A fundamental component of modeling crop interference is the effect of understory photosynthetic photon flux density (PPFD) on weed architecture and growth. The effect of decreased PPFD on spaced redroot pigweed dry matter accumulation, distribution, and plant architecture was quantified by increasing corn density and delaying weed planting date. As canopy-transmitted PPFD declined, total dry matter accumulation decreased and relative dry matter distribution was greater to main-stem components than to branch components. Increased rectangularity in understory weed architecture was associated with a concomitant decrease in branch number. The proportion of leaf area and dry matter in the upper segment of the redroot pigweed increased as PPFD declined with increased corn density. Results suggest that changes in plant architecture, as influenced by canopy-transmitted PPFD, may be as important as those of total dry matter and leaf area when describing and predicting the effects of crop-weed interference.

Spot treatments with hexazinone (0.03, 0.046, and 0.06% ai [v/v]) were evaluated for their effects on thistle density, seed production, and crown vetch vigor from 1991 to 1994 to determine if long-term usage of hexazinone caused damage to the vetch and exacerbated the thistle problem. Thistle densities were reduced within a year at any concentration of hexazinone and seed production was reduced or eliminated. Environment had a large impact because initial thistle densities varied widely among years. The amount of precipitation during July of the previous year accounted for 89% of the variation in initial thistle densities in the subsequent year. Vetch injury from hexazinone was also best explained by the amount of precipitation during July. Vigorously growing crown vetch in wet years may have suppressed the thistles enough to reduce density the following year while drought years reduced the competitiveness of the crown vetch, resulting in higher initial densities of thistles in the following year.

The suitability of several methods for estimating light conditions in the understorey of tropical forests, and of different sampling schedules was evaluated. Light conditions at 16 understorey sites in a Panamanian lowland forest were continuously measured for 9 mo with quantum sensors and photodiodes. Light conditions at the sites were also assessed indirectly with hemispherical fisheye photographs, plant canopy analysis, 38-mm photographs, 24-mm photographs and a spherical densiometer. Estimates from all indirect methods, except the spherical densiometer, were highly correlated with the direct measurements. Short-term direct light measurements for a day or a week also correlated with long-term light conditions. The indirect measures differed by up to c. 70% from the direct measures relative to single site measurements. Hence, the indirect methods are inadequate where single site light conditions have to be assessed accurately. However, because light conditions encountered in the understorey varied up to 13-fold, the indirect methods were found to be well suited to rank understorey light conditions among a large number of sites. The results from frequent and infrequent sampling schedules differed only slightly, suggesting that taking indirect measures at the beginning and the end of a study offers a reasonable compromise between accuracy and sampling effort.