Emanuel Epstein

Differential Solute Regulation in Leaf Blades of Various Ages in Salt-Sensitive Wheat and a Salt-Tolerant Wheat x Lophopyrum elongatum (Host) A. Love Amphiploid

Leaf blades of different ages from a salt-tolerant wheat x Lophopyrum elongatum (Host) A. Love (syn. Agropyron elongatum Host) amphiploid and its salt-sensitive wheat parent (Triticum aestivum L.cv Chinese Spring) were compared for their ionic relations, organic solute accumulation, and sap osmotic potential ([pi]sap). The plants were grown for 18 d in nonsaline (1.25 mM Na+) and salinized (200 mM NaCl) nutrient solutions. The response of leaf blades to NaCl salinity depended greatly on their age or position on the main stem. Na and proline levels were highest in the oldest leaf blade and progressively lower in younger ones. Glycine betaine and asparagine levels were highest in the youngest blade. The [pi]sap was similar for corresponding leaf blades of both genotypes, but contributions of various solutes to the difference in [pi]sap between blades from control and 200 mM NaCl treatments differed greatly. The NaCl-induced decline in [pi]sap of the youngest leaf blade of Chinese Spring was predominately due to the accumulation of Na and to a lesser extent asparagine; in the amphiploid, it was due to a combination of glycine betaine, K, Na, and asparagine. Proline contributed little in the youngest blade of either genotype. In the older blades Na was the major solute contributing to the decline in [pi]sap. Thus, the maintenance of low Na and high K levels and the accumulation of glycine betaine in the young leaf tissues contributed to the NaCl tolerance of the amphiploid. No such role was evident for proline.

Advances in salt tolerance

SummaryAdvances in and prospects for the development of salt tolerant crops are discussed. The genetic approach to the salinity problem is fairly new, but research has become quite active in a short span of time. Difficulties and opportunities are outlined. Salinity varies spatially, temporally, qualitatively, and quantitatively. In addition, the responses of plants to salt stress vary during their life cycle. Selection and breeding, including the use of wide crosses, are considered the best short-term approaches to the development of salt tolerant crops, but the new biotechnological and molecular biological techniques will make increasingly important contributions. Cooperation is called for among soil and water scientists, agronomists, plant physiologists and biochemists, cytologists, and plant geneticists, breeders, and biotechnologists. Given such cooperation and adequate support for these endeavors, the potential for increasing productivity in salt-affected areas can be realized.

Silicon deprivation causes physical abnormalities in wheat (Triticum aestivum L)

Summary The Earths surface including its soils consists largely of silicate minerals, and terrestrial plants absorb large amounts of silicon. Not so the plants grown for experimental purposes in solution culture, for not a single formulation of the widely used culture solutions includes silicon, the element not being considered generally «essential» for higher plants. We here show that Si-deprived wheat ( Triticum aestivum L.) plants (−Si) differ gready from Si-replete (+Si) ones in certain physical features. The friction force that must be overcome if the heads of +Si wheat are to slide down an inclined plane, a measure of roughness, was greater than for −Si wheat heads. Energy dispersive X-ray microanalysis and chemical analysis revealed the presence of Si in the awns of+Si plants but none or very little in those of −Si plants. The large physical differences measured in these experiments show wheat plants grown in conventional (−Si) cultures to be in some respect experimental artifacts. The over 130-year-old practice of growing experimental plants in −Si solution cultures should be reconsidered. We recommend that Si be included routinely in the formulation of nutrient solutions.

