Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/2010
Publication Date: 6/1/2010
Publication URL: http://www.ars.usda.gov/Services/docs.htm?docid=10135
Citation: Smith, T.E., Grattan, S.R., Grieve, C.M., Poss, J.A., Suarez, D.L. 2010. Salinity’s influence on boron toxicity in broccoli: II. Impacts on boron uptake, uptake mechanisms and tissue ion relations. Agricultural Water Management. 97(6):783-791. Interpretive Summary: Boron (B) is essential for crops, but the element has a narrow concentration window between that which is nutritionally adequate that which causes B toxicity symptoms, plant injury and subsequent yield reductions. Excess B often occurs in areas with saline soils and waters, where water reuse management strategies are practiced (e.g. the San Joaquin Valley of California). It is important, therefore, to evaluate B uptake by plants under saline conditions inasmuch as B toxicity may be confounded with the associated problems of salt accumulation. This report is a companion paper to one describing the effect of the dual stresses, boron and salinity, on the yield, biomass distribution and water use of broccoli, and focuses on plant uptake of mineral ions, their distribution within the plant, and the effect mineral ion relations may have on plant performance. We found that boron was sequestered in the lower broccoli leaves, away from the actively photosynthesizing leaves and developing head tissues. That the negative effects of boron were ameliorated as salinity increased suggests that boron may not be as limiting factor in saline water reuse systems as previously thought.
Technical Abstract: Limited research has been conducted on the interactive effects of salinity and boron stresses on plants despite their common occurrence in natural systems. The purpose of this research was to determine and quantify the interactive effects of salinity, salt composition and boron on broccoli (Brassica oleracea L.) performance, particularly, element accumulation, ion interactions and boron uptake processes. A greenhouse experiment was conducted using a sand tank system where salinity-B treatment solutions were supplemented with a complete nutrient solution. Chloride-dominated salinity and salinity characteristic of California’s San Joaquin valley (SJV), or sulfate-dominated, were tested at ECw levels of 2, 12 and 19 dS/m. Each salinity treatment consisted of boron treatments of 0.5, 12 and 24 mg L-1. Salinity, regardless of salt composition, reduced shoot boron concentration at the high boron concentration (24 mg L-1). However, increased salinity reduced shoot boron concentration when external boron concentration was low (0.5 mg L-1). Tissue Ca, Mg, Na, K, S and Cl concentrations were also affected by salinity level, chloride or sulfate salinity composition, and in some cases by substrate boron concentration. Calcium concentration in shoots were greater in chloride salinity as compared to SJV salinity; magnesium concentrations trended opposite and were greater in SJV salinity. Chloride and sodium shoot concentrations both increased with salinity, shoot chloride was greater with chloride substrate salinity and shoot sodium was greater with SJV substrate salinity. Using stable isotope analysis of solutions to separate transpiration from evapotranspiration (ET), we found that boron uptake and accumulation in the shoot was not simply the product of mass flow (solution concentration x cumulative transpiration), and the vast majority of the water lost from the tank system was by transpiration (> 90%) regardless of treatment. Under low substrate boron, the levels of boron in broccoli shoots could be not accounted for by simple passive uptake and transport in the transpiration stream, which suggests that some energy-dependent process was also occurring. However, under high boron treatments, broccoli plants exhibited a mechanism that limited boron uptake, transport and accumulation in the shoot.