RICE QUALITY AND PROCESSING. Effects of Nighttime Air Temperatures during Kernel Development on Rice Chalkiness

Transcription

1 RICE QUALITY AND PROCESSING Effects of Nighttime Air Temperatures during Kernel Development on Rice Chalkiness A.A. Ambardekar, T.J. Siebenmorgen, P.A. Counce, A. Mauromoustakos, and S. Lanning ABSTRACT Chalk is known to affect milling and processing quality of rice, and has been known to vary across cultivars and across years. Recent research has shown that elevated nighttime air temperatures (NTATs) could contribute to increased chalk and reduced milling quality. In an effort to develop a method to quantify the effects of NTATs, 95 th percentiles of NTAT frequencies occurring during reproductive (R) stages in Bengal, Jupiter, Cypress, LaGrue, Wells, and XL723 cultivars were correlated with chalk levels during the 2007 through 2010 harvest seasons. Chalk values were strongly correlated with 95 th percentiles of NTAT frequencies during the R7 and R8 stages for all cultivars, except Bengal and Jupiter. Although strong correlations of chalk levels with NTAT 95 th percentiles were observed during R7 and R8 stages of kernel development, it is speculated that while rice plants may be classified in the R8 stage, many kernels on the plants would lag in development and exist in the R6 and R7 grain-filling stages, when elevated NTATs are thought to have deleterious effects on kernel formation and thus, milling quality. INTRODUCTION Chalky rice kernels generally result in lower head rice yield (HRY), a quality indicator that determines the market value of rice, because they tend to be weaker and more prone to breakage during milling than translucent kernels (Lisle et al., 2000; Kadan et al., 2008). Warm nighttime air temperatures (NTATs) are reported to interfere with metabolic processes during developmental stages in rice, which may result 286

2 B.R. Wells Rice Research Studies 2010 in the formation of chalk (Lisle et al., 2000). Cooper et al. (2008) reported that rice cultivars grown in controlled-environment growth chambers showed different degrees of susceptibility to high NTATs with respect to chalkiness and further proposed that increased chalk formation in rice could be due to a reduction in the rates of enzymatic activity and physiological functioning during high-ntat exposure. The studies cited above were conducted using controlled-temperature growth chambers to simulate environmental conditions, but do not necessarily represent field conditions, in which irregular and non-systematic temperature fluctuations occur. Therefore, in order to assess the effect of NTATs during kernel development of field-grown rice samples on chalk, a method to quantify and correlate the occurrence of elevated NTATs to chalk was developed and used to indicate the reproductive growth stages in which the susceptibility to elevated NTATs was apparent. PROCEDURES Six cultivars ( Bengal, Jupiter, LaGrue, Cypress, Wells, and XL723 ) were grown in the locations shown in Table 1 each year from 2007 to 2010 as part of the Arkansas Rice Performance Trials (ARPT) system. At each location, each of the six cultivars was planted in three randomly-assigned, replicate plots within a field. The five pure-line cultivars (Bengal, Jupiter, LaGrue, Cypress, and Wells) were drill-seeded at a rate of 428 seeds/m 2 in nine-row (0.18-m spacing) plots, 4.57 m in length. The hybrid (XL729) was sown in plots of the same dimensions at a seeding rate of 171 seeds/m 2. During each of the four study years, the reproductive stages from R3 to R8 for each of the six cultivars grown at Stuttgart, Ark., were visually identified through fieldstage identification (Counce et al., 2000). The days upon which each of these stages initiated for each cultivar at the other growing locations were then estimated, based on staging data collected in Stuttgart and each location s respective temperature data. This procedure is described in the following section. Weather Data and Thermal Unit Calculation Ambient temperatures from 2007 to 2010 were recorded in 30-min increments using two temperature sensors (HOBO Pro/Temp Data Logger, Onset Computer Co., Bourne, Mass.) positioned at each growing location. Based on these 30-min temperatures, and using the following equation, degree-day-50 (DD50) thermal units ( F-day) over the course of each day were quantified: [{ Tmax (ºF) + Tmin (ºF) } - 50 ºF ] 30 - min 0.5 h x 1 day 48 DD50 = Σ i = h Eq. 1 Where TMAX and TMIN represent the maximum and minimum temperatures, respectively, during a 30-min interval. Maximum temperature was considered 94 F if the maximum temperature during a 30-min interval was greater than 94 F. Growth under 50 F was assumed negligible. 287

