The Ecophysiology Of Salvia Disorders And Adaptation

EMMANOUIL PANAGIOTOPOULOS, CHRISOSTOMOS KAPETANOS,

MARIA SKAPETI, CONSTANTINOS CHOLEVAS, JOHN DROSSOPOULOS, MICHAEL LOUKAS* AND SPIRIDON E.KINTZIOS

Department of Plant Physiology, Faculty of Agricultural Biotechnology, Agricultural University of Athens, Iera Odos 75, 11855 Athens, Greece

* Department of Genetics, Faculty of Agricultural Biotechnology, Agricultural University of Athens, Iera Odos 75, 11855 Athens, Greece

INTRODUCTION

Since only a few sage species are being intensely cultivated, there is a rather scarce, scientifically documented information on the interaction of Salvia plants with different environmental components, in particular stress factors potentially related to physiological disorders. In the present chapter several aspects of the ecophysiological adaptation of various Salvia species are reviewed, such as drought, heavy metal and herbicide tolerance and the physiology of seed germination. Furthermore, a descrip tion is given on the available data on the growth competition between plants of the same species as well as among different species.

DROUGHT TOLERANCE

Some species of coastal sage and chaparral shrubs of California are extremely tolerant of tissue dehydration, surviving water potentials as low as -9MPa during dry summer months. Such low water potentials (high tensions on xylem water) are known to cause severe embolism formation in the xylem vessels of woody plants, blocking water transport and potentially causing shoot dieback. Thus drought-hardy species of coastal sage and chaparral are either extremely resistant to water stress-inducted embolism or they become severely embolized during summer drought. An estimation of susceptibility to water stress-inducted embolism indicated that 50% loss in hydraulic conductivity would occur at -4.5 MPa for Salvia mellifera. Irrigation of S. mellifera for one summer reduced loss in conductivity from 78 to 38% and increased leaf areas 10-fold, indicating that xylem embolism and leaf drop were drought inducted. Results show that xylem tissues of S. mellifera are more sensitive

to water stress and tissue dehydration than those of co-occurring Ceanothus megacarpus (chaparral). The observed ability of S. mellifera to inhabit drier sites than C. megacarpus may result from drought deciduousness in summer and high growth rates in spring that facilitate the rapid construction of new xylem and leaf tissues. It may be that facultative drought deciduousness in coastal sage is tightly coupled to drought-inducted embolism of xylem tissues (Kolb and Davis 1994).

In another report on Salvia mellifera G., Hargrave et al. (1994) came with interesting conclusions while investigating the relationship between conduit (vessel and tracheid) diameter and water-stress-inducted air embolism using a double staining technique, between irrigated control plants at water potential of -1.3 MPa and water stressed plants at about -8 Mpa. More specifically, water stress was inducted either by natural drought conditions or by laboratory drying of shoots from previously irrigated shrubs. Diameters of non-embolized and embolized conduits were then measured microscopically in transverse stem sections. In irrigated controls there was little embolism and mean diameters were not significantly different for embolized vs. non-embolized conduits. For both artificially dehydrated and naturally droughted plants there was a 91% drop in kh due to embolism and the mean diameter of embolized conduits was about 30 um vs. 21 um for non-embolized conduits. With increasing conduit diameter there was an increasing probability of embolism. Wider conduits may have larger pores in their pit membranes, thus increasing their vulnerability to water-stress-inducted embolism. Alternatively, wider conduits may merely have more pits, thus increasing their statistical chances of having particularly large pore in an air-exposed pit membrane. Narrow vessels and tracheids provide an interwoven auxiliary transport system that appears to be of importance to transport when many of the wider, more efficient conduits become embolized.

Researchers working with Salvia reflexa Hornem. (Weerakoon and Lovett 1986) grown in pots of soil and subjected to drought treatments in glasshouse conditions, mention that quite short durations of water stress significantly decreased leaf area, top and root dry weight. The plants rapidly adjusted the water loss below potential evaporation rates. They survived and recovered from periods of drought for up to 30 days. However, dehydrated plants did not attain the growth rates of undroughted plants, even after short drought periods. Referring to the same species, Weerakoon and Lovett (1986) suggest that the rate of germination and total germination were decreased as the osmotic potential of germination medium increased from -0.4 to -1.4 MPa.

HEAVY METAL TOLERANCE

Zheljazkov and Zheljazkova (1996) investigated the simultaneous effect of soil heavy metals on the essential oil content and productivity of Salvia sclarea L. as well as the heavy metal accumulation in plant parts (roots, stems, inflorescence). As reported, the simultaneous contamination of soil (taken at a distance of 0.5 km from the source of pollution) with excessive amounts of Cd and Zn decreased the yield of fresh inflorescences by 14-19% and the yield of oil by 12-18%. There was no contamination of the oil and no visible toxicity injuries. Also, a cultivar response to Cd, Cu and Mn was determined. In most cases, Cd concentration in the plant parts was in the order: leaves > roots > inflorescence > stems and the concentration of Mn, Cu and Zn in the order: leaves >roots > inflorescence > stems, while the concentration order of Fe was leaves=roots > inforescence=stems. In a similar study, Zheljaskov and Nielsen (1996) came to the conclusion that clary sage can be grown on sites of severe air and soil heavy metal pollution as a substitute for some other edible crops.

Chromium toxicity in Salvia sclarea L. has also been investigated (Corradi et al. 1993), in respect of its effects on seed germination and seedling development. Seeds and seedlings of the plant were treated with different concentrations of hexavalent chromium (K2Cr2O7). In vitro seed germination was not affected, but when the emergent radicle came into contact with the Cr solution, its growth was inhibited although early shoots and cotyledons developed normally. If the seedlings were transferred to Eppendorf vials so that the root were completely immersed in the Cr solution, not only root elongation, but also shoot and cotyledon development were inhibited. After 48 hours, cotyledons appeared chlorotic and chlorophyll and carotenoid contents were reduced, but chloroplast ultra-structure was normal. The only ultrastructural alteration was a partial detachment of the plasma membrane from the cell wall.

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