Effect Of HCO₃ on Iron Nutrition Of Rice Under Drip Irrigation With Plastic Film Mulch

Aims:Rice cultivation by drip irrigation with plastic film mulch（DI-PFM） is a new type of rice cultivation method first proposed by Xinjiang Tianye Group. The system showed great water-saving potential and yield enhancement in practice. However, rice seedlings often develop chlorosis even died when grown with DI-PFM on calcareous soil. So, chlorosis is a threat to realize for high yield of DI-PFM and the mechanism of iron chlorosis is not clear. It is known that HCO₃^- is an important case of plant chlorosis in calcareous soil. In this paper, we compared the traditional flood irrigation cultivation（FI） and drip irrigation（DI）, to study the effect of HCO₃^- on rice growth, iron uptake and the different of physiological characteristics of these two cultivation systems. The aim is to figure out the mechanism of iron chlorosis of DI-PFM and to provide certain theoretical basis for alleviating rice chlorosis in DI-PFM.Methods:The research conducted at Agricultural Experiment Station in Shihezi University（pot culture）,Shihezi, Xinjiang province, China in 2014 and 2015. Lowland rice genotype T-43（Oryza sativa L.T43） was used in this study. Two potted experiment was composed. In pot experiment I（2014）,two-factor was designed as irrigation methods（conventional flooding irrigation（FI） and drip irrigation（DI）） and two HCO₃^- levels in irrigation water（2 mmol/L and 10 mmol/L）, aims to ascertain the different of iron uptake and growth by HCO₃^- under these two cultivation systems. In pot experiment II（2015）, two-factor was designed as irrigation methods（（FI） and（DI）） and four 0,2, 10, 40 mmol/L HCO₃^- levels in irrigation water（referred to as BC-0, BC-2, BC-10 and BC-40,respectively）, aims to figure out the mechanism of iron chlorosis under these two cultivation systems.Results:（1） The HCO₃^- concentrations of the soil solution were greater under FI than under DI, because more irrigation water was applied in the FI system. But soil pH was significantly higher in DI than in FI.（2） Soil DTPA-Fe concentration, leaf SPAD values, leaf total Fe concentration, leaf active Fe concentration, shoot biomass, and root biomass decreased as the HCO₃^- concentration of the irrigation water increased. The decreases were less under DI than under FI.（3） Xylem sap pH increased with HCO₃^- concentration increase and xylem sap pH of rice was higher in FI than in DI.（4） Soil DTPA-Fe concentration, root oxidation activity, leaf active Fe content, symplast Fe content, rice biomass, Fe accumulation and Fe content, decreased as the HCO₃^- concentration of the irrigation water increased both in these two cultivation systems, while the concentration of Fe in apolplast accumulated as the HCO₃^- concentration of the irrigation water.（5） Rice root and leaf total Fe concentration sometimes higher in higher HCO₃^- treatments than low HCO₃^- treatments, but rice total Fe content, leaf total Fe content, Fe accumulation, the grain harvest index and the Fe harvest index were decreased as the HCO₃^- concentration of the irrigation water increased.Conclusions:（1） The main cause of rice seedling chlorosis was that soil DTPA-Fe concentration significantly lower in DI than in FI. And low temperature and soil Eh changes were also the reason leadingchlorosis.（2） The HCO₃^-concentration leads soil pH increased higher in DI than in FI and soil DTPA-Fe decreased was a reason induces rice Fe chlorosis.（3） Irrigation water contain a high concentration of HCO₃^-will reduce the rice root activity, thus inhibiting rice roots on absorb iron nutrition.（4） Bicarbonate cause rice iron nutrition precipitate in apolplast due to it cause rice tissue fluid alkaline, and effect rice iron nutrient absorption and distribution.（5） Bicarbonate reduce rice growth in flooding irrigation is more severe than in drip irrigation on calcareous. The reason is xylem sap pH rise higher and yield decreased severe in FI than in DI. So,irrigation water contain a high concentration of HCO₃^- leads soil pH increased and induce soil DTPA-Fe decreased was a reason induces rice Fe chlorosis.