**3. The physiology of drought response**

The observed physiological changes in plants exposed to drought can be a direct consequence of drought, as well as of the response of the plant, in order to mitigate the stress. Drought typically occurs as a result of low and non-frequent precipitation, resulting in reduced soil water content that is first detected by plant roots [33]. Depending on drought duration and severity the water status of plants can be affected by insufficient water absorption due to low soil water availability, as well as to increased water loss in the process of transpiration. Water deficit in plants affects their normal physiological processes and hinders the development, growth and yield, ultimately resulting in wilting, senescence and plant death [33]. Drought can be potentiated by heat stress, which propagates the water loss from the plant by increased water evaporation from the leaves, and by soil salinity stress, together reducing soil water availability as well as having an additional toxic effect on the plant [34–36].

Drought responsive traits have been studied in common bean by evaluating various traits in field experiments as well as in more controlled environments, such as, greenhouses [37]. Phenological and yield-associated traits have been studied frequently because they are affected by drought stress, are an important indicator of yield output and are also measurable in a large phenotyping population. Phenological traits can also represent the adaptation of the life cycle of a plant to specific drought conditions in the environment. Breeding for earliness is an effective strategy for increasing the yield stability in regions such as the Mediterranean, where plants are exposed to increased drought in the summer time. Early flowering can help in drought avoidance; however, it is not effective in mitigating the drought stress once it occurs [9].

The response to drought in different common bean genotypes has been characterized in several studies based on physiological measurements such as photosynthesis and photosynthate acquisition as well as on partitioning indices [5, 38–44]. Photosynthesis and cell growth are primary processes influenced by drought due to decreased stomata conductivity in the early drought phases that limits evaporation and CO2 diffusion in the leaf mesophyll. The surplus of energy on the thylakoids in the photosynthesis apparatus results in photo inhibition—reduced photochemical efficiency [45]. Stomata closure is an effective strategy for shorter drought periods and for mild drought where photosynthesis is not affected in such a way as to reduce

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*Drought Stress Response in Agricultural Plants: A Case Study of Common Bean...*

with better partitioning indices expressing a higher yield [5, 40, 43].

Understanding the changes in metabolic pathways in plants under the influence

Screening genes with differential expression and proteins with changed abundance or activity in plants exposed to drought is greatly facilitated by modern transcriptomic and proteomic tools which have, together with other approaches, enabled rapid development of the field. Identification of detected genes and proteins is greatly facilitated by the recent advances in sequencing and the publication of full genome sequences of model legumes *Medicago truncatula* [48] and *Lotus japonicus* [49] and of crop legumes, such as, common bean [50], soybean [51], chickpea (*Cicer arietinum*) [52] and peanut [53]. Comprehensive lists of genes and proteins obtained from screening studies are then classified according to their

of drought, as well as the molecular mechanisms regulating their adaptation to this stress, is very important in identifying key molecular markers that could help distinguish between genotypes with different tolerance. On the molecular level, drought affects plant cells in different ways—through changes in gene expression and/or translation of transcripts to proteins, through posttranslational modification leading to protein activation and by further direct action on the protein itself. It is important to underline that these ways are interdependent and that only active key

**4. Response to drought on the molecular level**

proteins enable a response beneficial for the plant.

the yields [38]. When the duration of drought is longer, better drought tolerance and yields are enabled by a specific biochemical mechanism on the cell level. High yields of tolerant Durango genotypes, such as, 'Pinto Saltillo,' exposed to drought have been associated with early and fine regulation of the stomata response and with CO2 assimilation with stomata closure, limiting water loss during the day, maintaining higher relative water content (RWC) at night, with increased water use efficiency and limitation of reactive oxygen species (ROS) accumulation [41, 42, 44].

Screening selected genotypes adapted to Central European climatic conditions enabled us to identify genotypes with more drought tolerant traits [46, 47] thus becoming a starting point for studies on the mechanisms of drought. They were performed under controlled conditions with drought being induced by discontinuing irrigation and assessment of drought by soil water potential measurements. Observation of plant physiological changes, measurements of leaf water potential, relative water content and yield were employed for determining drought tolerance. Among the tested cultivars adapted to the growing conditions of the Central European region, the greatest difference in response to drought was observed between 'Tiber' and 'Starozagorski Čern,' the former being the most tolerant. For this reason these two cultivars were used in many of our studies of the response to drought [46, 47]. Studies on the level of physiology have confirmed that the ability to withstand drought is also related to the water consumption pattern of the plant. Some cultivars such as 'Starozagorski Čern' exhibit water spending behavior that enables them to thrive, and, when the water supply is sufficient, they produce high yields [47]. However, when exposed to drought, their yield can be significantly reduced. In contrast, cultivars adapted to harsher environments, regulate water more conservatively and their yield during drought is affected less, as is the case with 'Tiber.' Drought tolerance in water saving cultivars has been associated with great plasticity on the biochemical and cellular levels, being associated with stomatal conductance, photosynthesis rate, abscisic acid (ABA) synthesis and resistance to photoinhibition [39]. In addition, the distribution of photosynthetic products to developing pods and seeds is an important factor in determining the yield under stress, with genotypes

*DOI: http://dx.doi.org/10.5772/intechopen.86526*

#### *Drought Stress Response in Agricultural Plants: A Case Study of Common Bean... DOI: http://dx.doi.org/10.5772/intechopen.86526*

the yields [38]. When the duration of drought is longer, better drought tolerance and yields are enabled by a specific biochemical mechanism on the cell level. High yields of tolerant Durango genotypes, such as, 'Pinto Saltillo,' exposed to drought have been associated with early and fine regulation of the stomata response and with CO2 assimilation with stomata closure, limiting water loss during the day, maintaining higher relative water content (RWC) at night, with increased water use efficiency and limitation of reactive oxygen species (ROS) accumulation [41, 42, 44].

Screening selected genotypes adapted to Central European climatic conditions enabled us to identify genotypes with more drought tolerant traits [46, 47] thus becoming a starting point for studies on the mechanisms of drought. They were performed under controlled conditions with drought being induced by discontinuing irrigation and assessment of drought by soil water potential measurements. Observation of plant physiological changes, measurements of leaf water potential, relative water content and yield were employed for determining drought tolerance. Among the tested cultivars adapted to the growing conditions of the Central European region, the greatest difference in response to drought was observed between 'Tiber' and 'Starozagorski Čern,' the former being the most tolerant. For this reason these two cultivars were used in many of our studies of the response to drought [46, 47]. Studies on the level of physiology have confirmed that the ability to withstand drought is also related to the water consumption pattern of the plant. Some cultivars such as 'Starozagorski Čern' exhibit water spending behavior that enables them to thrive, and, when the water supply is sufficient, they produce high yields [47]. However, when exposed to drought, their yield can be significantly reduced. In contrast, cultivars adapted to harsher environments, regulate water more conservatively and their yield during drought is affected less, as is the case with 'Tiber.' Drought tolerance in water saving cultivars has been associated with great plasticity on the biochemical and cellular levels, being associated with stomatal conductance, photosynthesis rate, abscisic acid (ABA) synthesis and resistance to photoinhibition [39]. In addition, the distribution of photosynthetic products to developing pods and seeds is an important factor in determining the yield under stress, with genotypes with better partitioning indices expressing a higher yield [5, 40, 43].
