In the field, apple trees (Malus domestica L.) do not exhibit visible deficient symptoms
when affected by hidden zinc (Zn) deficiency, while internal injury in the leaves may prior occurred. We named the leaveson Zn deficient trees without and with visible rosette symptoms as ‘hidden rosette leaves’ and ‘rosetteleaves’, respectively. The healthy leaves of normal trees were named ‘normal leaves’ and selected as the control. The changes in leaf photosynthesis, physiology and internal structure were
analyzed to elucidatethe damage mechanism of Zn deficiency in apple leaves. We
found that the reduction in photosynthesisunder Zn deficiency was primarily due to
non-stomatal limitation, in particular the changes in photo-system II (PS II). Zinc deficiency significantly reduced chlorophyll content in the hidden rosette leaves without obvious changes to chloroplasts and grana number. The intrinsic efficiency
of PS II (Fv/Fm),actual photochemical efficiency of PS II (PS II) and electron transport
rate (ETR) were all significantlydecreased in both hidden rosette leaves and rosette
leaves, while a further significant increase in the minimal fluorescence (Fo) along
with reductions in maximum photochemical efficiency of PSII (Fv/Fm) and photochemical quenching (qP) occurred in rosette leaves. These results indicate that the decrease inchlorophyll content and inhibition of PS II light-harvesting activity
may account for the photosynthesisreduction during the initial stage of Zn deficiency. As Zn deficiency aggravated, cellular deficient charac-teristics like abnormal cell
arrangements and disorganization of thylakoids became more exaggerated;visible
external symptoms of Zn deficiency appeared with internal alterations, including
photosyntheticapparatus damage accompanied by subsequent chloroplast and
grana disintegration. We concluded thatthe PS II activity was relatively sensitive to
Zn deficiency, hence the chlorophyll fluorescence parameterslike Fv/Fm and PS II may be used as predictive indictors for hidden Zn deficiency on apple trees.