Open field experiment for the evaluation of Arundo donax ecotypes ecophysiology and yield as affected by soil water content

The high photosynthetic efficiency and the drought tolerance of Arundo donax L. contribute to its potential use as a suitable biomass crop for drought prone environments. A modelling approach to study net assimilation and stomatal conductance as influenced by available soil water content (ASWC) can be used to obtain threshold values to support the management of irrigation practice. Physiological model parameters, morphological (stem height and max diameter, number of leaves per stem), and productive traits (yield and biomass ash content), can support ecotypes selection activity, crucial for genetic improvement in a crop, such as A. donax, widely distributed in natural habitats, but characterized by no viable seeds. To this end, an open field experiment has been conducted comparing three A. donax L. clones collected from habitats representing a gradient of increasing temperature and declining water availability (Morocco; South Italy, and North Italy). Two irrigation treatments (rain-fed, and well-watered) and two different harvest dates (December and March) were also imposed as experimental factors. Physiological model involved light response curve approach measuring net assimilation and stomatal conductance under different light intensity and different actual ASWC. The obtained physiological curve parameters in relation to ASWC (maximum net assimilation - AN-sat, photosynthetic quantum efficiency - Qapp, and maximum stomatal conductance - Gs-sat) were well fitted by a two-segmented piecewise regressions. The segment breakpoints were computed at 43%, 39% and 65% of ASWC, for AN-sat, Qapp, and Gs-sat, respectively. This means that between 65% and 43% of ASWC, only stomatal conductance appeared to be influenced by the decrease of ASWC, reducing stomata opening, whereas the efficiency of net assimilation resulted unaffected by water depletion. The capacity to sustain high net assimilation rates untill low ASCW and the capacity to full recover net photosynthesis after a severe drought stress, emerged as crop traits contributing to explain the drought resistant aptitude of A. donax. No differences have been reported for any of the studied physiological parameters within ecotypes. On the contrary, the three clones exhibited differences in terms of biometric characters, biomass production and its ash content. The results of the study emphasize the multipurpose value of Moroccan ecotype reporting best performances in terms of both quantity and quality (highest biomass yield - 26.4 t ha−1 - lowest ash content 4.8%).