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Salinity Reduces Radiation Absorption and Use Efficiency in Soybean

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Salinity Reduces Radiation Absorption and Use Efficiency in Soybean Field Crops Research 69 (2001) 267±277 Salinity reduces radiation absorption and use ef®ciency in soybean D. Wanga,*, M.C. Shannonb, C.M. Grieveb aDepartment of Soil, Water, and Climate, University of Minnesota, St. Paul, MN 55108, USA bUSDA-ARS, George E. Brown, Jr., Salinity Laboratory, Riverside, CA 92507, USA Received 6 July 2000; received in revised form 1 October 2000; accepted 1 December 2000 Abstract The potential rate of plant development and biomass accumulation under conditions free of environmental stress depends on the amount of radiation absorption and the ef®ciency of utilizing the absorbed solar energy to drive photosynthetic processes that produce biomass materials. Salinity, as a form of soil and water stress, generally has a detrimental effect on plant growth, and crops such as soybean are usually sensitive to salinity. Field and greenhouse experiments were conducted to determine soybean growth characteristics and the relative impact of salinity on radiation absorptionP and radiation-use ef®ciency (RUE) at a whole plant level. Cumulative absorption of photosynthetically active radiation ( APAR) was estimated using hourly inputs of predicted canopy extinction coef®cients and measured leaf area indices (LAI) and global solarP radiation. On 110 days after planting, soybean plants grown under non-saline conditions in the ®eld accumulated 583 MJ APAR m2. A 20% P 1 reduction in APAR resulted from growing the plants in soil with a solution electrical conductivityP (EC) of about 10 dS m . Soybeans grown under non-saline conditions in the ®eld achieved a RUE of 1.89 g MJ1 APAR for above-ground biomass 1 P dry materials. The RUE reached only 1.08P g MJ APAR in the saline soil, about a 40% reduction from the non-saline control. Salinity also signi®cantly reduced APAR and RUE for soybeans in the greenhouse. The observed smaller plant and leaf sizes and darker green leaves under salinity stress were attributed to reductions in LAI and increases in unit leaf chlorophyll, respectively. Reductions in LAI exceeded small gains in leaf chlorophyll, which resulted in less total canopy chlorophyll perP unit ground area. Analyzing salinity effect on plant growth and biomass production using the relative importance of APAR and RUE is potentially useful because APAR and total canopy chlorophyll can be estimated with remote sensing techniques. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Salinity; Soybean; Radiation-use ef®ciency; Photosynthetically active radiation 1. Introduction tion with the amount of absorbed solar radiation (Monteith, 1977). The approach is fundamentally One of the most intriguing methods of forecasting sound because photosynthesis is the principal pathway the potential growth rate and yield of agricultural of plant carbohydrate assimilation, and radiation is the crops is to correlate plant growth and biomass produc- only source of expendable energy essential for photo- synthesis. For a crop species under non-stressed con- ditions, the rate of biomass accumulation can be * Corresponding author. Tel.: 1-612-625-5779; fax: 1-612-625-2208. expressed as the product of (1) the fraction of total E-mail address: [email protected] (D. Wang). photosynthetically active radiation (PAR) absorbed by 0378-4290/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0378-4290(00)00154-4 268 D. Wang et al. / Field Crops Research 69 (2001) 267±277 P the canopy and (2) the ef®ciency of utilizing the APAR analysis should provide an useful frame- absorbed PAR to drive photosynthesis for biomass work for understanding salinity effects on plant assimilation (Monteith, 1994). The amount of radia- growth. tion that may be absorbed by a plant canopy is strongly The overall goal of the study was to characterize the related to the vegetation cover or LAI, canopy struc- Prelative importance of salinity effects on soybean ture, and solar zenith angle. The ef®ciency of utilizing APAR and RUE at a whole plant level. More the absorbed PAR for biomass production, hereafter speci®cally, the study was conducted (1) to quantify termed radiation-use ef®ciency (RUE), can change canopy development,P leaf chlorophyll, and (2) to with variations in leaf chlorophyll content (Muchow determine APAR and RUE for soybeans grown and Davis, 1988), plant growth stage (Rochette et al., under either saline or non-saline conditions. For com- 1995), and ®eld management practices and environ- parison, ®eld and greenhouse experiments were con- mental stress levels. RUE increased with increasing ducted using similar salinity treatments and nitrogen fertilization in wheat (Green, 1987). Depend- measurement procedures. ing on the timing of stress, drought reduced either the amount of absorbed radiation or RUE in barley (Jamieson et al., 1995). RUE also varied with row 2. Materials and methods spacing in soybean (Board et al., 1994). High levels of salinity can signi®cantly reduce plant 2.1. Theoretical