The capacity of plants to tolerate high levels of salinity depends on the ability to exclude salt from the shoot, or to tolerate high concentrations of salt in the leaf (tissue tolerance). It is widely held that a major component of tissue tolerance is the capacity to compartmentalize salt into safe storage places such as vacuoles. This mechanism would avoid toxic effects of salt on photosynthesis and other key metabolic processes. To test this, the relationship between photosynthetic capacity and the cellular and subcellular distribution of Na + , K + and Cl -was studied in salt-sensitive durum wheat (cv. Wollaroi) and salt-tolerant barley (cv. Franklin) seedlings grown in a range of salinity treatments. Photosynthetic capacity parameters ( V cmax , J max ) of saltstressed Wollaroi decreased at a lower leaf Na + concentration than in Franklin. Vacuolar concentrations of Na + , K + and Cl -in mesophyll and epidermal cells were measured using cryo-scanning electron microscopy (SEM) X-ray microanalysis. In both species, the vacuolar Na + concentration was similar in mesophyll and epidermal cells, whereas K + was at higher concentrations in the mesophyll, and Cl -higher in the epidermis. The calculated cytoplasmic Na + concentration increased to higher concentrations with increasing bulk leaf Na + concentration in Wollaroi compared to Franklin. Vacuolar K + concentration was lower in the epidermal cells of Franklin than Wollaroi, resulting in higher cytoplasmic K + concentrations and a higher K + : Na + ratio. This study indicated that the maintenance of photosynthetic capacity (and the resulting greater salt tolerance) at higher leaf Na + levels of barley compared to durum wheat was associated with the maintenance of higher K + , lower Na + and the resulting higher K + : Na + in the cytoplasm of mesophyll cells of barley.

We have known for many years that the dehydration of plant cells can lead to accumulation of the plant growth regulator ABA. Application of this compound to well-watered plants mimics many of the effects of soil drymg on gene expression, physiology, growth, and development, making this compound a strong candidate for a role in the droughted plant. Dehydration of leaves can result in massive accumulations of ABA, and roots also synthesize the compound in increased amounts as they are exposed to drier and drier soil. Davies and Zhang (1991) argued that an important component of the drought responses of many plants can be an ABA signal moving from the roots to the shoots to regulate physiology and development as a function of soil water status/availability. Many recent reports show relationships between stomatal conductance and soil water status or xylem ABA concentration, which seem to support this view. Nevertheless, critica1 examination of the ABA-signaling hypothesis must show that enough extra ABA moves in the transpiration stream to the shoots to account for the changes in functioning that are recorded. Many other chemicals moving in the xylem to shoots can also provide shoots with 'information" conceming root functioning, and we must consider the nature of such signals. We should also be concemed with the nature of the information that might be transmitted by a root signal (eg. a measure of soil water status or soil water availability) and the form that such a chemical signal might take (e.g. the concentration of the signal molecule in the transpiration stream or the flux of signal molecules to the site of action in the leaf). IS IT REALLY NECESSARY TO TAKE ACCOUNT OF CHEMICAL SlCNALlNC OF S o l 1 DRYING?To provide firm evidence for root-to-shoot signaling of the effects of soil drying, it is necessary to artificially manipulate the plant to break the link between soil drying and reduced water uptake. Passioura (1987) has done this successfully using a pressure vessel placed around the roots of a plant growing in drying soil. As the soil dries, pressure is increased to balance the increase in soil suction. Pressurized plants show shoot water relations that are similar to those of wellwatered plants, even though the roots are in contact with drylng soil. Reductions in leaf growth rate and stomatal conductance (Passioura, 1988; Gollan et al., 1992) must, therefore, be attributed to an effect of soil drying that does not require a change in shoot water status.In another experiment, Gowing et al. (1990) divided the roots of small apple trees into two containers. Soil dryrng in one container restricted leaf expansion and leaf initiation with no obvious effect on shoot water relations. When roots in contact with drylng soil were severed from the plant, leaf growth rate recovered to that shown by well-watered plants. It seems unlikely that this treatment could make more water available to the shoots, and a more likely explanation for the restriction in shoot growth is the increased supply of an inhibitor origina...

Plants, as sessile organisms, adapt to different stressful conditions, such as drought, salinity, extreme temperatures, and nutrient deficiency, via plastic developmental and growth responses. Depending on the intensity and the developmental phase in which it is imposed, a stress condition may lead to a broad range of responses at the morphological, physiological, biochemical, and molecular levels. Transcription factors are key components of regulatory networks that integrate environmental cues and concert responses at the cellular level, including those that imply a stressful condition. Despite the fact that several studies have started to identify various members of the MADS-box gene family as important molecular components involved in different types of stress responses, we still lack an integrated view of their role in these processes. In this review, we analyze the function and regulation of MADS-box gene family members in response to drought, salt, cold, heat, and oxidative stress conditions in different developmental processes of several plants. In addition, we suggest that MADS-box genes are key components of gene regulatory networks involved in plant responses to stress and plant developmental plasticity in response to seasonal changes in environmental conditions.

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