Last few decades have witnessed emergence of Cr (III or VI) as major environmental pollutant. Among natural forms of Cr, Cr (VI) is highly phytotoxic owing to its greater mobility in soil solution surrounding rhizosphere 
. All biological systems have devised unique strategies to combat oxidative stress, an outcome of excessive ROS. Active participations of antioxidant system, PC, MT and several osmolytes in metal detoxification have been widely established 
. BRs have become a popular tool for oxidative stress amelioration 
. PA involvement in abiotic stress management, particularly metal excess, has given them a status of favorite metal detoxifiers in plants 
. Though significant amounts of information are available on the individual applications of BRs and PAs in heavy metal stress mitigation, their co-application potential has not been tested for their efficacy to induce metal stress tolerance. Thus, the present investigation is aimed to validate our hypothesis that BRs and PAs may interact to influence metal detoxification system and oxidative stress alleviation in general.
Reduced seedling growth (RL, SL and FW) under Cr (VI) stress might be associated with ability of Cr (VI) to compete for uptake transporters mainly used by indispensable divalent ions (Fe, P and S) which are required for normal plant growth 
. Combination of EBL and Spd was most effective in enhancing seedling growth parameters under Cr-stress. Improved morphological parameters could be attributed to BR (EBL) ability to regulate cell elongation and divisional activities through the up-regulation of xyloglucan endo-transglycosylase 
. Improved seedling growth may also be connected with potential of Spd to influence cell elongation process 
. Recently, a study has indicated the crucial involvement of Spd as a morphogenic determinant of cell fate in male gametophyte of water fern 
. It is interesting to observe that co-application of EBL and Spd has more pronounced effects on the improvement of seedling growth under Cr-stress, perhaps due to their synergistic or additive effects.
Participations of PGRs in heavy metal detoxification have been widely established 
. To further test the objective of our hypothesis that EBL and Spd may influence profiles of PGRs (PAs, auxins and ABA) to alleviate metal stress, we determined the endogenous titers of PGRs in Cr-stressed seedlings treated with EBL and/or Spd. PA biosynthesis and catabolism have been directly associated with abiotic stresses 
. Our results have validated the hypothesis on the influence of EBL and Spd interactions on PA pools and their involvement in Cr-stress mitigation. Enhanced Put content in Cr-stressed seedlings supplemented with EBL might contribute to, at least in part, improvement of seedling growth (). In addition, further improvement in Put titers for co-application of EBL and Spd under Cr-stress than their independent treatments suggests an additive or synergistic effect on PA metabolism.
It is well established that the levels of PAs, such as Put and Spd, in a plant tissue increase under various abiotic stresses 
. Being antioxidant in nature, the increased volume of Spd ascertains ROS and metal detoxification. Therefore, the exogenous application of Spd in our present experimental set up acted as a supplement to further enhance the overall level of Spd to mitigate Cr-induced oxidative stress. Spd is mostly expressed and synthesized in higher quantities under stress and is restored to normal level upon alleviation of stress. In the present study, significant increase in Spd content in Cr-stressed seedlings may be associated with the effort to combat or neutralize ROS generated under Cr-stress through utilization of more basic nature of Spd. We observed that EBL application alone could reduce the Spd titer significantly under Cr-stress. This downfall in Spd by EBL alone was more than the decline in Spd level brought by individual application of Spd under Cr-stress. Moreover, EBL and Spd co-application with Cr-stress could also lower Spd titers, though insignificantly, thus indirectly reflecting the diminution of Cr-stress (). Our previous studies also indicated significant impacts of EBL on the endogenous titers of PAs under Cu2+
. From the above observations, it may be suggested that EBL and Spd co-applications may have additive and/or synergistic interactions to influence endogenous Put and Spd pools through selective modulation of RsADC
gene expression to minimize the Cr-induced oxidative stress damages in radish.
Enhanced titers of free and bound IAA were recorded in seedlings fed with Cr-solution enriched with EBL alone. No significant increase in either free or bound IAA was observed for Spd application alone under Cr-stress. It was further noticed that co-application of EBL and Spd was more effective in improving the endogenous titers of free and bound IAA than their independent application, resulting in improved seedling growth even under Cr-stress (). This may be one of the strategies which EBL uses to enhance metal stress tolerance through increasing the endogenous levels of free and bound IAA. Moreover, active participation of auxins in metal stress alleviation can be supported by the observations of Dimpka et al. 