analysis development such as shoot and root growth for many plant species, including soybean (Shannon, 1994). For plants growing with suf®cient water and nutri- Growing soybeans in saline environments often leads ents, biomass dry matter (DM) production is propor- to excessive uptake and accumulation of salt ions such tional to the cumulative PAR (400±700 nm) absorbed as Na and Cl in plant tissues (LaÈuchli and Wieneke, by the plant canopy (Green, 1987). The relationship 1979). One explanation for the high tissue salt content between DM and RUE and PAR can be described with is that salt ¯ux to the shoot exceeds the rate of absolute a simple integral function ®rst proposed by Monteith shoot growth as shown in tomatoes (Dalton et al., (1977): Z 1997). Concentrations of the salt ions can reach toxic t1 levels that would cause leaf injury and impair basic DM ei afI bRs dt (1) functions of photosynthesis and regulation of bio- t0 2 1 chemical reactions and nutrient translocation (Green- where DM is in g m and is ei the RUE in g MJ . way and Munns, 1980). The osmotic effect of salinity The subscript i denotes experimental or salinity treat- stress is to reduce substrate water potential, similar to ments. Inside the integrand, parameters a and b are water stress, but induced by the high solute contents. canopy absorptivity for PAR and the ratio of PAR to 2 As indicated in Sionit and Kramer (1977), low levels global solar radiation (Rs,inWm ), respectively. of water stress can reduce soybean cell expansion and Parameter fI describes the fraction of radiation inter- cell wall synthesis, and high levels of water stress cepted by the plant canopy. All three parameters are would signi®cantly increase stomatal resistance and dimensionless. Variable t, including the integration reduce CO2 assimilation. The combined osmotic and limits (t0 and t1), represents time in second during the ion toxicity effect from salt stress often reduces growing season. canopy development in most plant species, and one Among a, b, and fI, parameter fI is the most dynamic would expectP a reduction in season total absorption of and changes with time, location, plant species, and the PAR or APAR. Salinity can also result in darker stage of growth. This parameter is easily estimated leaves such as in soybeans (Abel and MacKenzie, using Beer's law: 1964), suggesting the possibility of increased light f 1 eKLAI (2) capture that may compensate the canopy size reduc- I tion. The ion toxicity and osmotic effects from salt where K is the canopy extinction coef®cient, indica- stress may also have an impact on RUE, but no tive of canopy characteristics and light source, and literature values can be found. The RUE and LAI is the leaf area index. The extinction coef®cient D. Wang et al. / Field Crops Research 69 (2001) 267±277 269 for direct solar beam radiation (Kbe) can be calculated ECe values were normalized to a constant water con- from (Campbell and Norman, 1998): tent value of 0.15 cm3 cm3. This value corresponded p to the water content at 10 kPa, which was near ®eld x2 tan2c capacity. A weather station was installed at the ®eld Kbe (3) x 1:774 x 1:1820:733 site, and meteorological parameters such as incoming global solar radiation (Rs) were continuously recorded where x represents leaf angle distribution, a function during the experiment. of canopy structure (0.81 for soybeans under no To obtain plantP biophysical parameters for the salinity stress, Campbell and van Evert, 1994) and c is determination of APAR and RUE, nine soybean the solar zenith angle, which can be computed from plants were harvested from both the salinity and the latitude, longitude, and the time of measurement. control plots on DOY 201, 222, 243 and 264, respec- tively. To minimize dehydration and tissue break- 2.2. Field experiment down, the plants were stored in cooled ice chests immediately after harvest and transported to the A ®eld experiment was conducted between June laboratory where they were separated into leaf, stem, and October 1998 at the University of California root and pod (when present). Total leaf area of each Agriculture Experiment Station at Riverside, CA plant was measured by passing individual lea¯ets (3385802300N; 11782003000W; 258 m above sea level). through a LICOR LI-3100 leaf area meter (LI- The soil at the study site is an Arlington ®ne sandy COR, Lincoln, NE).1 LAI was calculated as the ratio loam (coarse-loamy, mixed, thermic, Haplic Durixer- of green leaf area divided by the total ground area each alf) with a particle size distribution consisting of 63% plant occupied. Leaf chlorophyll was determined from sand, 30% silt, and 7% clay. nine leaves found in the upper canopy of each plant To impose a salinity effect, the soil in the salinity using a SPAD-502 meter (Spectrum Technologies, treatment plot 12 m  76 m was salinized with a Plain®eld, IL).2 Calibration between the SPAD read- NaCl and CaCl2 mixture (at 1:1 weight ratio and ing and leaf chlorophyll was made from leaf extracts 1.8 Mg ha1 rate) using a sprinkler system before using a Beckman DU 7500 spectrophotometer (Beck- planting.
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