Photodynamic herbicides. Recent developments and molecular basis of selectivity

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Abstract
In 1984, we described the development of a new concept in the design of photodynamic herbicides and demonstrated the phenomenology of the process by using a harmless amino acid, δ‐aminoevulinic acid (see Reference 1 of this article). δ‐Aminolevulinic acid (ALA) is the precursor of all tetrapyrroles in plant and animal cells. By spraying plants with ALA, the latter is converted to tetrapyrroles which accumulate in the plant tissues. In light, the accumulated tetrapyrroles kill some plant species while other plant species are left unharmed. It has recently become apparent that susceptible plant species are divinyl or monovinyl dicotyledonous plants, while monovinyl mono‐cotyledonous plants are less susceptible. At night, monovinyl plant species accumulate mainly monovinyl tetrapyrroles, while divinyl plants accumulate mainly divinyl tetrapyrroles (see Reference 43 of this article). When divinyl plant species are induced to accumulate large amounts of divinyl tetrapyrroles or lesser amounts of monovinyl tetrapyrroles, they are killed by the photodynamic action of the tetrapyrroles in light. Monovinyl plant species appear to evade destruction by conversion of the divinyl tetrapyrroles to monovinyl tetrapyrroles. The latter are then readily converted to chlorophyll in light. Three groups of chemicals which can act in concert with ALA have now been identified, namely: (I) enhancers of ALA conversion to tetrapyrroles, (2) inducers of ALA formation by plant tissues, and (3) inhibitors of divinyl tetrapyrrole conversion to monovinyl tetrapyrroles. By combining ALA with a member(s) of one or more of the foregoing groups of chemicals, it has become possible to design herbicidal formulations which are effective against a wide range of field conditions.
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