This paper discusses whole-plant responses to salinity in order to answer the question of what process limits growth of non-halophytes in saline soils. Leaf growth is more sensitive to salinity than root growth, so we focus on the process or processes that might limit leaf expansion. Effects of short-term exposure (days) are considered separately from long-term exposure (weeks to years). The answer in the short term is probably the water status of the root and we suggest that a message from the root is regulating leaf expansion. The answer to what limits growth in the long term may be the maximum salt concentration tolerated by the fully expanded leaves of the shoot; if the rate of leaf death approaches the rate of new leaf expansion, the photosynthetic area will eventually become too low to support continued growth.

Whole-plant responses to salinity

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Responses of woody plants to flooding and salinity

This review deals with the adaptive mechanisms that plants can implement to cope with the challenge of salt stress. Plants tolerant to NaCl implement a series of adaptations to acclimate to salinity, including morphological, physiological and biochemical changes. These changes include increases in the root/canopy ratio and in the chlorophyll content in addition to changes in the leaf anatomy that ultimately lead to preventing leaf ion toxicity, thus maintaining the water status in order to limit water loss and protect the photosynthesis process. Furthermore, we deal with the effect of salt stress on photosynthesis and chlorophyll fluorescence and some of the mechanisms thought to protect the photosynthetic machinery, including the xanthophyll cycle, photorespiration pathway, and water-water cycle. Finally, we also provide an updated discussion on salt-induced oxidative stress at the subcellular level and its effect on the antioxidant machinery in both salt-tolerant and salt-sensitive plants. The aim is to extend our understanding of how salinity may affect the physiological characteristics of plants.

/pdf/plant-responses-to-salt-stress-adaptive-mechanisms-5b1puoq7eh.pdf

Plant Responses to Salt Stress: Adaptive Mechanisms

SUMMARY
Secretory tissues occur in most vascular plants. Some of these tissues, such as hydathodes, salt glands and nectaries, secrete unmodified or only slightly modified substances supplied directly or indirectly by the vascular tissues. Other tissues secreting, for instance, polysaccharides, proteins and lipophilic material, produce these substances in their cells. The cells of secretory tissues usually contain numerous mitochondria. The frequency of other cell organelles varies according to the material secreted. In most glandular trichomes the side wall of the lowest stalk cell is completely cutinized. This prevents the secreted material from flowing back into the plant.

The salt glands in Atriplex eliminate salt into the central vacuole of the bladder cell but, in other plants, the glands secrete salt to the outside. Different views exist as to the manner in which salt is eliminated from the cytoplasm. According to some authors, the mode of elimination is an eccrine one, while others suggest the involvement of membrane-bound vesicles.

Nectar is of phloem origin. The pre-nectar moves to the secretory cells through numerous plasmodesmata present in the nectariferous tissue. Nectar is eliminated from the secretory cells by vesicles of either KR or dictyosomal origin. In some cases, both organelles may be involved but an eccrine mode of nectar secretion has also been suggested by some authors.

Carbohydrate mucilages and gums are synthesized by dictyosomes but virtually every cell compartment has been suggested as having a role on the secretion of lipophilic substances. Most commonly, plastids are implicated in the synthesis of lipophilic materials but KR may also play a part. In some cases lipophilic materials may be transported towards the plasmalemma in the KR.

Resin and gum ducts of some plants develop normally or in response to external stimuli, such as microorganisms or growth substances. Among the latter, ethylene is the most effective.

During the course of evolution, secretory tissues seem to have developed from secretory idioblasts scattered among the cells of the ordinary tissues. Subsequently ducts and cavities developed and finally secretory trichomes.

Secretory tissues in vascular plants

Roots of virtually all vascular plants have an endodermis with a Casparian band, and the majority of angiosperm roots tested also have an exodermis with a Casparian band. Both the endodermis and exodermis may develop suberin lamellae and thick, tertiary walls. Each of these wall modifications has its own function(s). The endodermal Casparian band prevents the unimpeded movement of apoplastic substances into the stele and also prevents the backflow of ions that have moved into the stele symplastically and then were released into its apoplast. In roots with a mature exodermis, the barrier to apoplastic inflow of ions occurs near the root surface, but prevention of backflow of ions from the stele remains a function of the endodermis. The suberin lamellae protect against pathogen invasion and possibly root drying during times of stress. Tertiary walls of the endodermis and exodermis are believed to function in mechanical support of the root, but this idea remains to be tested. During stress, root growth rates decline, and the endodermis and exodermis develop closer to the root tip. In two cases, stress is known to induce the formation of an exodermis, and in several other cases to accelerate the development of both the exodermis and endodermis. The responses of the endodermis and exodermis to drought, exposure to moist air, flooding, salinity, ion deficiency, acidity, and mechanical impedance are discussed.

