Nutrient deficiency effects on root architecture and root-to-shoot ratio in arable crops

Abstract

Plant root traits play a crucial role in resource acquisition and crop performance when soil nutrient availability is low. However, the respective trait responses are complex, particularly at the field scale, and poorly understood due to difficulties in root phenotyping monitoring, inaccurate sampling, and environmental conditions. Here, we conducted a systematic review and meta-analysis of 50 field studies to identify the effects of nitrogen (N), phosphorous (P), or potassium (K) deficiencies on the root systems of common crops. Root length and biomass were generally reduced, while root length per shoot biomass was enhanced under N and P deficiency. Root length decreased by 9% under N deficiency and by 14% under P deficiency, while root biomass was reduced by 7% in N-deficient and by 25% in P-deficient soils. Root length per shoot biomass increased by 33% in N deficient and 51% in P deficient soils. The root-to-shoot ratio was often enhanced (44%) under N-poor conditions, but no consistent response of the root-to-shoot ratio to P-deficiency was found. Only a few K-deficiency studies suited our approach and, in those cases, no differences in morphological traits were reported. We encountered the following drawbacks when performing this analysis: limited number of root traits investigated at field scale, differences in the timing and severity of nutrient deficiencies, missing data (e.g., soil nutrient status and time of stress), and the impact of other conditions in the field. Nevertheless, our analysis indicates that, in general, nutrient deficiencies increased the root-length-to-shoot-biomass ratios of crops, with impacts decreasing in the order deficient P > deficient N > deficient K. Our review resolved inconsistencies that were often found in the individual field experiments, and led to a better understanding of the physiological mechanisms underlying root plasticity in fields with low nutrient availability.

Keywords: fertilizer; nitrogen; nutrient limitation; phosphorous; potassium; root morphology; root plasticity.

Figure 1

The relative change of the root traits under deficiency [(X0-X1)/X1] where X0 is the value in the treatment without any addition of the nutrient and X1 is the value of the treatment with the nutrient application. np stands for the number of publications/studies considered in the calculation, and nr for the total number of observations within these publications. The line within the boxes refers to the median. ** stands for significant differences at a 0.95 confidence level. Blue dots represent the mean.

Figure 2

Boxplot of the normalized root data under N deficiency and N non-deficiency. A t-test was performed; * stands for significant differences at a 0.9 confidence level and ** at a 0.95 confidence level. np stands for the number of publications/studies considered in the calculation, and nr is the total number of observations within these publications (np).

Figure 3

Boxplot of the normalized root data under P deficiency and P non-deficiency. A t-test was performed; ** stands for significant differences at a 0.95 confidence level. np stands for the number of publications/studies considered in the calculation, and nr is the total number of observations within these publications (np).

Figure 4

Boxplot of the normalized root data under K deficiency and K non-deficiency. A t-test was performed; no significant differences were found. np stands for the number of publications/studies considered in the calculation and nr the total number of observations within these publications (np).

Figure 5

Effects of N, P, and K deficiencies at field scale. The red arrows show a decrease, the blue arrows show an increase and the yellow arrows show similarity in that trait in case of deficiency of the respective nutrient. Lighter-colored arrows stand for few studies found investigating that specific parameter (2-4 studies), and na stands for not applicable.