PH dependent salinity-boron interactions impact yield, biomass, evapotranspiration and boron uptake in broccoli (Brassica oleracea L.)

Abstract

Soil pH is known to influence many important biochemical processes in plants and soils, however its role in salinity—boron interactions affecting plant growth and ion relations has not been examined. The purpose of this research was to evaluate the interactive effects of salinity, boron and soil solution pH on broccoli (Brassica oleracea L.) growth, yield, consumptive water use and shoot-boron accumulation. A greenhouse experiment was conducted using a sand tank system where salinity-B-pH treatment solutions were supplemented with a complete nutrient solution. Sulfate-dominated irrigation waters, characteristic of groundwater in California’s San Joaquin valley (SJV), were tested at EC levels of 2, 5, 8, 11 and 14 dS m−1. Each salinity treatment consisted of two boron treatments (0.5 and 21 mg L−1) and each of those treatments was tested under slightly basic (pH 8.0) and slightly acidic (pH 6.0) conditions. Results included multiple salinity-boron-pH interactions affecting shoot biomass, head yield and consumptive water use. Broccoli fresh head yields were significantly reduced by salinity and boron, but the degree of yield reductions was influenced by pH. Relative head yields were substantially reduced in treatments with high pH and high B, particularly under low and high salinity where head yields were decreased by 89 % and 96 %, respectively, relative to those at low salinity and low boron. Intermediate levels of salinity were far less damaging. Increased salinity and boron reduced evapotranspiration (ET) and there were no salinity-boron associated interactions on ET. However, increased salinity and boron concentrations increased water use efficiency (shoot biomass/cumulative volume ET). Shoot B concentration increased with increased boron and was greater at pH 6 as compared to pH 8. Shoot boron concentration decreased with increasing salinity at both pH levels but particularly at the high substrate boron concentration. It is likely that different mechanisms, yet unknown, are responsible for severe head-yield reductions at low and high salinity in the presence of high boron under alkaline conditions. We found that boron in the shoot did not accumulate by a simple passive process. Rather as boron increased from 0.5 to 21 mg L−1, there was a restrictive mechanism where total shoot boron (mg plant−1) was reduced by 10 to 40 times the amount potentially supplied to the shoot by passive transport via mass flow perhaps involving complex interactions with membrane channels and B exporters. Total shoot boron concentration was a poor indicator of plant growth response.