Root Endodermis and Exodermis: Structure, Function, and Responses to the Environment

Seedlings of Rangpur lime (Citrus reticulata var. austera hybrid?) and Etrog citron (C. medica) were treated in water culture with 0,25,50 or 100 mol m ~ 3 NaCl, and in sand culture with 0 or 100 mol m 3 NaCl. Leaf chloride analyses indicated that Etrog citron accumulated the most chloride at all levels of salinity. The structure, ultrastructure and concentrations of chloride and reserve assimilates of the primary root up to 50 mm back from the tip were compared between genotypes and between salt treatments. There were no differences in root anatomy in the absence of salt between the two genotypes. The hypodermal cells developed lignified and suberized walls which blocked the plasmodesmata and resulted in degeneration of the cell contents. Frequent thin-walled passage cells in the hypodermis had living contents and may represent major sites of ion uptake into the symplasm, which was connected with the stele via plasmodesmatal connections with and between the cortical cells. The primary endodermal cells had lignified casparian strips and plasmodesmata in other cell wall areas. These connections were blocked by secondary suberization of all except the endodermal passage cells opposite the protoxylem arcs. Suberization of the hypodermis and endodermis and the appearance of granular deposits in the vacuole occurred closer to the root tip of both genotypes after treatment with 100 mol m~3 NaCl. Levels of starch and triglyceride in 10 mm serial segments were similar between roots of control and salt-treated plants and increased with distance back from the root tip. Chloride concentrations increased with salt treatment but values (on a tissue water basis) were similar between genotypes and between the apical and proximal ends of the root. Reducing sugar concentrations decreased with salt treatment to a similar extent in roots of both genotypes.

Anatomy, Ultrastructure and Assimilate Concentrations of Roots of Citrus Genotypes Differing in Ability for Salt Exclusion

SummaryAvocado plants were kept in growth cabinets at 33°C day, 28°C night (33/28); 25°C day, 20°C night (25/20); and 17°C day, 12°C night (17/12), with a 12-h photoperiod and light intensity of 26000 1x. At 33/28 and 25/20 flowers opened in the afternoon as females and again the following morning as males (type B floral cycle) with some overlap of male and female stages at 25/20. At 17/12 very few flowers had a female stage, the majority opening once only as males. The duration of the flowering period decreased with a rise in temperature as did total number of flowers opening. Reproductive growth was inhibited in favour of vegetative growth at 33/28, as exhibited by smaller floral parts and abscission of buds and flowers. The rate of pollen tube growth increased with a rise in temperature, but abnormal growth was frequently observed at 33/28 and tubes failed to reach the ovary at 17/12. Endosperm and embryo development was observed to occur at 25/20 but not at 33/28 or 17/12 although a positive effect of...

The Effect of Temperature on Floral Behaviour, Pollen Tube Growth and Fruit Set in the Avocado

The effect of foreign pollen on the stigma of watermelon (Citrullus lanatus [Thunb] Matsum and Nakai) was studied by light and electron microscopy. Pollen of 14 of 15 tested species germinated on the stigma, and pollen tubes of eight species penetrated the style. Only watermelon pollen tubes reached the ovary. Pollen of all species stimulated stigma secretion, irrespective of germination capacity. Secretion was also stimulated by dead watermelon pollen, live yeast cells, and fixed red blood cells, but not by glass fragments. Prior foreign pollination did not affect germination or pollen-tube growth of subsequently applied watermelon pollen which produced fruit set. Foreign pollination did not result in fruit set, but stimulated the same sequence of papilla-cell senescence as watermelon pollen. We concluded that the processes of stigma secretion and papilla-cell death in response to pollination do not control the growth of foreign pollen on the watermelon stigma.

Foreign Pollination of the Stigma of Watermelon (Citrullus lanatus [Thunb.] Matsum and Nakai)

Arabinogalactan-proteins are localized extracellularly in the transmitting tissue of Nicotiana alata Link and Otto, an ornamental tobacco

Avocado pollen was stored at a range of temperatures and relative humidities (RH) for up to one year and the pollen was tested for viability in vivo.

Margaret Sedgley

Papers

Anatomy, Ultrastructure and Assimilate Concentrations of Roots of Citrus Genotypes Differing in Ability for Salt Exclusion

The Effect of Temperature on Floral Behaviour, Pollen Tube Growth and Fruit Set in the Avocado

Foreign Pollination of the Stigma of Watermelon (Citrullus lanatus [Thunb.] Matsum and Nakai)

Arabinogalactan-proteins are localized extracellularly in the transmitting tissue of Nicotiana alata Link and Otto, an ornamental tobacco

Storage of avocado pollen