LIST OF TABLES
2. INTRODUCTION
2.3 Major abiotic challenges and their effect in cowpea and related legumes
Drought, heat, and salinity are the major production constraints declining the cowpea yield more than all the pathogens combined [3, 6]. Drought is one of the most deleterious climatic challenges, adversely affecting the growth and yield, ranging from morphological, physiological, and biochemical alterations, evident at all phenological and developmental stages [260]. Salinity is another brutal abiotic threat to crop productivity, jeopardizing agricultural potential [261].
2.3.1 Terminal drought impair vegetative proliferation and seed development
In general, drought causes a decline in biomass, grain yield, photosynthesis, and nutrient partitioning, leading to wilting and impairment of metabolic functions. The foremost effect of drought is impaired germination and poor stand establishment. Water deficiency inhibits growth determining cellular events such as mitosis, cell elongation, and expansion by reducing cell turgor, causing reduced plant height, smaller leaves, and overall growth retardation. The cellular processes determining the yield are deleteriously affected by water deficiency. Most edible cowpea and other legumes varieties are susceptible to the pre-anthesis and terminal (post-anthesis) drought. The pre-anthesis water-stress shrinks the anthesis period, while the post-anthesis (terminal) stress shortens the grain-filling phase by curbing the enzymatic
activities that synthesize starch, hence detrimental to seed yield. Drought influences plant water relations by reducing relative water content (RWC), leaf water potential, stomatal resistance, and transpiration rate, with a concomitant increase in leaf and canopy temperature. Diminished water availability also limits nutrient acquisition and transportation. It also hampers photosynthetic activity mediated by decreased leaf area, impaired photosynthetic machinery, and pigments to invite premature leaf senescence. Drought alters stomatal oscillations in an ABA-dependent manner to prevent water loss in evapotranspiration, subsequently limiting CO2
influx in leaves. In addition, the low tissue water potential shrinks the cell volume, making the cellular components more viscous, which hampers the enzymatic activity of Rubisco and the photosynthetic machinery due to disrupted NADP-dependent linear electron flow [262, 263].
Consequently, the impaired carbohydrate metabolism affects the translocation of assimilates to that reproductive sinks, hampering seed-set and filling [264]. Moreover, drought increases susceptibility to photo-damage by disturbing the homeostasis of ROS levels and antioxidants [265].
Drought susceptibility of a genotype is often measured as a function of yield-reduction that depends on the timing, duration, and severity of the water deficit. As mentioned earlier, early maturing cowpea varieties can escape drought by completing their life cycle before the occurrence of terminal drought [266]. Also, intermittent moisture stress during vegetative and reproductive stage cause detrimental effects on many cultivars [267]. The most drought- sensitive growth stage in cowpea is just before and during the bloom, followed by pod-filling, vegetative, flowering, and fruiting [268]. Due to water deficiency, the flowering period terminates early, formation of new floral organs delays or aborted, leading to low productivity.
As mentioned earlier, in Sub-Saharan Africa, the productivity of cowpea can curb from 2000 kg/ha (yield potential) to 600 kg/ha when drought attacks the flowering stage [40]. Drought can result in up to 60% yield loss during the pod-filling and pod-setting stage [268, 269].
2.3.2 Terminal heat and freezing stress impair flowering and fertilization
Cowpea is somewhat adapted to dry conditions and high temperatures. However, night heat injury caused by 30°C can lead to complete floral bud abortion, pollen sterility, and indehiscence of anthers to prevent pod formation [258]. Some cultivars can exhibit a 4–14%
reduction in grain yield per °C increase in minimum night temperature during the reproductive phase. The combination of prevailing drought, heat, and long days can impede floral bud development and reduce grain filling by hampering assimilate partitioning. Furthermore, being
a warm-season crop, cowpea cannot thrive in cold temperatures and be killed by frost [270].
Chilling temperatures reduce seedling emergence by disrupting membrane organization in the embryo, degrades photosynthetic activity in the vegetative stage, which subsequently impair reproductive growth [271]. Cold tolerance can be traced in some sub-tropical varieties. Early sowing can benefit indeterminate crops like cowpea, enabling a longer growing period to compensate yield due to a second flush of fruit initiation and development [272]. However, this may invite imbibitional chilling injury leading to early seedling emergence with poor stand and low vigor.
