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Cereal Chem. 89(3):168-175Elevated nighttime air temperatures (NTATs) occurring during critical grain-filling stages affected rice physicochemical properties, which impacted functional quality. Six cultivars were grown at multiple field locations from northern to southern Arkansas during 2007 to 2010. Nighttime temperatures were recorded throughout production at each of the locations, and 95th percentiles of NTATs were calculated for each cultivar's reproductive (R) stages. Amylose content and crude protein content decreased linearly, whereas total lipid content increased linearly, with increasing NTATs occurring during the grain-filling stages (R6-R8). Effects of NTAT on proximate composition influenced functional properties. Peak viscosities increased linearly as NTAT increased, whereas setback viscosities decreased. Setback viscosities were linearly correlated to NTATs for medium-grain cultivars, but correlations were quadratic for the long-grain cultivars. Gelatinization temperatures increased linearly with increasing NTAT. The R stages in which correlations were strongest varied by cultivar and by property, hypothesized to result from differences in kernel development patterns among cultivars. These findings have significant implications for rice production scientists and processors, in that understanding the effects of NTAT on physicochemical and functional properties may help explain and reduce quality variation.Recent studies in both controlled-temperature and field-scale environments have established that elevated nighttime air temperatures (NTATs) occurring during critical grain-filling stages affect rice kernel development, resulting in reduced yield, increased kernel chalkiness, and reduced milling quality (Peng et al 2004; Cooper et al 2006, 2008; Ambardekar et al 2011; Lanning et al 2011). Other studies have shown that the chemical makeup of starch is affected by elevated NTAT, as evidenced by decreased amylose content (AC) and changes in ratios of long-to shortchain amylopectin (Counce et al 2005; Cooper et al 2008). Several hypotheses have been presented to explain the effects of NTAT stress, including reduced substrate supply to the endosperm, initiating slow starch granule growth and irregular granular organization (Fitzgerald and Resurreccion 2009) and disruption of enzymatic activity responsible for starch formation (Counce et al 2005).Although the underlying mechanisms that tie the effects of NTAT to the structural and functional changes of starch are not clearly established, these findings are critical to rice end-use applications, because functional properties of milled rice, which directly impact cooking and sensory quality, are primarily determined by starch physicochemical properties.AC of a given rice strain is determined by the expression of two alleles of a polymorphic waxy gene, Wx a and Wx b , which regulate amylose synthesis by controlling the activities of grain-bound starch synthase (GBSS) enzymes (Counce et al 2005). Suzuki et al (2003) reported that mutations to Wx a and Wx b a...

Rice quality and functionality are characterized in many ways, depending largely on the industry segment using the rice. These characteristics include appearance, milling, and cooking parameters. Recently, variable quality of rice grown in the United States has been reported, but the cause was not well documented. Agronomic impacts include planting time, irrigation and fertility, cultivar selection, and harvest conditions. However, recent research suggests that ambient air temperature, specifically elevated nighttime air temperature (NTAT) during grain filling, dramatically affects the variability of rice milling quality, in terms of milled- and head-rice yields; appearance, in terms of chalkiness; and functional characteristics, including viscosity profiles, gelatinization temperatures, and proximate concentrations. Future research is needed to develop cultivars that are resistant to stress resulting from elevated NTAT during the critical period of grain filling, and, for the near term, to develop altered production management practices that mitigate elevated-temperature stress.

Cereal Chem. 91(4):350-357The structural features of starch were examined to better understand the causes of variability in rice quality resulting from nighttime air temperature (NTAT) incidence during kernel development. Starch samples were isolated from head rice of four cultivars (Bengal, Cypress, LaGrue, and XL723) field-grown in four Arkansas locations (Keiser, Pine Tree, Rohwer, and Stuttgart) in 2009 and 2010. Average NTATs recorded during the grain-filling stages of rice reproductive growth in the four locations were 3.0-8.4°C greater in 2010 than 2009. Elevated NTATs altered the deposition of starch in the rice endosperm. Means pooled across cultivars and locations showed that amylose content was 3.1% (percentage points) less for the 2010 sample set. The elevated NTATs in 2010 resulted in a decrease in the percentage of amylopectin short chains (DP ≤ 18) and a corresponding increase in the percentage of long chains (DP ≥ 19) by an average of 1.3% (percentage points). The greater NTATs in 2010 also produced greater starch paste peak, final, and breakdown viscosities, whereas setback and total setback viscosities decreased. Changes in paste viscosity were highly correlated with the changes in the proportion of amylose and amylopectin. Onset gelatinization temperature was greater by 3.5°C, gelatinization enthalpy by 1.3 J/g, and relative crystallinity by 1.5% (percentage points) for the 2010 sample set. Changes in gelatinization parameters and granule relative crystallinity were highly correlated with the changes in amylopectin chain-length distribution. Year × cultivar × location interaction effects were statistically insignificant, indicating that the four cultivars evaluated all showed some degree of susceptibility to the effects of temperature incidence during kernel development, regardless of the growing location.