Root Growth and Branching of Two Cycas Species Are Influenced by Form of Nitrogen Fertilizer

Horticultural research into the group of plants known as cycads has been deficient, and this includes the study of root growth and function. The form of nitrogen (N) available to plants is known to influence root growth and morphology. The response of cycad roots to N has not been studied to date. Cycas revoluta and Cycas edentata seedlings were grown in hydroponic culture and provided urea, nitrate, or ammonium forms of N. Solutions with all three forms of N increased root growth and branching when compared with nutrient solution devoid of N, with ammonium eliciting the greatest increases. Ammonium increased lateral root length 210% for C. revoluta and 164% for C. edentata. Ammonium decreased specific root length 38% for C. revoluta and 39% for C. edentata. The influence of the N source on stem and leaf growth was minimal. Ammonium increased the root-to-shoot ratio 15% for C. revoluta and 51% for C. edentata, but urea and nitrate did not influence this plant trait. A mixture of nitrate and ammonium generated plant responses that were no different from ammonium alone. The plants supplied with N in the solution produced coralloid root growth that was 14% of the no-N plants for C. revoluta and 22% of the no-N plants for C. edentata. This initial determination of the cycad plant response to the N form indicated that root plasticity was considerable and ammonium stimulated root growth more so than urea or nitrate. Long-term growth studies in mineral soils and nursery container medium are needed to determine if these findings from the hydroponic culture of small seedlings translate to general recommendations for the preferential use of ammonium for cycad culture.