Effect of plant density on biomass yield of Cynara cardunculus

Cynara cardunculus L. is a perennial herb that has been proposed as an energy crop in Mediterranean environments because of its adaptation to dry and hot summers. Plant density is an important factor for biomass production but it is related to water availability; therefore, in water-constrained conditions it should be optimized. In this work the effect of plant density on the production of C. cardunculus is studied at field level in the rain-fed conditions of central Spain. The experiment was conducted under Mediterranean-continental conditions, 430 mm annual rainfall and 3-month dry period, on average. Sowing was carried out beginning autumn by means of a seed driller with 0.80 cm between rows. After one-year establishment, two plant densities were compared in 1-ha plots: the standard plant density and a sparse plant density, designed as a double-row system; it was achieved by lifting two out of four crop rows. Treatments were evaluated for plant density, stalk density, number of heads, plant height, biomass production and dry matter content in summertime. Cynara plants tended to be higher (215 vs. 205 cm) and more productive (0.83 vs. 0.69 kg dm pl-1) in double-rows, despite intra-variability. However, this apparent higher growth did not compensate the reduced plant density and finally, the yield in biomass was lower than in the single-row treatment (8 vs. 13 t dm ha-1 harvestable biomass). Moreover, a higher proportion of cauline leaves was recorded, which affected the quality of the biomass as a solid biofuel since N content is usually higher in leaves than in stems. Therefore, evidence was provided that the double-row design does not result in yield improvement.