not
range beyond –0.8 to –1.0 MPa. Bud.9 had the lowest stem water
potential of all rootstock genotypes (Figure 2). However, in 2018,
Ψm for B.9 was only significantly different from
CG.5257. In 2019, differences among rootstocks were greater but followed
the same trends as 2018. Less vigorous rootstocks like B.9, M.9, and
G.969 were significantly different from more vigorous rootstocks like
G.890, CG.5257, and G.814. Midday stem water potential was strongly
correlated with seasonal shoot growth (Figure 3) indicating that
rootstocks that confer higher vigor in the scion also contribute to
greater leaf stem water potential. Leaf gas exchange traits measured
mid-morning differed less among rootstocks than shoot growth or trunk
diameter. Carbon assimilation was the lowest for G.969 at approximately
13 µmol CO2 m-2 s-1 and the highest
for G.5257 at approximately 17.5 µmol CO2 m-2s-1 (Figure 4). There was a significant relationship
between maximal net carbon assimilation and seasonal shoot growth (r =
0.923; P<0.001; Figure 5). Stomatal conductance ranged from
0.11 µmol CO2 m-2 s-1 to 0.29µmol
CO2 m-2 s-1 among rootstocks.
Similar to net carbon assimilation, G.969 had the lowest stomatal
conductance and was significantly different from G.210. The same trend
was observed for transpiration rate (Figure 4). Transpiration was also
the lowest for G.969 but was the highest for G.4292. Patterns in mean
net carbon assimilation were similar to stomatal conductance and
transpiration rate. Although in 2018, B.9 and G.890 were not
significantly different, G.890 had higher net carbon assimilation
compared to B.9 in 2019. ‘Honeycrisp‘ grafted on G.969 was an outlier in
this relationship with low net carbon assimilation but intermediate
shoot growth values. There was a strong positive relationship between
rootstock-mediated vegetative vigor and water relations (Figure 3)
indicating that rootstocks limiting water and maintaining a lower
inherent stem water potential will also have less vigor.