Stomatal conductance (g s ) typically declines in response to increasing intercellular CO 2 concentration (c i ). However, the mechanisms underlying this response are not fully understood. Recent work suggests that stomatal responses to c i and red light (RL) are linked to photosynthetic electron transport. We investigated the role of photosynthetic electron transport in the stomatal response to c i in intact leaves of cocklebur (Xanthium strumarium) plants by examining the responses of g s and net CO 2 assimilation rate to c i in light and darkness, in the presence and absence of the photosystem II inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), and at 2% and 21% ambient oxygen. Our results indicate that (1) g s and assimilation rate decline concurrently and with similar spatial patterns in response to DCMU; (2) the response of g s to c i changes slope in concert with the transition from Rubisco-to electron transport-limited photosynthesis at various irradiances and oxygen concentrations; (3) the response of g s to c i is similar in darkness and in DCMU-treated leaves, whereas the response in light in non-DCMU-treated leaves is much larger and has a different shape; (4) the response of g s to c i is insensitive to oxygen in DCMU-treated leaves or in darkness; and (5) stomata respond normally to RL when c i is held constant, indicating the RL response does not require a reduction in c i by mesophyll photosynthesis. Together, these results suggest that part of the stomatal response to c i involves the balance between photosynthetic electron transport and carbon reduction either in the mesophyll or in guard cell chloroplasts.

It has been suggested that some biological processes are equivalent to computation, but quantitative evidence for that view is weak. Plants must solve the problem of adjusting stomatal apertures to allow sufficient CO2 uptake for photosynthesis while preventing excessive water loss. Under some conditions, stomatal apertures become synchronized into patches that exhibit richly complicated dynamics, similar to behaviors found in cellular automata that perform computational tasks. Using sequences of chlorophyll fluorescence images from leaves of Xanthium strumarium L. (cocklebur), we quantified spatial and temporal correlations in stomatal dynamics. Our values are statistically indistinguishable from those of the same correlations found in the dynamics of automata that compute. These results are consistent with the proposition that a plant solves its optimal gas exchange problem through an emergent, distributed computation performed by its leaves.A lthough biological and computational systems appear to share many analogous structures and processes (1), rigorous, nontrivial connections between life and computation remain elusive. One difficulty is that the biological systems that have been most actively investigated for evidence of computation (macroscopic organisms with neuronal networks, on the one hand, and proteins and nucleic acids participating in tangled webs of chemical reactions, on the other) are built from elements that interact irreducibly with vast numbers of each other. Such highly interconnected systems are notoriously hard to adequately describe mathematically. Support for the existence of sophisticated computation in biology (if it exists) is probably more likely to be found in systems in which the relevant elements interact sparsely. We propose that plants may be ideal for this purpose. Plants solve a gas exchange problem by using apparatus whose structure is reminiscent of cellular automata (CA), computerbased simulations of locally connected networks (2). Results of experiments we performed involving transient gas exchange and photosynthesis in intact, living leaves are remarkably similar to dynamical behavior observed in some CA that perform specific computational tasks. As we discuss below, these results are consistent with the hypothesis that plants engage in a form of emergent, distributed computation. Our argument is based on ideas drawn, in part, from plant biology and, in part, from computer science and physics. Because most readers will not be conversant with all of these, we first summarize the relevant (and well established) notions from each area. Salient Facts About PlantsIn bright light, a plant's stomata [variable aperture pores distributed over its leaves (and other parts) at densities of tens to hundreds per mm 2 (Fig. 1a)] tend to open. This creates a dilemma. The plant benefits from the opening because it is able to take in CO 2 , which it uses in photosynthetic energy storage. At the same time, it also experiences the detriment of increased water evaporation and the consequent ...

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Capitol Reef National Park, in southcentral Utah, contains 22 small orchards planted with antique fruit varieties by Mormon pioneers beginning over a century ago. The orchards continue to be managed in a pick-and-pay program, which includes spraying with phosmet to suppress codling moth (Cydia pomonella L.). The park is also home to a rich diversity of flowering plants, many of which are rare, bee-pollinated, and have populations within 1 km of the orchards. Over 3 yr, we studied the short-term effects of phosmet spraying on bee populations: (1) foraging on plants within the orchard understory and adjacent to it; and (2) nesting in, and at several distances from, the orchards. We recorded a rich bee fauna (47 taxa) in the orchards and on plants nearby. In 2 yr (2002 and 2004), we found no difference in the number of native bee visits to several species of plants flowering in and near to orchards immediately before and 1 d after spraying. Conversely, our nesting studies using the semidomesticated alfalfa leafcutting bee, Megachile rotundata (F.), showed strong significant declines in the number of adult males, nesting females, and progeny production subsequent to spraying at distances up to 160 m from sprayed orchards where the bees were presumably foraging. We showed that M. rotundata is negatively affected by phosmet spraying and suggest that caution should be exercised in its use in areas where bees are apt to forage.