Variety-Specific Responses of Mulberry to Copper-Induced Stress

Copper (Cu) is an essential plant micronutrient, but in excessive amounts it becomes phytotoxic, making Cu contamination of agricultural soils an important environmental concern. Because Cu tolerance mechanisms are complex and often genotype-specific, there is growing interest in multipurpose crops capable of thriving under metal stress. Mulberry is one of such species, which enables the combination of diverse agricultural uses with notable tolerance to heavy metals. In the present study, the tolerance to Cu-induced stress of five varieties belonging to Morus alba and M. indica was assessed in a pot experiment, in which the trees were watered with a CuSO4 solution that increased the soil Cu concentration to 150 mg kg−1, while the plants in the control group were watered with the same volume of tap water. In leaves sampled on days 0, 3, 7, and 21 after CuSO4 treatment, Cu content was analyzed by AAS; ascorbate, dehydroascorbate, thiols, oxidized thiols and xanthophyll pigments were analyzed by HPLC; and total phenolic content and photosynthetic pigment content were determined spectrophotometrically. The effect of CuSO4 treatment on the maximum quantum yield of photosystem II was analyzed by measuring the chlorophyll fluorescence of leaves on day 21 after CuSO4 treatment. The results showed a species-specific response to Cu-induced stress. Leaves of M. alba varieties exhibited a more pronounced increase in Cu content, higher antioxidant levels, more efficient redox regulation, a smaller decline in photosynthetic pigment contents and photosynthetic efficiency, resulting in more effective photoprotection through the xanthophyll cycle when compared to M. indica varieties under the conditions of this experiment. In particular, the varieties ‘Florio’ and ‘Giazzola’ exhibited high Cu tolerance and therefore show potential for cultivation in Cu-contaminated soils.