Morphology, photosynthesis, and internal structure alterations in field apple leaves under hidden and acute zinc deficiency

Abstract

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 leaves on Zn deficient trees without and with visible rosette symptoms as ‘hidden rosette leaves’ and ‘rosette leaves’, 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 elucidate the damage mechanism of Zn deficiency in apple leaves. We found that the reduction in photosynthesis under Zn deficiency was primarily due to non-stomatal limitation, in particular the changes in photosystem 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 (ϕPS II) and electron transport rate (ETR) were all significantly decreased 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 in chlorophyll content and inhibition of PS II light-harvesting activity may account for the photosynthesis reduction during the initial stage of Zn deficiency. As Zn deficiency aggravated, cellular deficient characteristics like abnormal cell arrangements and disorganization of thylakoids became more exaggerated; visible external symptoms of Zn deficiency appeared with internal alterations, including photosynthetic apparatus damage accompanied by subsequent chloroplast and grana disintegration. We concluded that the PS II activity was relatively sensitive to Zn deficiency, hence the chlorophyll fluorescence parameters like Fv′/Fm′ and ϕPS II may be used as predictive indictors for hidden Zn deficiency on apple trees.