High temperature during anthesis affects the salinity tolerance of quinoa under field conditions

In the arid landscapes of the Middle East and Central Asia, where the soil is often salty and the air is dry, farmers face a difficult choice: grow food or watch the land go fallow. For decades, the world has relied on a handful of staple crops like wheat and rice to feed its population, but these plants are fragile when the ground is salty or the air is too hot. As the climate shifts and temperatures rise, the search for a resilient alternative has turned toward a humble seed from the Andes: quinoa. This plant is not just a source of protein and nutrients; it is a survivor. It thrives in harsh conditions where other crops fail, tolerating salty soil that would kill a field of wheat. Scientists have long known that quinoa can handle salt, but they have struggled to predict exactly how much it can take in the real world, especially when the weather turns against it. The question is no longer just whether quinoa can grow in salty soil, but whether it can survive the double hit of salt and extreme heat, a combination that is becoming increasingly common in the regions where this crop is most needed.

To answer this, researchers in Iran set out to test five different varieties of quinoa under real field conditions over two consecutive years. They planted the seeds in a sandy-loam soil on the central plateau, a region known for its challenging climate. The team created a controlled environment where they could manipulate the saltiness of the water used for irrigation, testing levels ranging from fresh water to water as salty as the ocean. They grew the plants through two full seasons, 2017 and 2018, to see how the crops would perform. The first year was relatively mild, but the second year brought a surprise: a period of intense heat during the most critical time for the plants, when they were flowering and filling their seeds with grain. This natural variation allowed the scientists to compare how the same plants reacted to salt in a normal year versus a year when the heat was severe.

The results painted a clear picture of the plant's limits. In the cooler year of 2017, the quinoa performed well, producing a healthy harvest even in moderately salty soil. The plants grew tall, their seeds were heavy, and they yielded a significant amount of grain. However, the story changed dramatically in 2018. When high temperatures struck during the flowering stage, the plants' ability to produce seeds collapsed. The average seed yield dropped from nearly 400 grams per square meter in the first year to less than 76 grams per square meter in the second. The heat did not just reduce the amount of grain; it changed the plant's priorities. Instead of putting energy into making seeds, the plants focused on growing leaves and stems, resulting in a large amount of straw but very little harvest. The seeds that did form were smaller and lighter, and the plants became much more sensitive to the salt in the soil.

What the researchers found was that heat stress fundamentally altered how the quinoa handled salt. In the cooler year, the plants could tolerate high levels of salinity, with some varieties producing good yields even when the soil was quite salty. But when the heat arrived, the plants' tolerance dynamics shifted. The threshold value at which yield begins to decline showed mixed results under heat stress; for some genotypes like Sadoq, the threshold increased, while for others like NSRCQE, it decreased. However, the slope of the yield decline became much steeper across the board; for every unit of salt added beyond the threshold, the yield dropped much faster in the hot year than in the cool one. Essentially, while the starting point for stress varied by variety, the damage inflicted by salt became far more severe once that point was reached, making the plants more fragile overall. The study showed that the sensitivity of quinoa to salinity increased significantly when combined with heat stress, particularly during the flowering and seed-filling periods.

The experiment also highlighted that not all quinoa varieties are created equal. Some lines, like the one named Sadoq, showed a higher capacity to produce seeds in both years, though even this resilient variety suffered a massive drop in yield during the hot year. Other varieties were less consistent, with some performing well in one year and poorly in the next. The researchers observed that the plants' ability to produce large seeds was particularly vulnerable to the heat, with the percentage of large seeds dropping by nearly a quarter in the hot year. This inconsistency suggests that simply planting quinoa in salty regions is not a guaranteed solution. The timing of the planting and the specific genetic makeup of the seed matter just as much as the soil conditions.

The findings offer a sobering lesson for agriculture in a warming world. While quinoa remains a promising crop for saline soils, its success is not absolute. It is not a magic bullet that can ignore the weather. The study demonstrates that high temperatures during the flowering stage can disrupt the plant's reproductive process, causing it to abort seeds or produce tiny, light ones. This disruption happens because the heat interferes with pollination, a process that relies on specific temperature ranges. When the air is too hot, the plant's ability to turn sunlight and nutrients into grain is compromised, and the salt in the soil becomes a much more damaging factor. The researchers concluded that to make quinoa a reliable food source in these harsh environments, farmers and breeders must focus on selecting varieties that are less sensitive to heat during flowering and must adjust planting dates to avoid the hottest parts of the season. Without these adjustments, the promise of quinoa as a climate-resilient crop may remain unfulfilled in the very regions that need it most.