Mesophyll tissues

Hallam, N.D. Luff, S.E. (1980). Fine structure changes in mesophyll tissue of the leaves of Xerophyta villosa during desiccation. Botanical Gazette, 141,173-9. 

In monocotyledonous plants (grasses and cereals) and some others, there is no division of mesophyll tissue, and injury normally appears as a bifacial fleck.Some plants, after extended exposure to low concentrations of pollution (either continuously or intermittently), produce chlorotic patterns that may be distinctive of oxidant pollution or similar to symptoms of normal senescence. The early senescence seen in some plants may be a result of long-term exposure to ambient oxidants. 

Sulfur Dioxide. S02 injury on plants has received much attention, particularly during the past half century, and the toxic effects are well known. Symptoms of acute injury to specific crops have been described by investigators in several countries (2, 3, 4, 6, 7, 8, 15). Acute necrosis results from rapid absorption of S02. Once S02 enters the mesophyll tissue, it reacts with water to produce the sulfite ion which has strong phytotoxic properties. When lethal concentrations accumulate in the most susceptible areas of the leaf, a dark green, water-soaked discoloration develops. The affected area soon becomes flaccid, and upon drying becomes white to ivory on most plants. In some instances the dead tissue may turn red, brown, or almost black. 

Typically, PAN attacks spongy mesophyll tissue surrounding the substomatal chambers on the lower side of the leaf. When tissue immediately beneath the lower epidermis is killed, the epidermis dries, producing a glazed or bronzed appearance on the lower leaf surface. Usually there is no evidence of injury on the upper surface when the glazed or bronze symptom develops. Later, as the upper part of the leaf continues to grow, it cups downward, becomes rugose, and distorted. 

Nicotine is mainly located in the intercostal areas of the leaf (247, 289, 290). Chojecki (291), by microchemical technique, has arrived at the following depots of nicotine in the tobacco plant given in the order of decreasing concentration leaf epidermis, particularly at the base of the hairs, spongy mesophyll tissue of the leaves, primary cortex of roots, epidermis and parenchymatous tissue of the stem and palisade tissue of the leaves small amounts in the phloem and xylem of the stem, in the veins of the leaves, and in the medulla of the stem and traces in flowers and axial cylinders of the roots. 

The location and catabolism of the cyanogenic glycosides have been reviewed. These compounds are situated entirely in the epidermal layers of leaves of Sorghum bicolor whereas the glucosyl transferase enzyme that catalyses the last step of their formation is found in both epidermal and mesophyll tissue ... 

Comparative leaf histological responses — Lesion development in fronds of P. aquilinum and leaves of P. vulgaris and H. annuus was very similar. In these three species initial injury was characterized by collapse of one to six cells on the adaxial epidermis. Normal turgid cells which comprised a continuous epidermis became flaccid witL an initial increase in cellular stain intensity. This tissue collapse resulted in a depression on the leaf surface. Underlying mesophyll tissues showed little or no distortion at this stage. 

The histological stages of lesion development in leaves of G. max were similar to those in P. aquilinum, P. vulgaris> and H. annuus. Lesions were initiated by collapse of several cells on the adaxial epidermis. Epidermal cell collapse was followed by a distortion of palisade parenchyma cells. This mesophyll tissue layer exhibited extensive hyperplasia. Occasionally, slightly enlarged cells were observed concomitant with this hyperplasia. Hyperplasia and hypertrophy occurred prior to cell collapse. 

Leaves of pin oak showed a different sequence of steps in leaf injury from that seen in other plant species. Initially, injury began with collapse of adaxial epidermal cells. Initial lesions consisted of approximately one to six collapsed epidermal cells which resulted in a slight depression on the leaf surface. At this stage, all cells in the mesophyll layers, and the abaxial epidermis were unaffected. After several more simulated acid rainfalls larger lesions, with increased surface area and depth, developed from smaller lesions. These lesions encompassed five to fifteen collapsed epidermal cells. Penetration of acidic solutions into the mesophyll tissues resulted in collapse of epidermal and underlying palisade parenchyma cells. 

CO2 flux and H2O flux pass through stomata. Stomatal aperture depends on both internal and external factors. As Wong et a/., 1979, pointed out, the capacity of the mesophyll tissue to fix carbon allows the regulation of stomatal conductance (gc) For instance, DCMU, an inhibitor of photosynthetic electron transport, modifies both mesophyll CO2 assimilation and stomatal conductance in such a way that the intercellular concentration of CO2 (Ci) remains proportional to the air CO2 concentration (Ca) ... 

Fig. 6.28A-H. Changes in the protein reserves within cotyledon mesophyll tissue of seedlings of Lupinus albus L. cv. Agricultural White. Protein digestion occurs in a wave from the epidermis inwards so several stages of digestion can be seen in any cotyledon section. It is not possible to equate stage of digestion exactly with days after imbibition. (A) Is usually found in day 1 cotyledons (B) (C) and (D) in day 1-4. Plate (E) is characteristic of day 5 and (F) and (G) are found in day 6-8 cotyledons. In (H) can be seen the specially persistent type of protein body which remains in the mid-region of the cotyledon after other reserves have been withdrawn, n nucleus p pitfield pb protein body pbf protein body fragment ppf persistent protein fragment s starch v vacuole. From Parker, 1975 [115]...

Fig. 6.29A-C. Changes in the protein reserves and cell wall material within cotyledon mesophyll tissue of Lupinus angustifolius L. cv. New Zealand Bitter Blue. (A) Cotyledon material from dry seed imbibed in fixative (2 h, glutaraldehyde 4h, osmium tetroxide). (B) 5-day cotyledons. (C) 9-day cotyledons. Kindly provided by Dr. M. Parker...

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