Plant breeding for drought conditions

Abstract

Drought is considered one of the biggest problems in food production. Climate changes have a negative impact on the total food production in the World, regardless the increase achieved by breeding and new agronomy. Water deficiency usually leads to decreased plant growth, decreased photosynthesis intensity and metabolic disorders. The response of plants to drought is complex, because drought stress causes problems in the adoption of biogenic elements and the transport of nutrients and assimilates, which affects the overall metabolism of the plant. Drought management methods are multiple, complex and complementary, and breeding and development of yielding genotypes in drought conditions is particularly significant. Improved genetics are much more easily and quickly introduced into production than improved agronomy, which depend more on the input potentials, infrastructure, market access and experience in the agronomy. Drought resistance of plants is reflected in the ability to neutralize negative metabolic changes and maintain high synthetic ability. Resistance consists of resistance to high temperatures and resistance to water shortage. Resistance to soil water deficiency is a complex trait and cultivated plants can achieve it by avoiding drought, reducing dehydration and by tolerance to dehydration. The drought response starts by closing of stoma to prevent leaf drying and reduce water consumption, but it also leads to reduced CO2 uptake and photosynthesis. Due to osmotic regulation, stomas of resistant genotypes remain open, allowing photosynthesis to take place, partial leaf elongation, root growth, water uptake from soil, delayed leaf wilting, more efficient accumulation of dry matter and higher yield in stress conditions. In conditions of drought stress, delaying leaf death is especially important for increased tolerance to drought. An increase in the number of fertile flowers in flowering and, consequently, increase in the number of maturing grains, contributes to an increase in the capacity of the acceptors of assimilates. Breeding of cultivated plants almost exclusively used the empirical method in the 20th century. The physiological approach is used as the most significant support for empirical breeding today. The simultaneous application of both methods enables faster and more efficient development of drought resistant genotypes. This method enables easier and more efficient identification of traits that limit yield in drought conditions, multi-generation testing annually and faster selection, more successful testing of a large number of genotypes in work collection, crossing block, and segregation generations, based on which lower number ofselected lines is tested in comparative trials for yield. If the desirable trait is positively correlated with the yield, it is more favourable to perform selection on the trait in the younger segregation generations, than in older generations on yield, considering strong GxE interaction for yield. However, due to difficulty in evaluating physiological traits and their low heritability, breeding specific physiological traits that provide drought tolerance to plants is difficult and relatively modest results have been achieved so far. Phenology is a complex trait that affects the resistance of plants to drought, since changing the length of some phenophases can regulate the amount of water adopted before flowering and after flowering. Genetic manipulations in flowering time were of the greatest importance in adapting the vegetative and reproductive period to available water and evaporation. Early maturation is a physiological trait that avoids drought effect in many areas. Sowing genotypes that flower earlier does not always result in expected yield due to less accumulation of above-ground dry matter. Root architecture represents the trait of the plant that provides the most opportunities in creating drought tolerant genotypes. So far, studies of cultivated plants were least focused on root research, so there is no information on whether the root system of modern varieties is adapted to soil and ecological factors and whether it is necessary to make changes in roots trough breeding. A deep root system implies drought tolerance and the ability to absorb more water from the soil. The narrower xylem bundles in the seminal root reduce the use of water before flowering under conditions of drought stress, which contributes to an increase in the amount of water available during the grain filling period. Increasing the capacity of the root system, its depth and distribution in the soil is easiest to achieve by cultivating varieties with a longer vegetative period, which can be achieved by early sowing or sowing late varieties. In addition, selecting varieties with a higher early vigour can result in faster root growth, deeper penetration into the soil, and a more developed system of adventitious roots. Lower temperature of canopy or higher stoma conductance is indication of favourable soil water regime and deeper root system. Transpiration efficiency (TE) is an important component of water use efficiency. There are various ways of increasing TE in plants, and the most efficient of which is the cultivation of genotypes where the period of maximum biomass increase occurs during periods of moderate temperatures, when less water is used for growth. Processes that affect the extent of carbon isotope discrimination 13C (Δ) have a significant impact on TE, with discrimination being reduced when the TE value is high. Discrimination of carbon isotopes is closely related to TE throughout vegetation, which is why this method can be used to create varieties with higher TE. The advantage of using Δ, as a potential indirect method in breeding, is reflected in a much simpler and faster measurement compared to the measurement of TE. For some plant species, significant progress in higher yields breeding is achieved by increasing the harvest index (HI). The increase in HI did not lead to a significant change in the amount of water absorbed in small grains, but it did lead to a natural increase in water use efficiency. Under high temperature conditions, the average wheat grain yield is positively correlated with HI. Further increase of grain yield in cereals via HI change can not give significant results, which is why increased aboveground biomass while retaining the reached HI is achieved trough breeding. Less tillering, especially the formation of a smaller number of sterile spikes, contributes to the achievement of higher HI in drought conditions, as smaller leaf areas are formed before flowering, thus reducing transpiration and providing more water for the grain filling period. In large number of cultivated plants, the excess assimilates, which are synthesized until flowering, accumulates in the form of soluble carbohydrates in the steam, which plants use during the grain filling period (GFP). The extension of the GFP is a key factor in increasing the yield of small grains. During GFP, assimilates from the steam are translocated in the grain, and in extremely arid conditions can participate 100% in the final grain weight. In small grains, large genetic variation was found in the accumulation and remobilization of assimilates synthesized until flowering.