Apple trees utilize 3–5% of root biomass in the weathered rock layer to absorb 33–34% of their transpiration water consumption

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Rocky regolith layer can restrain plant root growth, which may affect soil water unitization of deep-rooted fruit trees in rocky mountain areas. However, how and to what magnitude the rocky regolith layer shapes root features and thus changes trees’ water consumption, remains unclear. Here, we investigated the root features and water uptake strategies of apple trees in three apple orchards with the underlying weakly (WWP), strongly (SWP), and completely (CK) weathered regolith layer within soil profiles, in the hilly areas of Jiaodong Peninsula, China. The results showed that the fine root biomass density (FRBD) of apple trees decreased while the specific root length (SRL) increased with increasing soil depths in the three orchards. The FRBD within the rocky regolith layer only accounted for 3–5% of the total FRBD in the WWP and SWP orchards, which were significantly lower than that in the CK orchard (3.31–5.11% vs. 29.96%) (P < 0.05). The maximum SRL was 76.61 m·g-1 and 72.38 m·g-1 in the WWP and SWP orchards, respectively, being significantly higher than that in the CK orchard (55.26 m·g-1). Both the FRBD and SRL within the rocky regolith layer showed no significant difference between the WWP and SWP, which indicated that it was the presence but not the weathering intensity of the rocky regolith layer shaped apple trees’ root features. The MixSIAR model estimated that the water uptake of roots in the rocky regolith layer accounted for an average of 33.14% of the total water uptake in the WWP and SWP orchards. During the fruit expansion period (July-August) with high water consumption, apple trees’ water uptake in the rocky regolith layer contributed 41.40% of water consumption in the WWP orchardand 34.77% in the SWP, which can be comparable to that in the corresponding soil depth in the CK orchard (46.70%). This suggested that apple trees may prime root activity to absorb more soil water with decreased root biomass in the rocky regolith layer. Overall, apple trees could utilize 3–5% of root biomass to absorb 33–34% of water consumption in the rocky regolith layer. Our study highlights the specific water use strategies in rocky mountain areas and will benefit the sustainable water management in the orchards in these areas.