Scientists screen for cultivars tolerant to high nighttime temps

high temperatures
Graphic courtesy University of Nebraska

The heat of the day is tough on crops, but it’s the heat of the night that keeps Arkansas rice growers worried.

Arkansas has experienced several weeks of hot, dry weather and that’s growing concern among rice producers, says Jarrod Hardke, rice agronomist for the University of Arkansas System Division of Agriculture.

“If we don’t see more rain soon, we will absolutely see growers who run out of water and are not able to maintain enough moisture to maximize yield,” he says. “We’re running through that at a very rapid pace.”

Rice has also been a victim of the warm temperatures. However, it’s not the beating sun that’s doing damage to the rice but rather the hours of the day that rice depends on to be cooler.

“The biggest point is not actually the daytime temperatures but the nighttime temperatures that don’t allow the plants to adequately cool themselves,” Hardke says.

Rice seeds are mostly composed of starch. When nighttime temperatures rise during seed filling development, chalk occurs. Chalk, which can cause white spots on the grain, is loosely packed starch granules and the air space between them. Although harmless to eat, chalk can prompt millers to dock loads with a high amount of the defect. Not only do consumers prefer rice grains to be translucent, but chalky rice also tends to be brittle, reducing the amount of whole kernels during milling.

Eshan Shakiba, assistant professor – rice breeding and genetics for the University of Arkansas System and one of the primary investigators of this study, says these high night time temperatures lead to chalky rice.

“Rice quality is so important. If the rice is not of good quality, it goes nowhere,” Shakiba says. “In 2010, we saw that a rise in nighttime temperatures leads to higher amounts of chalk.”

Shakiba says this discovery sparked a project that began in 2017.

“In 2017, we met with rice breeders, food scientists and other people at the University of Arkansas, and we decided we need to address this problem,” he says.

This meeting kicked off a five-year project that included three rice breeders and a plant physiologist from University of Arkansas System Division of Agriculture as well as three research scientists, Ramesh Dhakal and Manual Esguerra and program associate Courtland Cole Hemphill.

“We designed a study to test different cultivars in controlled conditions in greenhouses and growth chambers, as well as variable conditions in the field,” Shakiba says. “We’re trying to see which lines have tolerance to high nighttime temperatures.”

The study uses 72 different rice cultivars and three different planting dates in two separate stages: phenotyping, or judging the different cultivars based on physical attributes, and genotyping, or observing the genetic makeup.

“We will do phenotyping in the field, greenhouse and growth chambers,” Shakiba says. “The lines will be evaluated for grain quality such as chalk as well as yield and head rice yield.

“After phenotyping we will conduct an extensive genotypic study through advanced genetic approaches to identify genetic sources of tolerance to high nighttime temperature stress. For genotyping we will be using advanced molecular technology.”

The data collected from the genotyping will be used for breeding programs. As part of the effort, researchers will identify molecular markers linked to genes associated with tolerant to high nighttime temperatures. Breeders can then use the markers to screen for progeny lines with desirable genes.

This project is funded by Arkansas Rice Promotion Board.

The University of Arkansas contributed this article.

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