Chapter 1 Silicon in plants: Facts vs. concepts

The facts of silicon (Si) in plant life are one thing; the concepts regarding Si in plant physiology are another thing altogether. Most terrestrial plants grow in media dominated by silicates, and the soil solution bathing roots contains Si at concentrations exceeding those of phosphorus (P) by roughly a factor of 100. Plants absorb the element, and their Si content is of the same order of magnitude as that of the macronutrient elements. The general plant physiological literature, however, is nearly devoid of Si. The reason for this marked discrepancy is the conclusion that Si is not an “essential” element because most plants can grow in nutrient solutions lacking Si in their formulation. Such Si-deprived plants are, however, experimental artifacts. They may differ from Si-replete plants in (i) chemical composition; (ii) structural features; (iii) mechanical strength; (iv) various aspects of growth, including yield; (v) enzyme activities; (vi) surface characteristics; (vii) disease resistance; (viii) pest resistance; (ix) metal toxicity resistance; (x) salt tolerance; (xi) water relations; (xii) cold hardiness; and probably additional features. The gap between plant physiological facts and plant physiological concepts must be closed. The facts of Si in plant life will not change; hence it is the concepts regarding the element that need revising.

Potassium/sodium selectivity in wheat and the amphiploid cross wheat X Lophopyrum elongatum

The early response of K(+) and Na(+) net fluxes to different external NaCl and KCl levels has been studied in wheat (Triticum aestivum L.) and the amphiploid cross wheat X Lophopyrum elongatum (Host) Löve in culture solution experiments. We found that during the first 24 h of exposure to 100 or 200 mM NaCl, at low K(+) levels, the amphiploid absorbed, translocated and allocated to the youngest leaf less Na(+) than the wheat parental line. During that period, the amphiploid retained more K(+) than wheat. Short-term uptake studies with 86Rb and 22Na showed that K(+)(86Rb) and Na(+) influxes were not involved in genotypic differences in K(+)(86Rb) and Na(+) net uptake observed after 6 h of exposure to salt stress. Differences in K(+)(86Rb) net uptake could be attributed to differences in K(+)(86Rb) efflux and/or to K(+)(86Rb) accumulation by root vacuoles. The possibility that differential shrinkage of protoplast volume plays a role in the genotypic difference in K(+) retention cannot be ruled out. On the other hand, Na(+) efflux did not contribute significantly to differences in Na(+) net uptake between these genotypes. Hence, differences in Na(+) net uptake were attributed to differences in the transport of Na(+) to the shoot. The presence in the amphiploid of fast acting mechanisms able to enhance Na(+)/K(+) selectivity at different plant levels minimizes the early build-up of Na(+) concentration, and K(+) substitution by Na(+), in the growing tissue of the leaf.

The Role of Roots in the Chemical Economy of Life on Earth

Roots are the agents whereby the terrestrial biosphere acquires from soil most of the elements that go into the make-up of living things on land. In aquatic plants, all the cells accomplish the acquisition of carbon, water, and mineral nutrient elements. On land, only a thin surface layer of soil can supply these ingredients to plants and also expose them to the essential energy input in the form of sunlight. Small green plants, mainly algae, do in fact inhabit the top layer of soil. But this film is precisely the soil volume most quickly dried out and most readily shaded by all but the smallest plants. Evolutionary selective pressure, therefore, favored the elaboration of larger, bipolar structures, with a photosynthetic apparatus thrust up into the sunlit air and a system for absorption of water and mineral nutrients ramifying through the depths of the soil. The size of the evolving root systems had to keep pace with the expansion of the photosynthetic canopy for two reasons. The more extensive became the photosynthetic surface exposed to the drying air, the greater, too, became the unavoidable transpirational loss of water from the leaves, and that loss had to be made good through the absorption of water by the roots. Their capacity to take up water and transfer it to the shoot, and hence their size, had to increase in proportion to the increase in the size of the evolving aboveground systems. Secondly, the larger those

Photosynthesis, inorganic plant nutrition, solutions, and problems.

A brief account is given of the research that D.I. Arnon did before he ventured into the field of photosynthesis, viz. his work on inorganic plant nutrition in the laboratory of D.R. Hoagland. The connection between the two areas is indicated. In his work on plant nutrition Dr Arnon emphasized the role of specific nutrients and, with P.R. Stout, formulated a definition of essentiality that is used to this day. It is now necessary, however, to take into account elements not meeting their criteria of essentiality, as shown by a consideration of the element silicon.