The quality of seeds is also affected during heat stress by affecting the rate and duration of seed filling and macronutrient accumulation. Increased rate of seed filling has been shown to cause smaller and wrinkled seeds in lentil and chickpea production. Smaller seeds contain fewer nutrients to sustain the emerging seedling and thus tend to lead to less vigorous seedlings. Decreases in the duration of seed filling mean that the plant has less time to both translocate existing nutrients to the seed and synthesize new carbohydrates from photosynthesis to send to the seed. As a result, the seed will receive fewer nutrients to sustain the seedling. Starch and protein accumulation in seeds is particularly affected by heat and drought stress. This is cause for concern for some varieties, such as soybean, where there is an increased monetary value for increased protein content.
Another problem that can occur as a result of heat stress is that leaves may become scorched and even fall off. As leaves are the location of photosynthesis and consequently the main source of energy to the plant, reducing the number of leaves limits the ability to create carbohydrates which further decreases the amount of energy available to be stored in the seed.
Drought stress can also have a big impact on seed production. Since cell expansion begins with an increase in water uptake, drought during seed formation can negatively affect seed size and quality as the cell is not able to expand. In addition, water is essential for the movement of nutrients within the plant. During seed filling, energy reserves within the plant are transported, along with water, to the seeds. Without water, these nutrients are not able to be translocated, effectively reducing the size and quality of the seed. The endosperm is the part of the seed that feeds the embryo during germination. When it is reduced in size, germination and seedling survival is reduced because the seed has less food to start with. In soybeans, it has been shown that drought stress during the initial seed filling can cause approximately a 9% decrease in germination percentage.
With a predicted rise in global temperatures of 2°C by the end of the 21st century and with forecasts of increased frequency and severity of extreme climatic events including heat waves and droughts, protecting and preparing our seeds for adverse weather is a must. Farmers can diversify their crops to help adapt to these changing climatic conditions. Diversification has many lenses: a farmer could grow two varieties of hops, for example, one that is more heat and drought-resistant while the other, more cold and wet resistant. We can also increase the diversity within our seed varieties. While uniform plant populations can make farming more efficient in some cases, having a diverse plant population allows varieties to adapt to a wide range of climatic conditions. Hybrid varieties can easily be knocked out during an extreme climatic event, whereas with a diverse open-pollinated population there is a chance that part of the population may have genetic advantages in any given extreme climatic event This safeguards at least some of the production.
To create diverse open-pollinated populations it is important to ensure minimum plant population sizes. Seeds of Diversity offers an excellent Seed Saving Guide that differentiates the minimum population size of variety maintenance and genetic preservation. Another option to create a diverse plant population could be to pool seeds of the same variety from various farms in different climatic regions into a ‘composite’ seed mix to create flexible plant populations that are adaptable to many different climatic conditions.
Heat and drought certainly have negative effects on seed production. However, we can adapt through practices such as promoting diverse open-pollinated populations or increasing plant breeding efforts of plant lines that are more resistant to extreme climatic events.