A source of phytotoxic substances

In the discussion that follows the emphasis is placed on toxicity, even though the same substances may occasionally be stimulating under test conditions. 

Observations that indicated the possibility of,the presence of phytotoxic organic substances in soils were made at least as early as the first part of the nineteenth century. This possibility was not given very serious consideration for the remainder of the century, although there were occasional references to the subject. During the past 30 to 40 years, however, as methods have improved and as all research has intensified, the subject has been given ever-increasing attention. 

The literature on this subject has been thoroughly reviewed by several investigators (Evenari, 1949 Bonner, 1950 Stallings 1954 Zaumeyer, 1958 Brian, 1951, 1957a,b, 1960 Bomer, 1960 Woods, 1960 Garb, 1961 McCalla and Haskins, 1964 Garrett, 1965 Jackson, 1965 Patrick and Toussoun, 1965 Rovira, 1965, and others) and hence will not be considered here in detail except for a few of the more recent publications. 

Inhibitors are released from roots, chiefly, but the leaves of some plants, such as Encelia farinosa, a desert shrub, contain inhibitors. If in the leaves, the inhibitor may be either leached into the soil or the leaves may fall to the ground and undergo decay. The berries of some plants are also the source of growth-inhibiting substances, such as parasorbic acid found in the mountain ash (Sorbus aucuparius). 

PLANTS REPORTED TO PRODUCE DIFFERENTIAL GROWTH-INHIBITORS OF KNOWN COMPOSITION (from GARB, 1961) 

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Unfavorable effects of mulches, occasionally observed, are most likely to result from deficiencies of nitrogen, and sometimes of phosphorus, that may occur under mulches of materials of wide carbon—nitrogen ratios. Mulches may also be a source of phytotoxic substances naturally present or synthesized by certain microorganisms that are active in their decomposition (see Chapter 18). 

After a botrytis attack, grapes and must contain polyosides with phytotoxic and fungistatic activities. B. cinerea also produces divers antibiotic substances botrytidial, norbotryal acetate and botrylactone. Some of these substances can be the source of fermentation difficulties. 

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