Nitrate Use-efficiency: a Morphological Analysis of the Above- and Below-ground Functional Traits in Two Citrus Rootstocks

An increased knowledge of the potential morpho-physiological mechanisms of the nitrate use efficiency (NUE) of the crops is fundamental for reducing the excessive input of the N-fertilizers maintaining an acceptable plant yield and environmental health. The aim of this work was to identify above- and belowground morphological traits that improve nitrate acquisition efficiency in two citrus rootstocks, Sour Orange (Citrus aurantium L.) (SoO) and Sweet Orange (Citrus sinensis L.) (SwO), characterized as nitrate-use-efficient and –inefficient. Root length, tip numbers, leaf area and stem height, as functional traits, root mass ratio, leaf mass ratio and stem mass ratio, as allocation parameters, and specific root length, leaf mass per area and specific stem ratio, as cost parameters, were evaluated. Further, the functional, allocation and cost parameters of different root orders were analyzed. The results of this work revealed that, at low nitrate availability, SoO seedlings exhibited an above- and below-ground traits more competitive for the acquisition of this anion: wider leaf area, higher total length and tip numbers of the root system accompanied by higher total length of the 2nd order respect to the SwO. Further, the “competitive structure” of the SoO was reached by an increased biomass allocation and its better use efficiency towards the above- and below-ground parts of the seedlings. In conclusion, at the low nitrate level, the morphological changes of the below- and above-ground traits of the SoO could represent the mechanisms which define SoO as nitrate-use-efficient enhancing its capacity to absorb nitrate from soil environment.