Based on customer demands we are currently developing novel, cost-efficient cooling racks for rhizobox / small rhizotron studies. While roots are normally growing in soil at a lower temperature than the air aboveground, most studies place root boxes in climatized growth rooms where the root / rhizosphere compartment has the same temperature as the shoots - for technical reasons. This has inevitable consequences for growth and eco-physiological functioning of the root system, its symbionts and the soil microbiome. We are thus currently constructing novel, cost-efficient rhizobox cooling racks (see figure for a sketch) which will allow to adjust the temperature around rhizoboxes independently from the growth room (air) temperature, using standard lab cooling devises (i.e. circulating (cold) water in cooling coils) available in most research settings. In addition, the boxes will have a lid system to shade the transparent front plate and thus conveniently allow to grow the roots in darkness without the need of installing (and removing for imaging) additional shading cloths (or commonly "aluminum foil") - allowing for time efficient imaging. Stay tuned!
The effect of increased temperatures on aerial parts of the plants and their responses has been well studied, whereas their influence and response on roots (and root-to-shoot signaling) has been less explored. However, it is clear that root growth and physiology are severely affected by temperature - which is commonly higher in the rooting zone of pots and rhizoboxes as oposed to ex situ studies. For example, studies with several crops have shown different response of aquaporins and plasmatic membrane fluidity to higher temperatures in roots (Calleja-Cabrera et al., 2020, and references within). Similar, nutrient balance can also be altered by changes in temperature. Temperature effect on nutrient uptake varies, however, strongly depending on the crop. In tomato, for example, warmer soils limit root growth and decrease nutrient uptake causing a reduction in macro and micro-nutrient levels (Tindall et al., 1990; Giri et al., 2017). In the grass Agrostis stolonifera, high soil temperatures results in a lower number of roots and an increase in the uptake and partitioning of nitrogen, phosphorous and potassium (Huang and Xu, 2000). In Andropogon gerardii supra-optimal root temperatures cause a decrease in root and shoot growth. Further, higher temperatures moderately affects nitrogen uptake but its efficiency use is severely perturbed (DeLucia et al., 1992). In contrast, warm root-zone temperatures seem not to alter N, P and K uptake in maize to a large extend (Hussain et al., 2019). Several plant hormones that take part in root development and growth have been described to mediate temperature stress response. In particular, a role of BRs, salycilic acid (SA), ethylene (ET), abscisic acid (ABA) and cytokinin (CK) has been reported in several crops (Calleja-Cabrera et al., 2020, and references within). There is fewer and fragmentary data concerning secondary metabolts effects to rising temperatures in roots. In maize, for example, increase in temperature causes a decrease in the level of secondary metabolism compounds such as fitosterols and terpenoids (Sun et al., 2016). It goes without mentioning, that "unnormal" rooting zone temperatures can have also severe consequences on root development, morphology and overall root system architectures. Roots need an optimal temperature range to have a proper growth rate and function. In general, this optimal root temperature tends to be lower than optimal shoot temperature, however, roots have different optimal root temperature depending on the species. Within this range, a higher temperature is usually associated to altered root:shoot ratio, and a further increase in temperature would limit root development and alter root system architecture (RSA) reducing root:shoot ratio (Ribeiro et al., 2014; Koevoets et al., 2016). RSA is defined as the organization of the primary, lateral, and shoot-born roots. Each RSA is determined by parameters such as length, number and angle of these root types. RSA is the main factor that controls nutrient and water uptake efficiency since it determines the soil volume that roots are able to explore at different environmental situations (Lynch, 1995). Generally, the exposure of roots to temperatures higher than optimal causes a decrease in the PR length, number of lateral roots and their angle of emergence. Moreover, the increase in temperature may cause the initiation of second and third order lateral roots that are characterized by a larger diameter (see Figure for a graphical overview).
Overall it is thus clear that greater efforts are needed to control the temperature of soil in rhizobox experiments independently of the air temperature set in growth chambers or greenhouses. We expect the novel, cost-efficient VSI cooling racks to contribute to this efforts - helping to align the results of rhizobox studies better to ex situ studies coined by more realistic differences between soil (root) and air (shoot) temperatures. Keep you roots cool!