The photodynamic herbicidal performance of δ-aminolevulinic acid in combination with four chlorophyll biosynthesis modulators was evaluated under greenhouse conditions, using corn, soybean, and ten weed species. Treatments resulted in accumulation of various amounts of protoporphyrin IX and of monovinyl and divinyl Mg-protoporphyrin IX and protochlorophyllide. Accumulation of these tetrapyrroles was accompanied by various degrees of photodynamic injury, depending on treatment, plant species, and somewhat the modulator. The lower photodynamic susceptibility of dark monovinyl/light monovinyl and dark divinyl/light divinyl plants toward the accumulation of monovinyl and divinyl protochlotophyllide, respectively, was attributed to their greater abilities to metabolize these protochlorophyllides in the light. On the other hand, the higher photodynamic susceptibility of the dark monovinyl/light divinyl weed species toward the accumulation of monovinyl protochlorophyllide was attributed to their lower ability to metabolize the accumulated monovinyl protochlorophyllide in the light.

Dormant barnyardgrass seeds imbibed in darkness for 8 days at 38 C displayed a very low fluence response to phytochrome, and very little germination occurred in darkness. However, if seeds were imbibed at temperatures >42 C or exposed to 46 C for only 0.5 h, considerable germination was evoked in the absence of a light stimulus. It is proposed that the high temperatures caused a transition in dormancy from a light-requiring to a light-independent state. The finding may have relevance to field behavior of barnyardgrass seeds where burial could prevent reception of the required light stimulus.

Barnyardgrass seeded in field plots at monthly intervals from March through October at Shatter, CA, began to emerge in March when soil temperatures reached 17 C at a depth of 5 cm. For all plantings, most of the seeds germinated and produced seedlings within 2 weeks. Increasing day lengths favored vegetative growth, with April, May, and June plantings producing the tallest plants, the most dry matter, and the largest number of tillers. July and August plantings, the first to flower, began flowering and producing seeds within 5 to 6 weeks after planting, but the greatest number of seeds (15 000 to 25 000/plant) was produced by the larger plants of the April, May, and June plantings. September plantings produced few seeds/plant (37), when compared to March (5900), July (9800), and August (9700) plantings. The earliest that seed production would be expected from seedlings emerging in March and April would be early June, and seedlings emerging in September could produce seeds before killing frosts.