Stomatal density of cowpea correlates with carbon isotope discrimination in different phosphorus, water and CO₂ environments

Summary

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  Stomatal formation is affected by a plant's external environment, with long-distance signaling from mature to young leaves seemingly involved. However, it is still unclear what is responsible for this signal. To address this question, the relationship between carbon isotope discrimination (Δ) and stomatal density was examined in cowpea (Vigna sinensis).
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  Plants were grown under various environments that combined different amounts of soil phosphorus (P), soil water, and atmospheric CO₂. At harvest, stomatal density was measured in the youngest fully expanded leaf. The ¹³C : ¹²C ratio was measured in a young leaf to determine the Δ in mature leaves.
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  Results indicated that stomatal density is affected by P as well as by amounts of water and CO₂. However, stomatal responses to water and CO₂ were complex because of strong interactions with P. This suggests that the responses are relative, depending on some internal factor being affected by each external variable. Despite such complicated responses, a linear correlation was found between stomatal density and Δ across all environments examined.
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  It is proposed that the Δ value is a good surrogate for the long-term mean of the intercellular (C_i) to the atmospheric (C_a) CO₂ concentration ratio (C_i : C_a) and may be useful in understanding stomatal formation beyond complicated interactions.