Evaluation of germination characteristics and cadmium tolerance indices in various physalis (Physalis spp.) species

Introduction
The contamination of water, soil, and air by heavy metals has emerged as a major environmental concern driven by increasing anthropogenic activities. Major sources of heavy metals include urban and industrial wastewater and sludge, excessive chemical inputs, and metal mining activities. Cadmium (Cd) is recognized as one of the most hazardous heavy metals for both plants and humans. The permissible concentration of cadmium in agricultural soils is approximately 1 mg kg⁻¹. Cadmium has a low affinity for binding to soil stabilizing phases, such as oxides and solid organic matter; consequently, it can be readily absorbed by plants and translocated to aerial tissues. Cadmium stress inhibits several physiological and biochemical processes, including chlorophyll biosynthesis, photosynthesis, and nutrient uptake, ultimately leading to reduced plant growth and yield (Li et al., 2023). Cadmium accumulation and tolerance vary significantly among plant species and genotypes. Previous studies have reported a wide range of cadmium uptake and accumulation among different cultivars within the same plant species. Many members of the Solanaceae family, commonly grown in agricultural systems characterized by high inputs of organic and mineral fertilizers, exhibit high cadmium accumulation. Although Solanaceae species are regarded as cadmium-tolerant, their responses to cadmium stress vary among species. Physalis (Physalis spp.) is a member of the Solanaceae family valued for its bioactive compounds and its adaptability to a wide range of growing conditions. Physalis fruit possesses numerous nutritional, medicinal, and industrial benefits. The fruit is a rich source of provitamin A, vitamin C, and contains, to some extent, B‑complex vitamins, as well as essential minerals such as phosphorus, potassium, zinc, and iron. Consequently, physalis fruit has attracted considerable attention owing to its high nutritional value, antioxidant capacity, and reported anti‑inflammatory and anti‑stress properties, along with other medicinal benefits. The strong antioxidant properties of this plant are largely attributed to its high content of dihydroxy withanolides. This study aimed to evaluate the effects of different cadmium concentrations on the germination characteristics of physalis and to determine the relative tolerance and sensitivity of yellow, green, and purple cultivars to cadmium stress.
 
Materials and methods
The experiment was conducted in 2024 as a factorial arrangement based on a completely randomized design with four replications at the University of Torbat‑e Heydarieh. The experimental factors included six cadmium concentrations (0, 60, 120, 240, 500, and 750 μM) and three physalis cultivars (purple, yellow, and green).
 
Results and discussion
The results showed that the application of low cadmium concentrations significantly increased germination percentage and germination rate, possibly due to stimulation of seed germination. However, higher cadmium concentrations had a negative effect on all germination characteristics of the studied cultivars. Cadmium stress at 500 μM resulted in reductions of 76%, 83%, and 66% in seedling weight compared to non stressed conditions for the purple, yellow, and green cultivars, respectively. In all physalis cultivars, proline content increased with increasing cadmium stress intensity. For instance, at a cadmium concentration of 500 μM, proline content increased by 9%, 17%, and 23% compared to control conditions in the purple, green, and yellow cultivars, respectively. The highest contents of chlorophyll a, chlorophyll b, and carotenoids were observed in all three cultivars at a cadmium concentration of 60 μM; however, the contents of these photosynthetic pigments decreased significantly with increasing cadmium concentration. The sensitivity to cadmium stress in the purple, green, and yellow cultivars was approximately 16%, 66%, and 71%, respectively. The tolerance index indicated that the purple cultivar exhibited the highest tolerance to cadmium stress (0.049), whereas the yellow cultivar showed the lowest tolerance (0.017).
 
Conclusion
Overall, the results indicated that all physalis cultivars tolerated cadmium stress up to 500 μM; however, seedling growth was inhibited at concentrations above this level. In terms of tolerance, the purple cultivar was the most tolerant, followed by the green cultivar, while the yellow cultivar exhibited the highest sensitivity to cadmium stress.