Reproductive stage water stress leads to spikelet sterility in wheat. Whereas drought stress at anthesis affects mainly grain size, stress at the young microspore stage of pollen development is characterized by abortion of pollen development and reduction in grain number. We identified genetic variability for drought tolerance at the reproductive stage. Drought-tolerant wheat germplasm is able to maintain carbohydrate accumulation in the reproductive organs throughout the stress treatment. Starch depletion in the ovary of drought-sensitive wheat is reversible upon re-watering and cross-pollination experiments indicate that the ovary is more resilient than the anther. The effect on anthers and pollen fertility is irreversible, suggesting that pollen sterility is the main cause of grain loss during drought conditions in wheat. The difference in storage carbohydrate accumulation in drought-sensitive and droughttolerant wheat is correlated with differences in sugar profiles, cell wall invertase gene expression and expression of fructan biosynthesis genes in anther and ovary (sucrose : sucrose 1-fructosyl-transferase, 1-SST; sucrose : fructan 6-fructosyl-transferase, 6-SFT). Our results indicate that the ability to control and maintain sink strength and carbohydrate supply to anthers may be the key to maintaining pollen fertility and grain number in wheat and this mechanism may also provide protection against other abiotic stresses.

Agricultureisexpandingintoregionsthatareaffectedbysalinity.Thisreviewconsiderstheenergetic costs of salinity tolerance in crop plants and provides a framework for a quantitative assessment of costs. Different sources of energy, and modifications of root system architecture that would maximize water vs ion uptake are addressed. Energy requirements for transport of salt (NaCl) to leaf vacuoles for osmotic adjustment could be small if there are no substantial leaks back across plasma membrane and tonoplast in root and leaf. The coupling ratio of the H +-ATPase also is a critical component. One proposed leak, that of Na + influx across the plasma membrane through certain aquaporin channels, might be coupled to water flow, thus conserving energy. For the tonoplast, control of two types of cation channels is required for energy efficiency. Transporters controlling the Na + and Cl À concentrations in mitochondria and chloroplasts are largely unknown and could be a major energy cost. The complexity of the system will require a sophisticated modelling approach to identify critical transporters, apoplastic barriers and root structures. This modelling approach will informexperimentationandallowaquantitativeassessmentoftheenergycostsofNaCltoleranceto guide breeding and engineering of molecular components.

Water-soluble carbohydrates (WSCs; composed of mainly fructans, sucrose [Suc], glucose [Glc], and fructose) deposited in wheat (Triticum aestivum) stems are important carbon sources for grain filling. Variation in stem WSC concentrations among wheat genotypes is one of the genetic factors influencing grain weight and yield under water-limited environments. Here, we describe the molecular dissection of carbohydrate metabolism in stems, at the WSC accumulation phase, of recombinant inbred Seri/Babax lines of wheat differing in stem WSC concentrations. Affymetrix GeneChip analysis of carbohydrate metabolic enzymes revealed that the mRNA levels of two fructan synthetic enzyme families (Suc:Suc 1-fructosyltransferase and Suc:fructan 6-fructosyltransferase) in the stem were positively correlated with stem WSC and fructan concentrations, whereas the mRNA levels of enzyme families involved in Suc hydrolysis (Suc synthase and soluble acid invertase) were inversely correlated with WSC concentrations. Differential regulation of the mRNA levels of these Suc hydrolytic enzymes in Seri/Babax lines resulted in genotypic differences in these enzyme activities. Down-regulation of Suc synthase and soluble acid invertase in high WSC lines was accompanied by significant decreases in the mRNA levels of enzyme families related to sugar catabolic pathways (fructokinase and mitochondrion pyruvate dehydrogenase complex) and enzyme families involved in diverting UDP-Glc to cell wall synthesis (UDP-Glc 6-dehydrogenase, UDP-glucuronate decarboxylase, and cellulose synthase), resulting in a reduction in cell wall polysaccharide contents (mainly hemicellulose) in the stem of high WSC lines. These data suggest that differential carbon partitioning in the wheat stem is one mechanism that contributes to genotypic variation in WSC accumulation.Water-soluble carbohydrates (WSCs) can accumulate in the stem and leaf sheath of cool-season cereals (e.g. wheat [Triticum aestivum], barley [Hordeum vulgare], and oats [Avena sativa]) during the period from stem elongation to the early phase of grain filling and serve as temporary carbohydrate reserves, commonly called the stem carbohydrate reserves (Schnyder, 1993;