Effect Of Different P H Levels On Cadmium Accumulation In Rice (Oryza Sativa L.) Seedlings

Cadmium?Cd?is a potential threat to human beings while consuming food particularly rice as it is easily absorbed and translocated by rice.Cd uptake and transport is highly dependent on the pH of the medium in which plants grow.pH is a key variable in the bioavailability of Cd to plant.In this study,a hydroponic experiment was carried out to investigate the effect of five pH levels?3.5,5.0,6.0,7.0,and8.0?and three Cd concentrations?0,0.5,and 2.0?M CdCl₂?on Cd accumulation of two rice?Zhonjiazao17 and XZY?cultivars.qRT-PCR was performed in shoot and root of rice plants to understand the role of different metal transporters in Cd uptake and transport under different pH levels.The main results obtained from present study are listed below:1.The Cd content in root,shoot and total Cd accumulation rate increased as the external solution Cd concentration was increased from 0.0 to 2.0?M at each pH level.The tolerance index%?Ti?of both cultivars also showed that the ability of both cultivars was reduced significantly in high Cd?2.0?M?as compared to low?0.5?M?Cd concentration.Cd content in the root and shoot was increased as the pH of the medium was enhanced from 3.5 to 6.0,but the Cd content of the plants decreased when the pH reached at 7.0 and 8.0.The topmost Cd accumulation occurred at pH 6.0 at each of the Cd level in both cultivars,with the one exception of the roots of XZY seedlings exposed to 2.0?M Cd,where highest Cd accumulation occurred at pH 7.0.At a given level of Cd stress,ZJZ17 seedlings tended to accumulate less Cd than XZY seedlings,both in the shoot and in the root.Both cultivars showed more Cd content and Cd bio-concentration factor?BCF?in their root as compared to the shoot.2.The effect of raising the medium's pH from 3.5 to 6.0 was to up-regulate OsNRAMP1,while a further rise to pH 7.0 and 8.0 reduced the abundance of this transcript?the sole exception to this behavior was in the roots of XZY seedlings exposed to 2.0?M Cd,where the peak level of OsNRAMP1transcription occurred at pH 7.0 rather than at pH 6.0?.The transcriptional response of OsHMA2 was similar to that of OsNRAMP1 in shoot,although in this case the abundance transcript of OsHMA2 in the roots of XZY seedlings exposed to either 0.5 or 2.0?M Cd continued to increase up to a pH of 7.0.Generally,the abundance of both OsNRAMP1 and OsHMA2 transcript was higher in XZY than in ZJZ17 seedlings for a given pH/Cd environment.3.Zn content was largely unaffected by the medium's Cd concentration.In both shoot and root,the highest Zn content was observed at pH 6.0 and 7.0 as compared to rest of other pH levels under different Cd concentrations.However,the results showed that Fe translocation to shoot was suppressed by the outer Cd concentration especially in 2.0?M Cd.In shoot,a V-type pattern was formed under 0.5and 2.0?M Cd where the lowest Fe content was noticed at pH 6.0 in both cultivars.The ZJZ17 root Fe content was highest in seedlings grown at pH 7.0 either in 0.0?M or 0.5?M CdCl₂,and in those grown at pH 8.0 in the 2.0?M Cd,but that of XZY was highest at pH 6.0 at each level of Cd stress.4.The genes OsZIP5 and OsYSL15 shown to be most likely responsible for the accumulation and translocation of both these elements in rice seedlings.The peak transcription of OsZIP5 in the shoot occurred at pH 6.0 irrespective of the concentration of Cd in the growth medium.However,OsYSL15was transcribed at a low level at pH 6.0 in the shoot of plants exposed to either 0.5 or 2.0?M Cd stress.In the root,the transcriptional pattern of OsYSL15 was consistent with that of the uptake of Fe in root of both cultivars.5.Cd concentrations of the medium and its pH exerted a significant effect on seedling growth in both cultivars.Shoot and root fresh and dry weight,shoot and root lengths were decreased as the medium's Cd concentration was enhanced from 0 to 2.0?M Cd at each pH level.Instead of the Cd concentration,seedling growth was reduced as the medium's pH was raised from 3.5 to 6.0,and then rose when the pH was increased to 7.0 and to 8.0.The poorest performance for each of the traits occurred at pH 6.0.6.The stress induced by the presence of either 0.5?M or 2.0?M Cd had only a small?but nevertheless significant?effect on the leaf chlorophyll values?SPAD?in both cultivars.pH levels showed a V-type trend in acquiring SPAD chlorophyll values where the low SPAD values were observed at pH 6.0 of each Cd concentration in both rice cultivars.7.In summary,Understanding Cd uptake and transfer mechanism is pivotal for breading ground cadmium free rice and uncovering the role of pH response,is a key component for the understanding Cd uptake and transport mechanism for rice production.Tracking the performance of seedlings exposed to the three Cd concentrations and five pH levels in a growth medium confirmed that growth of two rice cultivars were impacted by both of these factors.Increasing the external Cd concentration resulted in an increased accumulation of Cd into the plants.The Cd accumulation was highest from a near-neutral?pH6.0?to neutral?pH 7.0?medium,driven by the up-regulation of the genes OsNRAMP1 and OsHMA2.The ZJZ17 cultivar proved to be grown best under low and high pH conditions in Cd stress than did XZY.Less Cd can be translocated to grains of rice if plants accumulate lower amount of Cd in their vegetative parts.Growing rice in a hydroponic culture with different pH levels until maturity is quite difficult to handle due to other factors affecting pH conditions.So,lime or sulfur application techniques can be used to better manage pH levels in Cd polluted soils to get less Cd contaminated grains under low or high pH levels.So,our results will help to lower the Cd contamination at the vegetative growth stage and will give an insight into the Cd uptake and transfer mechanism of rice,and implicit to limit Cd accumulation in grain would be through regulation of pH condition in rice growing soils.