Biochar increases plant water use efficiency and biomass production while reducing Cu concentration in Brassica juncea L. in a Cu-contaminated soil.

Biochar has been recently used as an alternative strategy to improve soil quality and plant growth in metal contaminated soils. However, the effects of biochar on gas exchange parameters such photosynthetic rate (A), water use efficiency (WUE) and instantaneous carboxylation efficiency (ICE) in metal tolerant plant species in contaminated soils is still unknown. Such information is important to understand how different biochar types can influence plant biomass production and metal uptake. Hence, a greenhouse experiment was set up as a completely randomized design combining two types of crop residue biochar (coconut husk (CB) and orange shell (OB) and two rates of application (30 t ha-1 and 60 t ha-1)). A control treatment (no biochar) was also included. The aim of the study was to investigate the influence of biochar on the physiological performance, growth and concentration of copper (Cu) in the shoot of B. juncea plants in a Cu-contaminated soil. Besides reduced growth on the control soil, all other treatments increased plant growth. No toxicity symptom was observed in the plant, confirming its Cu tolerance. Biochar increased plant biomass by approximately 170% and reduced Cu concentration up to 51%. Application of 30 t ha-1 biochar significantly increased net photosynthesis in 59% (CB) and in 34% (OB) while reducing stomatal conductance in 40% and transpiration rate in 14% (CB) and 19% (OB). Application of 60 t ha-1 of biochar reduced stomatal conductance and transpiration rate in 51% and 60% (CB) and 26% and 7% (OB), respectively. All biochars improved the water use efficiency (WUE) in the plants (52-182%) for the instantaneous WUE and 80-162% for the intrinsic (IWUE). Therefore, biochar, especially CB, caused an improved regulation of the stomata aperture so that plant could maintain a high photosynthetic rate while efficiently controlling the use of water. This could be an important mechanism to reduce the excessive uptake of Cu by the plant.