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.