. These authors have found stress protective role of auxins with siderophores in heavy metal stress diminution in Streptomyces spp
ABA is a stress hormone protecting plants under abiotic stresses. Homeostasis of ABA endogenous titers is very crucial for the normal metabolic activities, such as opening and closing of stomata, dormancy and senescence. Excess of ABA synthesis leads to closure of stomata with net outcome of reduced photosynthesis 
. In the present investigation, Cr-stress was found to enhance both free and bound ABA concentrations over untreated control (). In our present study augmented ABA profiles under Cr-stress might be connected with reduced seedling growth, associated with enhanced dormancy and low quantum yield recorded in young radish seedlings. Slight decrease in free and bound ABA levels in Cr-stressed seedlings supplemented with either EBL or Spd might partly contribute to improved seedling growth under metal stress tolerance. We observed that co-application of EBL and Spd was able to reduce ABA titers from stressful regime to nearly normal levels (), which may be correlated with normalization of metabolic activities by lowering ABA levels. Recently, interactions of BRs and ABA have been shown to improve thermotolerance of A. thaliana
. Additionally, another independent study has indicated the possible interactions of ABA with PAs to orchestrate PA metabolism and stress responses in grapevine 
Improved antioxidant system is a key player in alleviation of oxidative stress posed by ROS 
. We also verified our hypothesis by checking the effects of EBL and Spd co-applications on the major components of antioxidant system. Modulated concentrations/activities of antioxidants/antioxidant enzymes, such as GSH, ASA, PL, GB and TP, and enzyme activities (GPOX, CAT, SOD and GR), noted in Cr-stressed seedlings demonstrated their active participation in oxidative stress management (, ). Application of either EBL or Spd with Cr-stress increased antioxidant titers, with ultimate aim to improve seedling growth under Cr-stress. Furthermore, improvement in antioxidant system in seedlings fed with both EBL and Spd together under Cr-stress was more evident than their individual application. Our findings are in concordance with the observations reported previously 
, which showed improvement/modulation in antioxidant (ASA, GSH and PL) titers and enzyme activities in R. sativus
upon 28-homobrassinolide treatment under Cr-stress.
Cr-stressed seedlings showed significant downfall in the levels of examined stress indicators, including MDA, TSS, Chl a, Chl b and Cart, and H2
, when compared with untreated control (). Application of EBL and Spd together was able to increase the levels of TSS, Chl a, Chl b, Cart and TP more significantly than their individual treatments, thereby more effectively improving the Cr-stress management capacity of radish seedlings. The active implication of these stress indices as oxidative stress markers has been widely documented in plants 
PCs minimize metal toxicity through complex formation and conjugation. Enhanced titers of PCs observed under Cr-stress (, 
) were found to be improved in Cr-stressed seedlings fed with either EBL or Spd. Moreover, co-application of EBL and Spd has more prominent effects on PC biosynthesis than their individual applications (). Furthermore, applications of EBL and Spd, either together or alone, were also able to reduce the Cr (VI) uptake (), through a less known phenomenon affecting adventitious roots of seedlings 
Additionally, radical scavenging assays performed in this study demonstrate strong impact of EBL and Spd applications on the abilities of radish seedlings to scavenge ROS. Reduced values of DPPH, deoxyribose and reducing power assays and higher contents of H2O2 content were recorded under Cr-stress (). Positive impact of EBL and Spd alone/together on reduction in ROS volume also revealed Cr-stress alleviation. Both NBT and DAB staining methods also showed that EBL and Spd applications, alone or together, could reduce the production of O2− and H2O2 under Cr-stress (). However, applications of EBL and Spd were able to remarkably improve the DPPH, deoxyribose and FRAP values, while significantly reduce H2O2 content in Cr-stressed seedlings. Co-application of EBL and Spd was found to have better effect on ROS scavenging than their individual applications (). In addition, EBL and Spd induced decline in H2O2 content under Cr-stress was associated with a significant decrease in NADPH oxidase activity ().
Cr-induced ROS caused severe damages to membrane permeability as shown by increased EC value (). However, applications of EBL and Spd, either individually or together, were able to reduce the EC values, perhaps by enhancement of antioxidant system and various stress-related parameters as discussed above. This result indicates the stabilizing effects of EBL and Spd, especially when co-applied, on the membrane permeability under Cr-stress.
Our results demonstrate that co-application of BRs and PAs is more effective in alleviation of Cr-stress than their individual treatments. Improved Cr-stress mitigation with EBL and Spd co-application involves physiological and molecular interactions in a synergistic and/or additive manner. Therefore, these findings provide a unique and eco-friendly strategy employing interplay of BRs and PAs to overcome heavy metal stress mitigation, and abiotic stress in general, in radish.