3.3. Root system architecture
The root system is exposed to a heterogenous matrix of soil in a range of environmental conditions, including a soil temperature gradient (Lundholm, 2009; Onwuka, 2016; Ulrich et al., 2014). The responses to supra-optimum temperature include a decrease in primary root length, lateral root density and changes in angle under which roots emerge (McMichael & Quisenberry, 1993; Nagel et al., 2009; Seiler, 1998). These changes in the root system architecture impact on nutrient uptake, but also affect response effectiveness to other stresses, such as drought (Hendrick & Pregitzer, 1996). Different species show distinct optimum temperatures for root system architecture responses (Gray & Brady, 2016; Luo, Xu, Chu, He, & Fang, 2020; Walter, Silk, & Schurr, 2009) (Table 1 ). In contrast to aboveground optimum temperature responses, the root system usually shows a narrower range for suitable growth (Table 1 ). Garden pea displays a decreasing trend in primary root growth rate and lateral root development is inhibited as the temperature shifts from cold (15°C) to warm (32°C) temperature (Gladish & Rost, 1993). However, garden pea seedlings exposed to 32°C for 17 days that showed inhibition of primary root elongation, are capable of restoring root growth to a normal state when transferred to 25°C (Gladish & Rost, 1993). Similarly, soybean shows a considerable decrease in parameters related to general root growth (e.g. root surface area, cumulative root length and root volume) when submitted to a heat-temperature regime (40℃/32℃) compared to the control (30℃/22℃) (Alsajri et al., 2019). In contrast, root secondary developmental parameters are enriched in the high temperature regime (e.g. number of root tips and root forks) (Alsajri et al., 2019). With respect to cereal crops, twenty one-day old wheat plants submitted to 36℃/28℃ (day/night regime) show a significant reduction in several root parameters when compared to the control at 25℃/20℃, such as a decrease in root biomass, shoot-to-root ratio, primary root length, root surface area and root volume (Rehman, Farooq, Asif, & Ozturk, 2019).
4. reproductive growth and development upon ambient temperature changes
Reproductive tissues and organs are developmentally more responsive to increased ambient temperature than vegetative parts (Boden, Kavanová, Finnegan, & Wigge, 2013), and some of the underlying molecular mechanisms are known.