Expression of triploid heterosis in the biomass productivity of energy willow plants under salinity stress

Bioenergy production from short rotation woody species grown on saline soil provides multiple environmental advantages. Breeding of energy willow for salinity tolerance can support green energy utilization. Here, we quantify triploid heterosis in biomass traits under salt stresses. Crosses between diploid cultivars (Tora and Inger) and our autotetraploid plants (PPE-2/6 and PPE-7) resulted in TH16/24 and TH21/2 triploid hybrid genotypes. Growth characteristics of shoots and roots of triploid hybrids and their parental plants were monitored by pixel-based phenotyping under high salt stress (NaCl 2.0 g kg-1/EC: 8.71 mS cm-1) in greenhouse. Different values of Mid-Parent Heterosis (MPH %) were recorded by comparing these two crossing combinations and top or above views (for TH16/24 plants: 3.66% (side pixels) and 22.25% (above pixels) and for TH21/2 plants: 63.30% (side pixels) and 72.10% (above pixels). At the end of the growing period, considerable MPH values were detected in biomass parameters of TH16/24 and TH21/2 plants (shoot length: 18.26%/26.28%; green biomass weight: 40.28%/52.40%; stem diameter: 15.40%/13.98%) in soil with 2.0 g NaCl kg-1. The TH21/2 hybrid plants expressed higher hybrid vigor than plants from the TH16/24 genotype. The TH21/2 hybrid plants exhibited better water use efficiency than their parental plants and showed high MPH values for K+/Na+ ratio in soil containing 2.0 g NaCl kg-1. The presented data support a conclusion that heterosis depending on crossing partners can ensure improvement in salinity tolerance for triploid hybrids. Field experiment has been initiated for the characterization of these hybrids under natural saline soil.