Nitrogen (N) is an essential element for plant growth, development, and
metabolism. In apple production, the excessive use of N fertilizer may
cause high N stress. Whether high N stress can be alleviated by
regulating potassium (K) supply is unclear. A hydroponics experiment was
conducted herein to test the influence of different N and K conditions
on the root morphology and physiology of apple M9T337 rootstocks. The
results showed that high nitrate-N conditions caused the dysregulation
of N/K balance, hormone content, and antioxidant metabolism, resulting
in damage to the root cortex structure. High nitrate conditions caused
stomatal closure and reduced mesophyll conductance ( g
m) and the maximum carboxylation efficiency ( V
c,max), thus increasing photosynthetic limitations, and
also reduced the expression of sugar transporter proteins (
MdSOT1, MdSOT2, and MdSUT1), leading to a lower
soluble sugar content in the root system, which was detrimental to root
growth. Further analysis found that appropriately increasing the K
supply (10 mM) enhanced the superoxide dismutase, peroxidase, and
ascorbate peroxidase activities, reduced the malondialdehyde and H
2O 2 contents, and protected the root
structure. It also optimized the net photosynthetic rate ( A
n), increased sugar transporter protein expression and
sugar-metabolizing enzyme activity in the roots, promoted the transport
of photosynthetic products such as sorbitol and sucrose from the leaves
to the roots, and increased the soluble sugar content in the roots. In
summary, our data show that an appropriate increase in the K supply (10
mM) alleviated th