Plant form and function

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A favourite phrase in architectural debate is that form follows function– that the form of a building should be based on the functional activities housed within. I am sure that everyone can readily imagine buildings where this tight coupling does not occur. However, in biological organisms the connection should be much more certain, as the form of an organism primarily depends on internal function, as represented by the genome and its translation into cellular metabolism. Modern plant biology easily demonstrates the connection, for example when genetic changes in starch metabolism lead to orders of magnitude changes in plant size (Rasse & Tocquin, 2006), when herbicide-resistance genes allow virulent weeds to survive in the face of applied herbicides (Menchari et al., 2006) and in identifying how many genes appear to be required to confer the capacity to hyperaccumulate toxic heavy metals (Hammond et al., 2006). Processes that are difficult to investigate become much easier with modern genomic approaches. Examples include the involvement of auxin in establishing ectomycorrhizal symbiosis (Reddy et al., 2006) and the action of calmodulin for antioxidant defense in leaves (Hu et al., 2007).

Research published in New Phytologist covers many orders of magnitude in time and space (Woodward, 2007), and aspects of the form and function relationships are generally visible but not always in the direction of form following function. Function often follows form and in a manner that does not appear easily possible to dissect by the modern techniques of molecular plant biology. Invisible to the naked eye is the critically important convective flow of air through leaves to rhizomes in anoxic, waterlogged soils (Armstrong et al., 2006) and the lateral movement of carbon dioxide within the air spaces of leaves, important for photosynthesis during drought (Pieruschka et al., 2006). In forest canopies form follows function that follows form, in that the light intercepted by an individual tree is influenced by its size and form and also by the size and form of surrounding trees. However, effective optimization of form and light interception require flexibility in branch form as the tree grows up into the forest canopy (Osada, 2006).

The interaction of form and function can change sign. Nurse plants can protect smaller individuals during their early stages of establishment, a feature that may benefit either genetically related or genetically unrelated species. Nurse plant protection is primarily an above-ground function of shelter that appears to be most effective during drought (Sthultz et al., 2007). If well-watered, then the nurse plant protection changes to a competitive function, outgrowing and increasing the mortality of establishing plants. This change in plant form is an aspect of phenotypic plasticity. A major feature of plasticity in growth form is that it is reduced as environmental conditions become more limiting for growth (Valladares et al., 2007). As function becomes more restricted, so also does the capacity to change form in response to environmental challenges. So, for example, change in form through leaf herbivory affects function and reduces the capacity to obtain the optimum form for a particular environment (Valladares et al., 2007).

The reverse situation of function following form has a long history in plant science that still continues, with the specific aim of attempting to discern critical, but hidden, aspects of function that constrain observable form. The area of the xylem lumen in 51 woody species present in California is positively correlated with plant height (Preston et al., 2006), a functional relationship perhaps associated with the increasing resistance to water flow with plant height. However, in tropical rainforests of Central America, vessel diameter was uncorrelated with precipitation, while vulnerability to drought-induced embolism increased as precipitation decreased (Choat et al., 2007). Species-specific differences in the responses of both form and function to the environment diminish the capacity to generalize accurately. Yet, surveys of large numbers of species do reveal some generalities. For example leaf mass per area is positively correlated with shade tolerance in evergreen tree species (Lusk & Warton, 2007). The close tie between leaf form and function is an area of high current activity (Leishman et al., 2007), again with a focus on leaf structure, but also in terms of functional components – leaf photosynthesis, nitrogen and phosphorus. When applied to invasive plants, the approach clearly identified that exotic invasive species modified function to enable faster growth than native invasive species, with a potential connection to reductions in leaf defence.

Form and function are much like yin and yang, opposing but complementary forces in plant development. The field of ‘eco-devo’ investigates this relationship in situations where changing functional responses to the environment (opposing current development) elicit a response of form to something potentially adaptive. Casson & Gray (2008) describe such responses by stomata in changing environments. Deep investigation of plant systems also indicate that functions are connected as networks, from the interactions of systemic signals in plants (Demidchik & Maathuis, 2007; Roberts et al., 2007), to mycorrhizal symbioses (Paszkowski, 2006) and to the spread of pathogens through plant populations (Jeger et al., 2007). So, in fact, form and function are joined within a complex network of co-occurring components, interacting on a range of timescales yet to be fully investigated. New Phytologist, with the breadth of subjects that it publishes, is constantly working at identifying these network components, through our emphasis not only on mechanisms but also on the diversity of plant form.

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