3 AAES Research Series 591 Thermal unit accumulation at the initiation of the R3 stage was assigned a value of zero. DD50 values were computed using the 30-min temperature data and accumulated to determine the progression through R-stages. To determine the thermal units required for a rice cultivar to advance from one R-stage to another, the accumulated DD50 values computed from the temperature readings at Stuttgart were first aligned with the initiation of each stage at Stuttgart; this process was repeated for each cultivar in each study year. The rate of R-stage development for each cultivar was assumed to be constant across locations during the same growing season. Thus, the thermal unit accumulation versus R-stage progression pattern determined for each cultivar in each year at Stuttgart was utilized, in conjunction with the 30-min temperature data collected at each location, to determine the day-of-the-year (DOY) initiations of each R-stage of each cultivar at the other locations. The visually-observed R3 DOY for each cultivar at each location was used as starting data. This process yielded the initiation DOY and duration of each cultivar s R-stages at each location in each year, upon which subsequent nighttime temperature analysis was based. Ambient temperatures during the time of the day extending from 8:00 pm to 6:00 am were considered as NTATs. Chalk During each harvest year and at each location, rice samples of each cultivar were harvested in triplicate over a range of moisture contents (MCs). Chalk values were determined and averaged across all sample lots within each harvest year/location/cultivar combination. In brief, duplicate 100-g rough rice samples from each harvest lot were de-hulled to produce brown rice. One hundred brown rice kernels from each sample were randomly selected and placed on a tray (152 mm 100 mm 20 mm) made from 32-mm thick, clear acrylic sheet, so that no single kernel touched another kernel. A digital image of kernels was created by placing the tray on the scanner of an image analysis system (WinSeedle Pro 2005aTM, Regent Instruments Inc., Sainte-Foy, Quebec, Canada). Prior to analysis, the imaging system was configured to color-classify chalk by selecting and scanning a brown rice kernel considered to be completely chalky into the imaging system as a reference color for chalk. The imaging system measured and recorded the number of pixels representing the entire kernel area from the scanned images, as well as the number of pixels corresponding to those areas color-classified for chalk on a kernel. Percent chalk in a sample was determined as the ratio of the total chalky area (pixels) of the 100-kernel set to the total area of the kernels, multiplied by 100. Statistics Frequencies of the observed NTATs during each R-stage (see Fig. 1, representing frequency of NTAT during the R7 stage) were tallied and the 95 th percentiles of 288

4 B.R. Wells Rice Research Studies 2010 NTAT frequency, below which 95% of the NTATs occurred, were calculated for all year/location/cultivar/r-stage combinations using a cumulative frequency distribution model (JMP release 8.2, SAS institute, Cary, N.C.). The statistical significance of each correlation was determined by analysis of variance at α = 0.05 using polynomial regression analysis (JMP release 8.2, SAS institute, Cary, N.C.). RESULTS AND DISCUSSION Chalk values of cultivars harvested during 2007 through 2010 are shown in Table 1. In general, chalk values tended to be greater in years 2007 and 2010 than in 2008 and Pairwise correlation coefficients at α = 0.05 significance level were calculated to describe trends in chalk levels versus 95 th percentiles of NTAT frequencies occurring during the R5 to R8 reproductive stages (Table 2). Positive correlations were observed between chalk levels and 95 th percentiles of NTAT frequencies during the R8 stage in all cultivars. Hybrid cultivar XL723 showed the strongest correlations, followed by Wells and LaGrue. Cypress, Jupiter, and Bengal showed weaker, but significant correlations. Results similar to the findings of this study were observed by Cooper et al. (2008) wherein chalk formation of LaGrue was observed to be significantly greater than those of Bengal and Cypress with increasing NTAT. Figure 2 reveals a particularly strong quadratic relationship between chalk and NTAT frequency percentiles during the R8 stage for all cultivars. Similar quadratic trends were also observed in the chalk values of all the cultivars plotted against the 95 th percentiles of NTAT frequency during the R7 stage (data not shown). This relationship suggests an optimum temperature, below and above which chalk formation was triggered. Thus, findings of this study agree with the theory that an optimal temperature exists for the enzymes responsible for packing of starch during the grain-filling stages. In a controlled-temperature study, Yoshida and Hara (1977) observed a similar secondorder relationship between NTAT and chalk formation in Indica (IR20) and Japonica rice (Fujisaka 5). In this study, high chalk levels were observed at NTATs below and above 18 C (65 F). Counce et al. (2005) noted eight enzymes required to convert sucrose into fully-branched starch molecules in developing rice kernels. Starch synthesis enzymes, particularly starch synthase, are sensitive to temperatures above 25 C (77 F), as observed in wheat and maize (Keeling et al., 1994). Kernel maturation, or progression through R-stages, is asynchronous among the kernels on a rice plant, and certainly among kernels on different plants within a field (Hollaway et al., 1995; Counce et al., 1996). This would indicate that a particular R-stage may not be representative of all kernels on the plant. For example, the R6 reproductive stage represents the stage in which the caryopsis of the first observed kernel on the main stem panicle completely elongates to the end of the hull (Counce et al., 2000). Subsequent kernels lag behind in the maturation process, passing through this grain-filling stage after the first kernel. Therefore, while results of this study indicate that NTATs during the R7 and R8 stages have the most pronounced effect on chalk formation, the R6 grain-filling stage is hypothesized to be the developmental stage during which NTAT 289

