Discuss ion Sources of Lead in Perennial Ryegrass and Radishes C. Stafford Brandt Research Chemist, Crops Research Division, Agricultural Research Service, U S . Department of Agriculture, Beltsville, Md.
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f the authors had limited themselves to a report of their experimental protocol and data, I would have the difficult task of reviewing one more paper confirming the obvious: Atmospheric lead increases the lead content of vegetation. Fortunately, the authors have chosen to discuss some other implications of their work and even indulged in some speculation. This makes my job much easier, since I can now indulge in speculation and the simple task of second-guessing. The studies with filtered and unfiltered air are the major contributions of this paper. From these data, to my knowledge the first available, the message seems clear: Atmospheric lead contributes to the lead content of leaves, but other sources make a significant contribution. Actually, one-fourth of the filtered cs. unfiltered air data (the data on radish a t Argonne) indicates that atmospheric lead is not a contributor to vegetation lead. Intuition and the limited data presented on the possible contribution from water implicate the soil as the other source. The authors make no statement about the contribution of lead from the seed, but intuition helps us to dismiss this as a major contribution. The field data by themselves could be misleading, since soil lead and atmospheric lead are confounded. In ignorance of air pollution, one could make a fairly good case, from this data, for a passive accumulation of lead by vegetation, directly relatable to the soil lead content. I n view of the experimental protocol used, one cannot dismiss the possibility of significant variation in lead content of vegetation caused by variation in the lead content of the soil. There are a couple of changes in the protocol that I wish the authors might have made. Had they used a high and a low lead soil from the field experiment, in either or both of the filtered us. unfiltered air studies, we would have a far better basis for estimating the contributions of soil lead. Any two (or more) different soils, in either the chamber or the greenhouse study, or both, would have helped. As it is, location (and therefore environment) and soil are confounded. Perhaps this can be resolved next time. The rates of lead uptake (as related to atmospheric lead) are markedly different at the three locations. The authors suggest that this is due to growth rate. Some estimates of growth, such as yield, would help the reader to estimate the environmental conditions a t the three locations. The soils scientist would also like to see a better description of the soils than merely source and texture. “What was the pH?”-“Was any fertilizer added ?”-are two immediate questions. Perhaps this information is available in the author’s file. Such second-guessing is easy and more useful to the future than to the analysis of the present study. 1 must, however, take serious exception to the treatment of the data as presented in Figures 4 and 5. The first point, perhaps a minor one, has to d o with the justification of the logarithmic transform of the data
224 Environmental Science & Technology
o n lead concentration in the leaf. There is hardly enough data to demonstrate that such a transform is necessary to satisfy statistical needs. The second point questions the justification for combining all the data. We have three totally different environments, including soil, and two totally different classes of plant tissue. Combining these data hardly seems to serve the interest of pragmatism. By themselves, the two filtered air studies demonstrate quite well that there is a major contribution to the lead content of vegetation from the soil. The field study cannot do this by itself. The filtered air studies indicate that soil derived lead in the leaf tissue of the two classes of plants is not markedly different, and that the contribution from the two different soils was similar. The field data neither confirm nor deny this hypothesis. Combining all the data, especially with the obviously different rates of uptake at the three locations, does not contribute to the hypothesis. There is one other point of significance upon which the authors have not commented. They indicate that a foliar spray of water, containing considerable soluble lead, did not increase the lead content of the leaves. This raises an interesting question as to how atmospheric lead does increase the lead content of leaves. Most soluble materials applied as foliar sprays do enter the leaf in significant amounts. Soluble fluorides enter the leaf readily under such conditions. Generally speaking, particulate fluorides do not readily enter the leaf but remain on the surface, except in cases where the particulate is readily soluble, and where dew, rain, o r other moisture forms dissolve the particulate. We generally consider atmospheric lead to be primarily particulate and not readily soluble. We are therefore faced with a contradiction. Although a soluble lead compound, sprayed on a plant, does not enter the leaf, a supposedly insoluble particulate readily enters the leaf. There is a problem that should keep some graduate students busy for a while. Putting aside the second-guesses and my exception to the manner of data presentation, I must agree with the authors that plant tissues in our time (and probably earlier) contain significant quantities of lead of soil origin. If, as has been suggested, ancient man did have a dietary lead intake lower by one or more orders of magnitude than present man, we are indeed faced with a dilemma. Perhaps, despite the evidence afforded by his dental structure, he was almost strictly a carnivore. Considering our present knowledge of lead in animal tissue, I a m not sure that this would account for the suggested lower dietary lead. Certainly, plant material and therefore soil-derived lead, entered earlier man’s food chain somewhere. Perhaps the ancient bones of animals other than man’s should be analyzed for lead. However, this may only compound the dilemma.
