Solar radiation use efficiency and the amount of water-soluble carbohydrates in the stem as factors in improving wheat yields

In current conditions, characterized by a decrease in the precipitation amount in spring and summer, its uneven amounts with the increased frequency and duration of periods of high temperatures, stable production of wheat grain requires the investigation of the possibilities of improving its yield. The search for the traits, related to high yields, resistance, and adaptivity to stresses in wheat genotypes is the object of many studies. The aim of this review is to analyze two promising traits which can potentially be improved. One of the main factors for wheat yield growth is known to be the absorption of photosynthetically active radiation by crops of wheat. It has now approached the maximum possible level, so a promising way for further breeding is to enhance its use efficiency. In this regard, the review discusses the current state of research on the efficiency of photosynthetically active radiation use by this important food crop. Based on literature data and the results of our own research in Ukraine and taking into account that genetic variations in the values of radiation use efficiency can differ by a factor of 2–3, the role of genotypic characteristics of wheat varieties as an important resource for enhancing radiation use efficiency is analyzed. The relationships between radiation use efficiency and plant dry matter formation for different genotypes and under different growing conditions are considered. It is shown that the main factor that affects enhancing solar radiation use efficiency is the increase in plant dry matter formation. Given the complexity of determining the efficiency of radiation use, the biomass of winter wheat plants in the early stages of spring vegetation is also discussed as a sign of a variety with a higher efficiency of converting light energy into biomass. It is substantiated that the effect of this trait on yield may be associated with a better supply of assimilates, which will further contribute to the formation of a well-grained spike. Another important trait in enhancing the grain productivity of wheat is the content or amount of water-soluble carbohydrates deposited in the stem. The ability to accumulate reserve assimilates in the stems before intensive grain filling is often considered a sign of drought tolerance of the variety, since the level of moisture supply significantly affects the accumulation of water-soluble carbohydrates in the stem. Moreover, there are evidences of a positive effect of increased amounts of water-soluble carbohydrates in the stem on yield, regardless of water supply conditions. Therefore, this review discusses the role of both growing conditions and genotype for the accumulation and remobilization of reserve assimilates from the stem or its individual internodes to grain filling. Based on the literature data on the increase in the amount of water-soluble carbohydrates in the stem of wheat varieties with different breeding periods and on the genetic yield increase in them, it has been substantiated that the content of water-soluble carbohydrates in the stem can be a potential trait related to yield. One of the factors influencing the increase in the content of water-soluble carbohydrates in the stem on the yield is the depositary role of the stem as an intermediate reserve for further remobilization of accumulated assimilates. Although the positive effect of water-soluble carbohydrates in the wheat stem in compensating for grain yield has been proven mainly under water stress, the sufficient genetic diversity of their content, as well as the medium and high heritability under different conditions indicate the prospects for further research into this trait in increasing wheat yield.