Physiological processes in plant ecology

Arid lands in the Mediterranean Basin harbour populations of saltbushes over substantial areas. These have been grazed for centuries and often cleared. There are also some 100,000 hectares of artificially established saltbushes, both native and exotic; they include no more than half a dozen species, when large scale plantations are concerned. Another dozen of exotic species, introduced from Australia, Southern and Northern America, have successfully undergone field trials and could be used on a large scale in a near future. The present paper attempts to review the current state of knowledge on Atriplex spp. in the Mediterranean Basin as a means of Arid Land rehabilitation at a time where huge areas in the region have undergone processes of severe degradation or have been subjected to desertization, particularly over the past four decades. In spite of a number of constraints in their establishment, management and utilization, the plantation of Atriplex spp. appears as one of the best way, if not the best one, to rehabilitate desertized areas and restore them to production, under the present state of knowledge on arid land rehabilitation. They, in particular, are amenable to inclusion into new, man-made agro-sylvo-pastoral systems of production well adapted to arid lands and to the needs of their populations.

The role of saltbushes (Atriplex spp.) in arid land rehabilitation in the Mediterranean Basin: a review

Summary
Plant hydraulic characteristics were studied in diploid, tetraploid and hexaploid cytotypes of Atriplex canescens (Chenopodiaceae) to investigate the potential physiological basis underlying the intraspecific habitat differentiation among plants of different ploidy levels
Populations of A canescens from different habitats of the Chihuahuan Desert (New Mexico, USA) were analyzed using flow cytometry to determine ploidy levels Traits related to xylem water transport efficiency and safety against drought-induced hydraulic failure were measured in both stems and leaves
At the stem level, cytotypes of higher ploidy showed consistently lower leaf-specific hydraulic conductivity but greater resistance to drought-induced loss of hydraulic conductivity At the leaf level, comparisons in hydraulics between cytotypes did not show a consistent pattern, but exhibited high plasticity to proximal environmental conditions related to soil water availability
The results suggest that a trade-off between stem hydraulic efficiency and safety across ploidy levels underlies niche differentiation among different cytotypes of A canescens Polyploidization may have been facilitated by environmental heterogeneity related to water availability, and variation in water-related physiology found in the present study suggests an important functional basis for the niche differentiation and coexistence of A canescens cytotypes in desert environments

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Polyploidy enhances the occupation of heterogeneous environments through hydraulic related trade-offs in Atriplex canescens (Chenopodiaceae).

The subgenus Tridentatae of Artemisia (Asteraceae: Anthemideae) is composed of 11 species of various taxonomic and geographic complexities. It is centered on Artemisia tridentata with its three widespread common subspecies and two more geographically confined ones. Meiotic chromosome counts on pollen mother cells and mitotic chromosome counts on root tips were made on 364 populations ( = 3.1 plants per population). These population counts are ∼60% of all Tridentatae counts. Some are first records for taxa. The Tridentatae are a polyploid complex (x = 9) with ploidy levels from 2x to 8x, but mostly 2x (48%) and 4x (46%). Polyploidy occurs in nine of the 11 species and in many subspecies as well. Supernumerary or b chromosomes are present only at a low frequency. In the principal species, A. tridentata, 2x plants are larger than 4x ones, which are adapted to drier conditions, probably in consequence of their slower growth rates. Gigas diploidy is a phenomenon shared by some other woody genera, but is in contrast to the gigas polyploid nature of many herbaceous genera. Polyploidy occurs within populations and is essentially autoploid. Hybridization sometimes occurs at taxa interfaces in stable hybrid zones. Stable Tridentatae hybrid zones coupled with the group's inherent propensity for polyploidization has led to the establishment of a geographically and numerically large and successful complex of species.

Cytogeography and chromosome evolution of subgenus Tridentatae of Artemisia (Asteraceae)

Na and k accumulation and salt tolerance of Atriplex canescens (Chenopodiaceae) genotypes

chromosomes; the gigas form has 2n = 18 chromosomes. Several lines of evidence suggest that the gigas form is a relic diploid and the normal form is an autotetraploid derived from it. The growth rate of seedlings and new twigs is nearly twice as great in the diploid as in the tetraploid. Seed germination is faster and much better in the diploid. The tetraploid is reproductively isolated from the diploid because of a much earlier flowering period. The diploid plants possess many attributes which make them uniquely adapted to the drifting sand dune

James M. Melby

Papers

Evolutionary studies of atriplex: a relic gigas diploid population of atriplex canescens