2.3.3 High salinity cause lethal toxicity
Sub-lethal salinity reversibly retards growth without indicating any injury. However, high concentrations (100-200 mM NaCl) diminish yield, severely inhibit growth, or even kill the plants [273]. High salt levels in soil exert ionic stress in plants, individually, or in composite to drought. Indeed, irrigation can induce an accumulation of salt at the soil surface to negatively affect seed germination, seedling stand, vegetative development, and yield. In cowpea, high salinity can cause water deficiency, ion toxicity, nutritional imbalance, oxidative damage [274, 275]. Salt sensitivity varies with the plant’s developmental stage. Pod and seed yield is significantly reduced when the salinity is applied in the vegetative stage, producing fewer or smaller seeds. In contrast, only biomass growth was hampered by salt stress imposed during the flowering and pod-filling stage [257]. Thus, the vegetative stage of cowpea is most severely susceptible to salt stress, and the sensitivity decreases later with development.
Salinity affects plants in two phases, resulting in disturbed physiological and biochemical interactions that affect almost all the developmental stages [276]. In the first phase, the roots deplete water-extraction, exerting osmotic imbalance, which is expressed as reduced leaf area and stunted growth due to loss in cell-turgor, water-content, and limited cell-expansion and stomatal closure. The second phase is characterized by the cytotoxicity caused by the accumulation of Na+ andCl- ionsin the leaf blade, resulting in the death of sensitive, old, and non-expanding leaves,while the new leaves cope by diluting the ions. The ion toxicity destroys water interactions via kosmotropic as well as chaotropic effects. The influx of Na+ disturbs the K+/Na+ ratio, which is further aggravated by K+ efflux mediated by ion transporters or leakage due to increased membrane permeability. K+ ions are the most abundant cation in the cytosol, an essential nutrient and cofactors for many enzymes (such as pyruvate kinase) required to maintain adequate membrane potential and ionic and pH homeostasis [277]. The higher charge
density of Na+ compared to K+ makes it a weaker kosmotrope, affecting the biochemistry of enzymes, other proteins, and DNA. Na+ cannot compensate for the biochemical requirement of K+ in enzymatic and polysome activity, which can also lead to K+ associated programmed cell death (PCD) [278]. Salinity also transiently arrests cell-cycle by diminishing the activity of cyclins and cyclin-dependent kinases (CDKs) to conserve energy and augment defense and repair cellular systems [279]. Despite osmotic stress and ion toxicity, salinity also imposes oxidative damage, photosynthetic degradation, nutrient deficiency, and deteriorates the water- uptake ability to induce water deficiency. High salt levels upset the nutrient balance by interfering with the uptake of nitrogen, phosphorus, calcium, potassium, etc. [280, 281].
Salinity declines photosynthesis efficiency mainly through shrinking leaf area, chlorophyll content, and stomatal conductance [282]. Salinity adversely affects reproductive development by inhibiting microsporogenesis, ovule abortion, and senescence of fertilized embryos [283, 284].
2.3.4 Nutrition deficiency and oxidative stress inhibit growth and survival
Dehydration and salt stress often impose nutrition deficiency in the stressed plants. The physiological damage caused by the stress degrades the plant’s ability to assimilate and transport the available sources of nutrition. Besides channeling of plant energy towards defense and repair, nutrition starvation can also be one of the factors leading to growth retardation during stress. Moreover, oxidative stress may also occur as a concomitant effect, resulting in the generation of reactive oxygen species (ROS) such as superoxide anion radicals (O2-), hydroxyl radicals (OH-), hydrogen peroxide (H2O2), alkoxy radicals (RO), and singlet oxygen (O21) [285]. Chloroplasts are the primary source of strong oxidants generated due to excited pigments in thylakoid membranes. Their excess accumulation can react with cellular biomolecules, leading to lipid peroxidation, membrane injuries, protein degradation, and enzyme inactivation.