Macronutrient inputs to annual cropping systems can benefit weeds as well as crops, sometimes decreasing or eliminating the benefits of fertilization. This interaction between fertility management and integrated weed management is becoming increasingly important as these fields focus on efficiency and prevention, respectively. The risk of increased weed competition reflects the fact that weed biomass and height may be highly responsive to nitrogen, phosphorus, and/or potassium. This generalization is supported by monoculture studies of species such as redroot pigweed (Amaranthus retroflexus L.), common lambsquarters (Chenopodium album L.), and barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.], and by ecological theory. However, field studies indicate variation in the effects of macronutrients on weed-crop competition and crop yield, even within species groups. To address challenges in interpreting, comparing, and extrapolating from these diverse reports, we propose a conceptual framework that summarizes the mechanisms underlying observed variation within and between studies. The framework highlights functional traits and trends that help predict yield outcomes in binary weed-crop interactions. Important factors include timing of emergence, maximum heights of the weed and crop, and relative responsiveness to the added nutrient. We also survey recent work on the effects of nutrient source (for example, the composition of organic amendments) on weed-crop competition. Because different sources vary in their nutrient release dynamics and supplied nutrient ratios, they may have dramatically different effects on weed-crop competition and crop yield. Finally, we offer a guide to best practices for studies of fertility effects on weed-crop competition. Although this review highlights several topics requiring further research, including fertility effects on multispecies interactions and interactions with other environmental factors, emerging methods offer considerable promise. Ultimately, an improved understanding of nutrient effects on weed-crop competition will contribute to the efficient and effective management of diverse cropping systems.

Living mulches are cover crops grown simultaneously with and in close proximity to cash crops. Advantages of living mulches over killed cover crops may include increased weed suppression, erosion and leaching control, soil health, and resource-use efficiency. Advantages of living mulches over synthetic mulches may include enhanced agroecosystem biodiversity and suitability for a wider range of cropping systems. A major disadvantage of this practice is the potential for competition between living mulches and cash crops. The intensity and outcome of mulch-crop competition depend on agroecosystem management as well as climate and other factors. In this review, we consider the management of living mulches for weed control in field and vegetable cropping systems of temperate environments. More than 50 years of research have demonstrated that mechanical or chemical suppression of a living mulch can limit mulch-crop competition without killing the mulch and thereby losing its benefits. Such tactics can also contribute to weed suppression. Mechanical and chemical regulation should be combined with cultural practices that give the main crop a competitive advantage over the living mulch, which in turn outcompetes the weeds. Promising approaches include crop and mulch cultivar selection, changes to planting time, density, and planting pattern, and changes to fertilization or irrigation regimes. A systems approach to living mulch management, including an increased emphasis on the interactions between management methods, may increase the benefits and lower the risks associated with this practice.

Angiosperm dominance in terrestrial landscapes is partially attributable to high photosynthetic capacities. Angiosperms benefit from diverse anatomical and physiological adaptations, making it difficult to determine which factors may have been prerequisites for the evolution of enhanced photosynthetic rates in this group. We employed a novel approach to this problem: comparisons between angiosperms and Marsileaceae, a family of semi-aquatic ferns that are among the only land plants to match angiosperm photosynthetic rates. We found that Marsileaceae have very high stomatal densities and, like angiosperms but unlike all other ferns previously studied, exhibit wrong-way stomatal responses to excision. These results suggest that stomatal density and a little-studied angiosperm trait, the capacity for lateral displacement of guard cells into neighboring epidermal cells, are crucial for facilitating high rates of gas exchange. Our analysis also associates these adaptations in Marsileaceae with an increased risk of excessive water loss during drought. Our findings indicate that evolution in stomatal physiology was a prerequisite for high photosynthetic capacities in vascular plants and a key driver of the abrupt Cretaceous rise of the angiosperms.

A living mulch is a cover crop grown during the growing season of a main (cash) crop.Living mulches may reduce soil erosion and nitrate leaching more effectively than bare ground or non-living mulches. Living mulches can also make substantial contributions to non-chemical or diversified weed management programmes. However, living mulches sometimes fail to suppress weeds, or they succeed at the cost of main crop yield. Seeking to understand these divergent outcomes, we survey the effects of living mulches on weeds in temperate field and vegetable crops. Primary purposes of this review include (a) outlining mechanisms of weed control by living mulches, (b) identifying major sources of variation in weed control outcomes, and (c) highlighting knowledge gaps. The effects of living mulches on weeds are largely (but not exclusively) via competition and typically strongest when living mulch vigour is high at sensitive stages of weed life cycles. Best management practices depend on living mulch, main crop, and weed community characteristics. Living mulches often have different effects on different weed species, which may have far-reaching consequences for weed diversity and community composition. Future research on long-term changes to weed communities will clarify how and when living mulches should be adopted to increase agroecosystem biodiversity and promote sustainability.

Stomata respond to changes in light environments through multiple mechanisms that jointly regulate the tradeoff between carbon assimilation and water loss. The stomatal response to blue light is highly sensitive, rapid, not driven by photosynthesis, and present in most vascular plant groups but is believed to have been lost in the ancestor of leptosporangiate ferns. Schizaeales and Salviniales are the only leptosporangiate orders that have not been tested for stomatal responses to low fluences of blue light. We report that these stomatal responses are absent in Lygodium japonicum (Schizaeales). In contrast, we observed stomatal responses to low fluences of blue light in Regnellidium diphyllum and Marsilea minuta (Marsileaceae, Salviniales). In R. diphyllum, blue light triggered stomatal oscillations. The oscillations were more sensitive to atmospheric carbon dioxide concentration than to humidity, suggesting that the blue light responses of Marsileaceae stomata differ from those of angiosperms. Our findings suggest that Marsileaceae have physiologically diverged from other leptosporangiate ferns, achieving unusually high photosynthetic capacities through amphibious lifestyles and numerous anatomical convergences with angiosperms. Blue light stomatal responses may have contributed to this divergence by enabling high rates of leaf gas exchange in Marsileaceae.