Comparative assessment of nickel stress responses in <i>zinnia</i> , <i>celosia</i> , and <i>tuberose</i> plants: insights into physiological adaptation, oxidative stress, and metal uptake dynamics

Nickel (Ni) is an essential micronutrient that becomes toxic at higher concentrations, affecting plant growth and physio-biochemical attributes. Understanding the dose-dependent responses of ornamental plants to Ni is vital for sustainable cultivation and phytoremediation of Ni-contaminated soils. The present pot study investigated the responses of zinnia (Zinnia elegans), celosia (Celosia plumose), and tuberose (Polianthes tuberosa) to soil-applied Ni (0, 50, 100, 150, and 200 mg kg−1). The experiment was conducted under open field conditions using a factorial completely randomized design (CRD) with three replications per treatment. Ni at 100 mg kg−1 significantly improved shoot and root dry weights in zinnia (36% and 60%), celosia (19% and 45%), and tuberose (23% and 57%) compared to the control; however, it decreased by 56% and 90% (zinnia), 47% and 87% (celosia), and 43% and 80% (tuberose) at a higher concentration (200 mg kg−1) compared to the control Ni 100 mg kg−1. Similarly, relative water content and membrane stability index, and total chlorophyll were improved in zinnia (17%, 13%, and 43%), celosia (21%, 27%, and 40%), and tuberose (15%, 13%, and 25%) at Ni 100 mg kg−1 compared to the control, hence reduced to 40%, 31%, and 79% (zinnia), 31%, 49%, and 42% (celosia), and 25%, 33%, and 32% (tuberose) at 200 mg kg−1 than the Ni 100 mg kg−1. Highest photosynthetic and transpiration rates were recorded in zinnia (37% and 38%), celosia (29% and 57%), and tuberose (35% and 56%) at 100 mg kg−1 compared to the control, which declined to 57% and 59% (zinnia), 50% and 64% (celosia), and 52% and 61% (tuberose) at 200 mg kg−1 compared to the control 100 mg kg−1. Likewise, catalase, peroxidase, and superoxide dismutase activities were markedly increased in zinnia (29%, 18%, and 35%), celosia (41%, 14%, and 11%), and tuberose (34%, 19%, and 62%) at 100 mg kg−1 comparing with control, with highest decline of 5%, 10%, and 37% (zinnia), 3%, 11%, and 17% (celosia), and 17%, 10%, and 37% (tuberose) at 200 mg kg−1 than the 100 mg kg−1. In contrast, maximum bioconcentration factor and translocation factor were recorded in zinnia (85% and 57%), celosia (91% and 40%), and tuberose (90% and 84%) at Ni 200 mg kg−1, whereas Ni 100 mg kg−1 enhanced values to 83% and 44% in zinnia, 90% and 27% in celosia, and 89% and 35% in tuberose compared to the control. Overall, moderate Ni concentration improved plant growth and physiological performance, whereas higher Ni disrupted physiological homeostasis and induced oxidative stress. These results also highlight the suitability of zinnia, celosia, and tuberose as promising plant species for the remediation of Ni-contaminated soils.