Salinity reduces radiation absorption and use efficiency in soybean

The potential rate of plant development and biomass accumulation under cond itions free of environmental stress depends on the amount of radiation abso rption and the efficiency of utilizing the absorbed solar energy to drive p hotosynthetic processes that produce biomass materials. Salinity, as a form of soil and water stress, generally has a detrimental effect on plant grow th, and crops such as soybean are usually sensitive to salinity. Field and greenhouse experiments were conducted to determine soybean growth character istics and the relative impact of salinity on radiation absorption and radi ation-use efficiency (RUE) at a whole plant level. Cumulative absorption of photosynthetically active radiation (Sigma APAR) was estimated using hourl y inputs of predicted canopy extinction coefficients and measured leaf area indices (LAI) and global solar radiation. On 110 days after planting, soyb ean plants grown under non-saline conditions in the field accumulated 583 M J Sigma APAR m(-2). A 20% reduction in Sigma APAR resulted from growing the plants in soil with a solution electrical conductivity (EC) of about 10 dS m(-1) Soybeans grown under non-saline conditions in the field achieved a R UE of 1.89 g MJ(-1) Sigma APAR for above-ground biomass dry materials. The RUE reached only 1.08 g MJ-L Sigma APAR in the saline soil, about a 40% red uction from the non-saline control. Salinity also significantly reduced Sig ma APAR and RUE for soybeans in the greenhouse. The observed smaller plant and leaf sizes and darker green leaves under salinity stress were attribute d to reductions in LAI and increases in unit leaf chlorophyll, respectively . Reductions in LAI exceeded small gains in leaf chlorophyll, which resulte d in less total canopy chlorophyll per unit ground area. Analyzing salinity effect on plant growth and biomass production using the relative importanc e of Sigma APAR and RUE is potentially useful because APAR and total canopy chlorophyll can be estimated with remote sensing techniques. (C) 2001 Else vier Science B.V. All rights reserved.