Physiology and yield of field‐grown Brassica oleracea L. exposed to acidic fog

Abstract

SUMMARYField‐grown Brassica oleracea L. ev. Emperor (broccoli) plants were exposed to a range of treatments with simulated acidic fog in open‐field plots to examine the effect of fog chemistry on yield and growth, and to determine whether alterations in yield or growth were related to the development of foliar injury and/or the inhibition of net photosynthesis. Simulated fogs (i.e. pH values of 1.76, 2.23, 2.72, 3.22 or 5.48) with a liquid water content of 0.20–0.25 g m−3 were applied twice weekly for 10 weeks. Applications of pH 1.76 fog caused significant yield reductions (37%) relative to pH 5.48. Leaf dry weight responses were more sensitive to fog chemistry than yield. Leaf dry weight was significantly inhibited by 53, 25 and 9% in the pH 1.76, 2.23, and 2.72 fog treatment groups, respectively.The depression of yield by fog of pH 1.76 was closely correlated with the development of foliar injury and the inhibition of net photosynthesis on a percentage basis. Plants exposed to pH 2.23 or 1.76 fog consistently displayed amounts of injury ranging from 5 to 7% or 20 to 31%, respectively, whereas plants exposed to pH 2.72 fog or higher exhibited less than 5% injury throughout the study. Whole‐study average rates of net photosynthesis were significantly depressed by pH 1.76 fog (30% lower). The inhibition of net photosynthesis was coincident with increases in both mesophyll and stomatal resistances to CO2, but the ratio of mesophyll to stomatal resistance was not significantly altered.These findings provide evidence that repeated exposures to highly acidic fog (pH 1.76 or 2.23) may cause significant reductions in marketable yield and/or growth of B. oleracea, but substantive adverse effects would not be expected from exposures to fog of lower acidic strength. Although the inhibition of net phososynthesis in B. oleracea by acidic fog was coincident with decreased yield, reduced rates of CO2 assimilation appear to be due to the development of foliar injury (i.e. reduced photosynthetically active leaf area per plant) rather than a specific alteration in mesophyll or stomatal resistance to CO2.