The bzh root system reacts highly economically when N is scarce

In oilseed rape (Brassica napus) bzh dwarf plants have improved nitrogen (N) uptake, N-utilization and N-use efficiency compared to normal types. Is this due to their shoots, or do roots play a role?

bzh semi-dwarf hybrids have a reduced root system at a very early stage of plant development, according to a new paper in Annals of Botany. The plants have the adaptability to be more efficient in nitrogen use, as in N-poor conditions the semi-dwarf plant reduces the shoot growth to improve the root:shoot ratio, whereas a normal plant would increase root growth. “We suggest that bzh semi-dwarfs were not only more N efficient than normal types when N is scarce due to a higher harvest index but also due to a higher root: shoot ratio,” write Antke Schierholt and colleagues in their paper. “The higher root biomass (root EC) of normal-types is probably needed under low N to maintain biomass and yield, whereas the semi-dwarf hybrids have less biomass to be maintained.”

Root growth in vitro experiment. Five seedlings per genotype were grown in 12 × 12-cm Petri dishes under controlled conditions. Source: Schierholt et al. 2019.

Ironically, dwarfing genes are used in many species where farmers want to increase yield. This is done to reduce lodging, where the stem of the plant bends over, making it more difficult to harvest. While the effects of dwarfing are easy to see on the shoots, it is more difficult to see the effects elsewhere. “As roots grow hidden in the soil, they are very difficult to characterize in the field. Root biomass and root distribution in the soil profile can be estimated destructively or non-destructively in various artificial environments ranging from in vitro gel plates to rhizotrons, as reviewed by Fiorani and Schurr (2013). However, the more precise techniques are very laborious and expensive and therefore are not applied in plant breeding, where non-destructive methods for high numbers of genotypes in the field are required, “said Schierholt and colleagues in their paper. 

Root electrical capacitance measurements in the field. The plant electrode was attached with a clamp about 2 cm above ground at the hypocotyl of the rapeseed plant and the soil electrode was pushed approx. 40 cm into the ground. Source: Schierholt et al. 2019.

To get around this problem, the team turned to root electrical capacitance. This approach looks at roots as leaky capacitors, so examining electric flow through soil can reveal how plant biomass varies below ground without having to dig up the plant. This technique showed the surprisingly low root biomass of the semi-dwarf hybrids.

“We conclude that the opposite root growth reaction of the bzh semi-dwarf and the normal type under N deficiency contributes to the higher N efficiency of semi-dwarf genotypes due to better economic use of N and assimilates. However, the effect of the bzh gene on the root system is complex and could be explained by a greater fine root system, altered lateral roots, changes in the tap root biomass or other shifts in root architecture,” said Schierholt and colleagues in their paper, though they caution their findings have limitations.

“The initial question of whether root EC may explain the higher N efficiency of bzh semi-dwarfs cannot be finally answered based on the available data. Consequently, an analysis of the root architecture of semi-dwarf and normal-type hybrids, as well as an analysis of the molecular and physiological plant reactions, would be needed to fully explain differences in N efficiency between the growth types.”