Food Legumes and Rising Temperatures: Effects, Adaptive Functional Mechanisms Specific to Reproductive Growth Stage and Strategies to Improve Heat Tolerance

Abstract

Ambient temperatures are predicted to rise in the future owing to several reasons associated with global climate changes. These temperature increases can result in heat stress- a severe threat to crop production in most countries. Legumes are well-known for their impact on agricultural sustainability as well as their nutritional and health benefits. Heat stress imposes challenges for legume crops and has deleterious effects on the morphology, physiology, and reproductive growth of plants. High-temperature stress at the time of the reproductive stage is becoming a severe limitation for production of grain legumes as their cultivation expands to warmer environments and temperature variability increases due to climate change. The reproductive period is vital in the life cycle of all plants and is susceptible to high-temperature stress as various metabolic processes are adversely impacted during this phase, which reduces crop yield. Food legumes exposed to high-temperature stress during reproduction show flower abortion, pollen and ovule infertility, impaired fertilization, and reduced seed filling, leading to smaller seeds and poor yields. Through various breeding techniques, heat tolerance in major legumes can be enhanced to improve performance in the field. Omics approaches unravel different mechanisms underlying thermotolerance, which is imperative to understand the processes of molecular responses toward high-temperature stress.

Keywords: food legumes; functional mechanisms; high temperature stress; reproductive function; ‘Omics’ approach.

FIGURE 1

Total legume production and area harvested worldwide an in Asia in 2014–2015 (modified from FAOSTAT, 2014).

FIGURE 2

Sensing and signaling in plants in response to heat stress. Heat stress affects the plasma membrane to activate calcium channels, which induces Ca²⁺ influx and activates the heat shock response. Thus, the MAPK cascade leads to gene expression. Secondary signals such as ROS, H₂O₂, NO, and ABA lead to stress tolerance. CaM3, calmodulin; HSFs, heat shock factors; CDPKs, calcium-dependent protein kinases; MAPKs, mitogen-activated protein kinases; ROS, reactive oxygen species; NO, nitric oxide; HK, histidine kinase; UPR, unfolded protein response; ER-UPR, endoplasmic reticulum unfolded proteins; Cyt-UPR, cytosolic unfolded proteins.

FIGURE 3

The life cycle of a typical angiosperm showing target sites of heat stress. The sporophyte phase is the main phase, which generates microspores that produce pollen grains as the male gametophytes (microgametophyte), and megagametophytes (megaspores), which form an ovule that contains female gametophytes.

FIGURE 4

Effects of heat stress during the reproductive phase (at different functional stages).

FIGURE 5

Effect of heat stress in normal-sown and late-sown (heat-stressed) plants Chickpea [(A: Biomass in control (a) and heat-stressed (b), Pollen load in control (c) and heat-stressed (d), Pollen viability in control (e) and heat-stressed (f) pollen viability in control (g) and heat-stressed (h), Stigm receptivity in control (i) and heat-stressed (j) (Kaushal et al., 2013)], Mungbean [(B; Pollen viability in control (a) and heat-stressed (b), pollen germination in control (c) and heat-stressed (d), and SEM observations on pollen morphology in control (e) and heat-stressed (f) (Kaur et al., 2015)], and lentil [(C; Pollen viability in control (a) and heat-stressed (b), Pollen germination in control (c) and heat-stressed (d), Pollen load in control (e) and heat-stressed (f), stigma receptivity in control (g) and heat-stressed (h), ovule viablity in control (i) and heat-stressed (j)]. Notice reduction in pollen load, pollen viability, in vitro pollen germination, stigma receptivity and ovule viabilty in heat-stressed plants of all the legumes (Kaushal et al., 2013; Kaur et al., 2015). Figures are being reproduced with the permission from the copyright holder.