Salinityʹs influence on boron toxicity in broccoli: I. Impacts on yield, biomass distribution, and water use

Research addressing the interactive effects of the dual plant stress factors, excess boron and salinity, on crop productivity has expanded considerably over the past few years. The purpose of this research was to determine and quantify the interactive effects of salinity, salt composition and boron (B) on broccoli (Brassica oleracea L.) fresh head yield, biomass distribution and consumptive water use. A greenhouse experiment was conducted using a sand-tank system in which salinity–B treatment solutions were supplemented with a complete nutrient solution. Chloride-dominated salinity and salinity characteristic of Californiaʹs San Joaquin valley (SJV), or sulfate-dominated, were tested at ECw levels of 2, 12 and 19 dS m−1. Each salinity treatment consisted of boron treatments of 0.5, 12 and 24 mg L−1. Plant head yield and shoot biomass were significantly reduced by both salinity and boron. Moreover, there was a significant salinity–boron interaction where increased boron was relatively less detrimental under saline conditions. These results occurred regardless of the salt solution composition (chloride or SJV). We found that an ‘interactive model’ better described our growth response than did a ‘single stressor yield model’. Salinity and boron also affected the distribution of shoot biomass. Regardless of salt type, as salinity increased, the fraction of biomass as leaf tissue increased while the biomass fraction as stems and particularly heads, decreased. However, an increase in B at low or high salinity with the SJV composition, decreased the head biomass fraction. This was not observed at moderate salinity, nor on any plants treated with Cl-dominated salinity. Cumulative evapotranspiration (ET) was also reduced by increased salinity but water use efficiency (WUE) was not. WUE was reduced by increased boron, but only at the low and high salinity levels.