User:Southhillwill/sandbox

""- "So you're a botanist, huh?"

- "An aspiring botanist."

- "Oh... Well then you HAVE GOT to check out Florida.

- "Why's that?"

- "Well... the flora, duh."

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Soil salinization, the accumulation of water-soluble salts to levels that negatively impact plant production, is a global phenomenon affecting approximately 831 million hectares of land. More specifically, the phenomenon threatens 19.5% of the world's irrigated agricultural land and 2.1% of the world's non-irrigated (dry-land) agricultural lands. High soil salinity content can be harmful to plants because water-soluble salts can alter osmotic potential gradients and consequently inhibit many cellular functions. For example, high soil salinity content can inhibit the process of photosynthesis by limiting a plant’s water uptake; high levels of water-soluble salts in the soil can decrease the osmotic potential of the soil and consequently decrease the difference in water potential between the soil and the plant’s roots, thereby limiting electron flow from H2O to P680 in Photosystem II’s reaction center.

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--- Introduction

Paraheliotropism refers to the phenomenon in which plants orient their leaves parallel to incoming rays of light, usuually as a means of minimizing excess light absorption. Excess light absorption can cause a variety of physiological problems for plants, including overheating, dehydration, loss of turgor, photoinhibition, photo-oxidation, and photorespiration, so paraheliotropism can be viewed as an advantageous behavior in high light environments. Not all plants exhibit this behavior, but it has developed in multiple lineages.

--- Physiological Basis

While all mechanistic aspects of this behavior have yet to be elucidated (e.g., evidence indicates differential gene expression is involved, but the specifics have yet to be determined), many of the physiological aspects of paraheliotropic movement, at least in Phasoleus vulgaris (the common bean), are well understood. In this plant, daily leaf movements are influenced by two main factors: an endogenous circadian oscillator and light-induced signals. (Satter 1981). Physically, the movement is carried out by turgor-dependent changes in the volume of cortical parenchyma cells (called motor cells) in a turgor-sensitive part of the plant called the pulvinus, located at the juncture of the leaf base and the petiole (Mayer, Koler). The cumulative effect of volume-changes in these motor cells manifests itself on the tissue/organ level as a swelling or shrinking of one or both sides of the pulvinus, which results in the reorientation of the adjacent leaf (Mayer, Koler). Potassium and chloride have been shown to be the major osmolytes involved in the process, and plasma membrane-located proton pumps and ion transporters have been shown to play a critical role in creating osmotic potential (Suh, moshelion). The hormones IAA and ABA are also involved in the process and play antagonistic roles, with IAA inducing pulvinar swelling and ABA inducing pulvinar shrinking (moritosh). Blue light has also been shown to induce rapid pulvinar shrinking (Wang).