Water stress affects many physiological processes in plants. Photosynthesis, transpiration rate, stomatal conductance, osmotic adjustment, accumulation of stress related proteins and
Literature review
accumulation of abscisic acid (ABA) are some of the physiological processes affected by
water stress (Yordanov et al., 2000). Changes to some of these key physiological processes
may affect biomass accumulation, cell turgor, leaf water potential, reduced relative water
content and consequently yield in crops (Reddy et al., 2004). This section examines the main
physiological processes affecting photosynthetic capacity of plants affected by water stress.
2.5.3.1 Stomatal conductance
Stomatal conductance is the most important mechanism regulating carbon and water exchange and consequently controls photosynthesis and transpiration in plants. Stomatal conductance is a function of density, size and opening of stomata and it acts as a plant’s
primary defence mechanism when exposed to drought conditions (Chaves et al., 2003).
Drought tolerant genotypes ensure that water loss is reduced through minimal stomatal
opening and at the same time allowing carbon dioxide in for photosynthesis (Agbicodo et al.,
2009; Cruz de Carvalho et al., 1998). Due to its critical role in regulating water and gas,
stomatal conductance has been recommended as a reliable parameter in screening for drought tolerance. Stomatal conductance has been used in selecting drought tolerant genotypes in
various crops. In cowpea and faba bean (Anyia & Herzog, 2004a; Hamidou et al, 2007; Khan
et al., 2007; 2010), significant genotypic variations were observed in stomatal conductance when exposed to drought conditions, providing room for the selection of genotypes adapted to drought conditions. In shorter term drought, it is better to select genotypes with high stomatal conductance for optimised yield. However, in an event of prolonged drought, low stomatal conductance would enhance survival of genotypes.
2.5.3.2 Transpiration
Transpiration under water stress varies between drought tolerant and susceptible genotypes. Some genotypes exhibit high transpiration, while others significantly reduce transpiration. In
terms of drought adaptation, genotypes which reduce transpiration, when exposed to drought conditions, show their ability to tolerate drought (Hall & Schulze, 1980). This reduction in transpiration results from reduced leaf area, low stomatal frequency and orientation of leaves,
to ensure low radiation loading and evaporative water loss to the environment (Farooq et al.,
2009). However, reduction in transpiration, due to reduced stomatal conductance may reflect limited photosynthetic capacity, resulting in reduced carbon assimilation. Genotypes with reduced transpiration resulting from low stomatal conductance may only be important in areas with short periods of water stress, because such genotypes stop growing under prolonged drought conditions (Liu & Stützel, 2002).
2.5.3.3 Net photosynthesis
Generally, photosynthesis reduces with water stress due to several factors including stomatal
conductance, carbon assimilation, and transpiration (Chaves et al., 2003). However, variation
among genotypes may explain varying responses to drought conditions. Genotypes which maintain high net photosynthesis under water stress conditions generally indicate an ability to
tolerate drought conditions (Farooq et al., 2009). High net photosynthesis is also associated
with high chlorophyll maintenance under water stress conditions (Bertolli et al., 2012).
Therefore, the selection of genotypes with high net photosynthesis due to high chlorophyll concentration may contribute to an improvement in the yield performance of cowpea under water stress conditions.
2.5.3.4 Water use efficiency and transpiration efficiency
Water use efficiency (WUE) and transpiration efficiency (TE) have been used as indicators of drought tolerance. WUE is defined as the biomass accumulated per unit of water used and TE
is the amount of biomass accumulated per unit of water transpired (Manavalan et al., 2009).
Literature review
WUE and TE were affected by changes in stomatal conductance and photosynthetic capacity in cowpea (Ahmed & Suliman, 2010). Genotypes with high transpiration efficiency and high water use efficiency under drought conditions reflect their ability to photosynthesise and accumulate more dry matter compared to genotypes with low WUE and low TE. High transpiration efficiency was positively correlated with yield under drought conditions in
cowpea (Anyia & Herzog, 2004a) and groundnuts (Arunyanark et al., 2008), making it a key
selection criterion for drought tolerance. Consequently, the selection of genotypes with high efficient use of water and high TE would be beneficial for crop production in drought prone areas.
2.5.3.5 Relative water content
Leaf relative leaf water content (RWC) is the amount of water in leaf tissues expressed as a ratio in relation to the maximum amount of water the leaf can hold at the point of saturation
(Suriya-arunroj et al. 2004). Relative water content is calculated as:
RWC =
where FW = Fresh weight of leaves; DW = Dry weight of leaves; TW =
Turgid weight of leaves i.e weight of fresh leaves at saturation point.
And RWC has been widely used in evaluating genotypes for drought tolerance due to a high positive correlation with yield in crops. Generally, RWC decreases with an increase in moisture stress. However, some genotypes show relatively higher RWC than others indicating an ability to tolerate drought. High RWC indicates the ability of genotypes to
retain plant tissue water under moisture stress. In cowpea (Kumar et al., 2008) and wheat
(Bayoumi et al., 2008; Rampino et al., 2006) wide variation in RWC among genotypes
suggests that it is one of the traits that could be used in the identification of dehydration tolerant genotypes. Correlations of 0.8 and 0.87 between RWC and pod set ratio and number
of pods, respectively, were observed in cowpeas evaluated under both moisture stress and
non-stress conditions (Kumar et al., 2008). The cowpea drought tolerant genotypes
maintained minimal differences in RWC, between stress and non-stress conditions, resulting in high pod set ratio and number of pods per plant, which are directly related to yield.
Bayoumi et al. (2008) also found a strong positive correlation (0.84) between RWC and yield
under water stress in wheat. Kumar et al. (2008) indicated that cowpea plants are able to
maintain high RWC either through efficient water uptake from the soil or reduced water loss through stomatal closure. Considering that RWC strongly correlated with yield in both cowpea and wheat, it can be used as a trait for germplasm selection under drought conditions. The RWC has the added advantage of simplicity in measurement because it does not require sophisticated equipment although laborious process.