Can Broccoli Tolerate Higher Concentrations of Boron under Saline Conditions?.
Stephen Grattan1, Catherine Grieve2, Timothy E. Smith1, Andre Lauchli1, James A. Poss2, and Donald Suarez2. (1) University of California, Davis, One Shields Ave - Dept of LAWR, 231 Veihmeyer Hall, Davis, CA 95616, (2) US Salinity Laboratory, 450 W. Big Springs rd, Riverside, CA 92507
Reuse of saline drainage water is a management option that is necessary for reducing the volume of drainage water produced on the west side of California's San Joaquin Valley (SJV). A potential limitation in implementing a drainage water reuse system is determining the extent by which boron, a naturally occurring element in the drainage water, affects the selection, growth and yield of crops in the reuse system. Boron is a concern for several reasons. First, boron is an element that is essential for crops but has a small concentration window between that what is considered deficient and that which is potentially toxic. Second, it has a higher affinity to the soil than common salts requiring much more water to reclaim soil B to pre-existing levels than it does to reduce the salinity to pre-salinization levels. Furthermore, the B concentration in San Joaquin Valley drainage water varies widely but in nearly all cases, it far exceeds levels that would result in toxic conditions based on B-tolerance guidelines. Greenhouse studies using an elaborate sand tank system at the USDA-ARS, George E. Brown, Jr. Salinity Laboratory are underway to evaluate B tolerance particularly in relation to salinity and pH of the soil environment. The experiments were designed to determine the interactive effects of salinity, pH, and boron on broccoli performance including growth, yield, injury, and ion relations. Broccoli (Brassica oleracea L., botrytis group, cv Seminis PX511018) was selected because it is a crop common to the Westside of the SJV and is known to be moderately sensitive to salinity and moderately sensitive to B in non-saline systems. In one study, the irrigation treatments consisted of three salinity levels representing non-saline (1.5 dS/m), moderately saline (11dS/m) and saline (18 dS/m) conditions. At each salinity, the composition was either chloride dominated or sulfate dominated. The pH of the solutions was maintained between (5.7 and 6.7) using additions of sulfuric acid. Results from this study indicate that both Cl-based salts and those characteristic of shallow saline drainage water (i.e. a mixture of salts dominated by sodium sulfate) showed a significant salinity-boron interaction. At high salinity, increased B concentration was less detrimental, both visually and quantitatively (i.e. biomass), than it was at low salinity. That is, plants could tolerate a higher solution B-concentration at higher salinity. However there was no significant difference between salt types. Regardless of the composition of the salinizing solution, increased salinity increased shoot B concentration when B concentrations in the solution were relatively low (i.e. 0.5 mg/L). At the highest solution B concentration (28 mg/L), increased salinity reduced shoot B concentration. Solution B in itself had very little influence on shoot ion accumulation but both salinity (i.e. EC) and salinity composition had very strong influences on shoot tissue ion composition. Cumulative water use of broccoli was evaluated in relation to the various treatments. Cumulative ET was, for the most part, directly related to cumulative biomass; the higher the cumulative biomass the higher the cumulative ET (data not shown). Stable isotopic ratios of oxygen in the solution were used to separate evaporation and transpiration. With these estimates, we were able to provide insight into whether B uptake is truly passive with the transpiration stream as many have suggested in the literature or whether the plant is able to regulate the amount of B it absorbs and transports to the shoot. In no treatment did shoot B accumulate to a level predicted based on transpiration volume times solution B-concentration. Plants treated with low B contained the largest percent of B uptake (10-60%), expressed relative to predicted passive uptake. Salinity treatments, regardless of composition, represented the higher percentage range. On the other hand, plants treated with high B (14 or 28 mg/L) only accumulated 1-2% of that predicted if uptake and accumulation were truly passive. Therefore, under conditions of high external B, the amount of B that is absorbed and/or transported to shoots of broccoli (and perhaps other species) is controlled by some unknown mechanism, a finding that deserves further investigation. Another study is currently underway to investigate the influence of pH (6.0 and 8.0) on salinity - B interactions. pH was included as a treatment because of its influence on B uptake by plants and that SJV soils generally are slightly basic in nature. Preliminary observations indicate that broccoli performs better in slightly acidic conditions rather than under slightly basic conditions. Similar to what was observed in the previous study, plants at low salinity and high boron are currently showing symptoms where older leaves cup upward in succession. Leaf and stem tissue with be analyzed for key elements at the end of the study.