Starch Synthesis in the Kernel of Wheat Under High Temperature Conditions

Abstract

As temperature rises above 18-22�C, the observed decrease in the duration of deposition of dry matter in the kernel is not accompanied by a compensating increase in the rate of grain filling with the result that grain weight (and yield) is diminished at high temperature. Reduced starch content accounts for most of the reduction in grain dry matter at high temperature. Responses to temperature in the low temperature range, 20-30�C (the LTR), could possibly be ascribed to the temperature response characteristics of the reaction catalysed by soluble starch synthase (SSS), the enzyme synthesising starch. However, the rate of cell enlargement and the rate of accumulation of nitrogen in the grain also do not increase much as temperature rises, so other explanations are conceivable for the temperature responses in the LTR. Variation amongst cultivars of wheat in tolerance of high temperature is evident in the LTR. At temperatures above 30�C (in the high temperature range (HTR) between 30 and 40�C), even for short periods, the rate of starch deposition is slower than that observed at lower temperatures, an effect which is carried over after transfer from high to lower temperatures. This response is attributable to a reduction in the activity, possibly due to thermal denaturation, of SSS. Several forms of SSS are found in cereal endosperm, and some forms may be more tolerant of high temperature than others. Loss of enzyme activity at high temperature is swift, but is partly restored some time after transfer from hot to cool conditions. There appear to be two distinct mechanisms of response to elevated temperature, both resulting in a reduced grain weight through reduced starch deposition, but one of them is important only in the range of temperature above 30�C.

II. Heat stress in Triticum: kinetics of Cu and Zn accumulation

“…Yet, at a cellular level, although about 80% of cellular enlargement occurs between the end of cellular division and the conclusion of dry matter accumulation (Nicolas et al, 1984), the rate of cellular volumetric augmentation does not compensate the decreasing extent of cellular enlargement (Stone et al, 1995. Caley et al (1990 and Jenner (1994) further confirmed these data, and Shpiler and Blum (1986), working with field trials, also reported that grain maturity develops earlier, producing smaller and shriveled grains, in heat stressed genotypes.…”

Truncation of grain filling in wheat (Triticum aestivum) triggered by brief heat stress during early grain filling: association with senescence responses and reductions in stem reserves

“…These findings challenge the notion that the rapid reduction in photosynthetic capacity during the heat treatment in the susceptible genotypes (Drysdale, Frame, Lyallpur-73 and Reeves) directly reduced their grain weights (by limiting source supply), as the responses of their developing grains to heat were characterised by a reduction in GFD rather than a reduction in grain filling rate. Similarly, the consistent trend for heat to increase grain filling rate during the treatment, and the absence of a significant reduction in MGR following heat treatment (except in Waagan) argues against damage to the grain soluble starch synthase as being very influential under these conditions, despite other evidence that this enzyme is heat-sensitive (Jenner 1994). An alternative explanation for why grain filling was cut short in these varieties is that heat accelerated senescence in the grain, curtailing the development of the grains and their ability to convert the delivered sugars into starch.…”

“…Soluble starch synthase has been identified as both the ratelimiting and most heat-sensitive component of the starch biosynthetic machinery of the developing wheat grain, although the effect of heat on soluble starch synthase appears to be reversible (Jenner 1994). Elevated temperatures raise respiration rates in the grains and spike, thereby limiting the sugar pools available for starch deposition (Wardlaw et al 1980).…”

“…In addition to its effect on grain number when it occurs before anthesis (Saini and Aspinall 1982), heat can affect grain size when it occurs around anthesis and early grain filling. Heat reduces grain size mainly by affecting the deposition of starch, which normally makes up~70% of the grain mass (Jenner 1994). Heat may affect grain size by reducing sugar supply to the developing grain or by impacting processes within the grain that convert the delivered sugars into starch.…”

Quality of winter wheat in relation to heat and drought shock after anthesis

“…The yield losses are caused by a reduction in the starch content, since over 65% of cereal kernels is composed of starch (Jenner 1994;Barnabás et al 2008). Starch accumulation is correlated with the sucrose content of the kernels and with the activity of sucrose synthase and other enzymes with an important role in starch synthesis .…”