Physiological and ecological constraints play key roles in the evolution of plant growth patterns, especially in relation to defenses against herbivores. Phenotypic and life history theories are unified within the growth-differentiation balance (GDB) framework, forming an integrated system of theories explaining and predicting patterns of plant defense and competitive interactions in ecological and evolutionary time. Plant activity at the cellular level can be classified as growth (cell division and enlargement) of differentiation (chemical and morphological changes leading to cell maturation and specialization). The GDB hypothesis of plant defense is premised upon a physiological trade-off between growth and differentiation processes. The trade-off between growth and defense exists because secondary metabolism and structural reinforcement are physiologically constrained in dividing and enlarging cells, and because they divert resources from the production of new leaf area. Hence the dilemma of plants: Th...

https://www.jstor.org/stable/pdfplus/2830650.pdf

The dilemma of plants: To grow or defend.

The degree of herbivory and the effectiveness of defense varies widely among plant species. Resource availability in the environment is proposed as the major determinant of both the amount and type of plant defense. When resource are limited, plants with inherently slow growth are favored over those with fast growth rates; slow rates in turn favor large investments in antiherbivore defenses. Leaf lifetime, also determined by resource availability, affects the relative advantages of defenses with different turnover rates. Relative limitation of different resources also constrains the types of defenses. The proposals are compared with other theories on the evolution of plant defenses.

https://science.sciencemag.org/content/sci/230/4728/895.full.pdf

Resource Availability and Plant Antiherbivore Defense

Expansion and intensification of cultivation are among the predominant global changes of this century. Intensification of agriculture by use of high-yielding crop varieties, fertilization, irrigation, and pesticides has contributed substantially to the tremendous increases in food production over the past 50 years. Land conversion and intensification, however, also alter the biotic interactions and patterns of resource availability in ecosystems and can have serious local, regional, and global environmental consequences. The use of ecologically based management strategies can increase the sustainability of agricultural production while reducing off-site consequences.

/pdf/agricultural-intensification-and-ecosystem-properties-3ptxhboitj.pdf

Agricultural Intensification and Ecosystem Properties

The evolutionary response of plants to herbivory is constrained by the availability of resources in the environment. Woody plants adapted to low-resource environments have intrinsically slow growth rates that limit their capacity to grow rapidly beyond the reach of most browsing mammals. Their low capacity to acquire resources limits their potential for compensatory growth which would otherwise enable them to replace tissue destroyed by browsing. Plants adapted to low-resource environments have responded to browsing by evolving strong constitutive defenses with relatively low ontogenetic plasticity. Because nutrients are often more limiting than light in boreal forests, slowly growing boreal forest trees utilize carbon-based rather than nitrogen-based defenses. More rapidly growing shade-intolerant trees that are adapted to growth in high-resource environments are selected for competitive ability and can grow rapidly beyond the range of most browsing mammals. Moreover, these plants have the carbon and nutrient reserves necessary to replace tissue lost to browsing through compensatory growth. However, because browsing of juvenile plants reduces vertical growth and thus competitive ability, these plants are selected for resistance to browsing during the juvenile growth phase. Consequently, early successional boreal forest trees have responded to browsing by evolving strong defenses during juvenility only. Because severe pruning causes woody plants to revert to a juvenile form, resistance of woody plants to hares increases after severe hare browsing as occurs during hare population outbreaks. This increase in browsing resistance may play a significant role in boreal forest plant-hare interactions. Unlike woody plants, graminoids retain large reserves of carbon and nutrients below ground in both low-resource and high-resource environments and can respond to severe grazing through compensatory growth. These fundamental differences between the response of woody plants and graminoids to vertebrate herbivory suggest that the dynamics of browsing systems and grazing systems are qualitatively different.

Carbon/nutrient balance of boreal plants in relation to vertebrate herbivory

Herbivorous insects use diverse feeding strategies to obtain nutrients from their host plants. Rather than acting as passive victims in these interactions, plants respond to herbivory with the production of toxins and defensive proteins that target physiological processes in the insect. Herbivore-challenged plants also emit volatiles that attract insect predators and bolster resistance to future threats. This highly dynamic form of immunity is initiated by the recognition of insect oral secretions and signals from injured plant cells. These initial cues are transmitted within the plant by signal transduction pathways that include calcium ion fluxes, phosphorylation cascades, and, in particular, the jasmonate pathway, which plays a central and conserved role in promoting resistance to a broad spectrum of insects. A detailed understanding of plant immunity to arthropod herbivores will provide new insights into basic mechanisms of chemical communication and plant-animal coevolution and may also facilitate new approaches to crop protection and improvement.

Plant Immunity to Insect Herbivores

The nitrogen content of a plant is only one of the many plant characteristics that are vitally important to herbivores. However, because of its central role in all metabolic processes as well as in cellular structure and genetic coding, nitrogen is a critical element in the growth of all organisms. Supplementary N often elicits enhanced health, growth, reproduction, and survival in many organisms. This suggests that N is a limiting factor. Since N makes up a large portion of the earth's atmosphere (about 78%), the problem is not an absolute but a relative shortage-that is, a scarcity of usable or metaboliza­ ble N during critical growth periods (159, 328). Plants encounter shortages of inorganic nitrogen (nitrate and/or ammonium ions); animals experience shortages of organic nitrogen (specific proteins and/or amino acids). This article reviews and examines the evidence (a) that N is scarce and perhaps a limiting nutrient for many herbivores, and (b) that in response to this selection pressure, many herbivores have evolved specific behavioral, morphological, physiological, and other adaptations to cope with and uti­ lize the ambient N levels of their normal haunts. McNeill & Southwood (201) and White (328) have also reviewed these general questions. There­ fore, this review explores additional evidence and further develops the fundamental arguments. The review is organized into three major divisions. The first focuses on important sources of variation in plant N (seasonal and ontogenetic trends, different tissues and species, etc) because such variation may be the basis

W. J. J. Mattson

Papers

Herbivory in relation to plant nitrogen content