5 AAES Research Series 591 effects are manifested in the kernels. These speculations are supported by the findings of a historical analysis relating milling quality to NTATs by Cooper et al., 2006, in which it was similarly concluded that, although NTATs were shown to affect kernels during the R8 stage, most of the kernels during that stage were actually observed to be in earlier reproductive (grain-filling) stages. SIGNIFICANCE OF FINDINGS Chalk levels were directly correlated to 95 th percentiles of NTAT frequencies during the R8, and to a lesser degree, R6 and R7, reproductive stages. Since the staging system developed by Counce et al. (2000) is based on a visual rating of the most mature kernel s development on the main stem panicle, strong correlations at the R8 stage may actually indicate that NTAT effects are incurred by kernels at the R6 and R7 grain-filling stages. Medium-grain cultivars Bengal and Jupiter exhibited weak correlations of chalk with 95 th percentiles of NTAT frequency, indicating that they may be somewhat resistant to the impacts of elevated NTATs occurring during critical reproductive growth stages. Long-grain cultivars XL723, Wells, and LaGrue, showed greater increases in chalk with increasing 95 th percentiles during the R7 and R8 stages. Cypress exhibited the weakest correlations among the long-grains, suggesting that it may be less susceptible to the impacts of elevated NTATs than other long-grains. The quadratic trend observed between chalk values and 95 th percentiles of NTAT frequencies during the R7 and R8 stages of all cultivars may suggest an optimal temperature for the grain-filling process, below or above which chalk formation is apparent. ACKNOWLEDGMENTS The authors wish to thank the Arkansas Rice Research and Promotion Board and the corporate sponsors of the University of Arkansas Rice Processing Program for their financial support. LITERATURE CITED Cooper, N.T.W., T.J. Siebenmorgen, and P.A. Counce Effects of nighttime temperature during kernel development on rice physicochemical properties. Cereal Chem. 85: Cooper, N.T.W., T.J. Siebenmorgen, P.A. Counce, and J.F. Meullenet Explaining rice milling quality variation using historical weather data analysis. Cereal Chem. 83: Counce, P.A., R.J. Bryant, C.J. Bergman, R.C. Bautista, Y.J. Wang, T.J. Siebenmorgen, K.A.K. Moldenhauer, and J.F. Meullenet Rice milling quality, grain dimensions, and starch branching as affected by high night temperatures. Cereal Chem. 82:

6 B.R. Wells Rice Research Studies 2010 Counce, P.A., T.C. Keisling, and A.J. Mitchell A uniform, objective, and adaptive system for expressing rice development. Crop Sci. 40: Counce, P.A., T.J. Siebenmorgen, M.A. Poag, G.E. Holloway, M.F. Kocher, and R. Lu Panicle emergence of tiller types and grain yield of tiller order for direct-seeded rice cultivars. Field Crops Res. 47: Hollaway, G.E., T.J. Siebenmorgen, P.A. Counce, and R. Lu Causes of multimodal moisture content frequency distributions among rice kernel. Trans. Am. Soc. Agri. Eng. 11: Kadan, R.S., R.J. Bryant, and J.A. Miller Effects of milling on functional properties of rice flour. J. Food Sci. 73:E151-E154. Keeling, P.L., R. Banisadr, L. Barone, B.P. Wasserman, and G.W. Singletary Effect of temperature on enzymes in the pathway of starch biosynthesis in developing wheat and maize grain. Func. Plant Biol. 21: Lisle, A.J., M. Martin, and M.A. Fitzgerald Chalky and translucent rice grains differ in starch composition and structure and cooking properties. Cereal Chem. 77: Pereira, T., N.T.W. Cooper, and T.J. Siebenmorgen Effect of storage temmperature and milling duration on storage properties of rice. Discovery 9: Yoshida, S. and T. Hara Effects of air temperature and light on grain-filling of an indica and a japonica rice (Oryza sativa L.) under controlled environmental conditions. Soil Sci. Plant Nutr. 23:

7 AAES Research Series 591 Harvest Table 1. Chalk z values for cultivars harvested from different locations during 2007, 2008, 2009, and Cultivars season Locations y Bengal Cypress Jupiter LaGrue Wells XL Chalk (%) Corning Newport Stuttgart Rohwer Corning Pine Tree Stuttgart Rohwer Keiser Pine Tree Stuttgart Rohwer Keiser Newport Pine Tree Stuttgart Rohwer z Chalk values were averaged across all samples analyzed for each year/location/cultivar combination (Table 1). y All locations are in Arkansas, USA. Table 2. Correlation coefficients of chalk z with the 95 th percentiles of nighttime air temperature frequencies during the R5 to R8 reproductive stages of the indicated long- and medium-grain rice cultivars grown in Arkansas in 2007, 2008, 2009, and Cultivars R-stage Bengal Cypress Jupiter LaGrue Wells XL723 cultivars R5 NS y NS NS 0.51 NS R6 NS R R z Chalk values of each sample were measured in duplicate and then averaged across all samples within each year/location/cultivar combination (Table1). y NS = not significant (p > 0.05). All 292

8 B.R. Wells Rice Research Studies 2010 Fig. 1. Nighttime air temperature frequencies during the R7 reproductive stage of long-grain hybrid XL723 grown at Stuttgart, Arkansas in 2007, 2008, 2009 and Mean peak head rice yields (phrys) and chalk levels for each year are indicated (Table 1). Fig. 2. Relationships of chalk and the 95th percentiles of nighttime air temperature frequencies during the R8 stages of all cultivars grown during 2007, 2008, 2009, and