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Growth Dynamics and Spatial Variation of the Eggplant Root System Under Elevated Co2 and Soil Moisture Stress

Bikash C. Sarker
Department of Agricultural Chemistry, Hajee Mohammad Danesh Science and Technology University, Dinajpur- 5200, Bangladesh

Michihiro Hara
Faculty of Agriculture, Iwate University, Morioka 020-8550, Japan

Abstract/ Executive Summary:

A complementary root growth under the interactive effect of elevated CO₂ and water stress has been hypothesized and increased root biomass and surface area for economical use of water and mineral, resulted in a better drought survival morphological characteristic. Eggplants were grown under two atmospheric CO2 concentrations, ambient 365~370 ppm for control and 700 ppm for elevated CO₂ concentration, using environment control growth chambers. In addition to the two levels of atmospheric CO₂, two levels of conditioned water treatments; frequently irrigated control and 21-d lag of irrigation for water stress was maintained for a 63-d period which consisted of three irrigation-drying cycles. The objectives of this study were to quantify the interactive effects of rising CO₂ and water stress on root morphological growth in special reference to root biomass and surface area. The temporal trend in root growth and spatial distribution at different growth profiles were investigated at three different growth stages. Eggplants showed positive regulation with an increased root growth under elevated CO₂ at any developmental stages. Eggplants subjected to periodically subsequent water stress had a complementary root growth (root biomass and root surface area) under elevated CO₂ environment. Roots of water stressed eggplants grown with 700 ppm CO₂ had a greater dry biomass and surface area than roots of well-watered eggplants grown with ambient CO₂ environment. The relative increase in percentage (RE) due to elevated CO₂ was transient and varied during any plant developmental stage. A maximum of 82% RE in root dry biomass (RERDW) in well-watered eggplants and also additional 130% RERDW in the eggplants at 21-d lag water supply were due only to elevated CO₂ over ambient CO₂. The strongest stimulation in root growth occurred in water stressed eggplants under elevated CO₂. Elevated CO₂ markedly stimulated the root biomass and exposed more root surface area at all depths of soil profiles but the greatest stimulation occurred at the top soil layer (0-20 cm), during all growth stages. Eggplants subjected to periodically subsequent water stress had greater stimulation at deeper bottom soil profiles during the reproductive stage. The results suggested that the elevated CO₂ (700 ppm) can compensate for restriction in root growth by water stress in improving morphological feature, and can increase net input organic C in soil.

Keywords: Acclimation, Eggplant, Elevated CO2, Root growth, Root surface area, Water stress.

Location: Laboratory of Agro-Environmental Sciences in the Iwate University, Morioka (North-eastern Japan)

Start Date: 13-05-2004

End Date: 03-08-2004

Program Area: Variety and Species

Commodity: Brinjal

Objectives:

1. To determine to what extent root morphology and net C input in soil are altered by changing of two important environmental factors, elevated atmospheric CO₂ and soil moisture stress, in agricultural crop production.

Methodology:

Pot experiments were conducted in the environment controlled growth chamber at the Laboratory of Agro-Environmental Sciences in the Iwate University, Morioka (North-eastern Japan) using ambient CO2 environment (365-370 ppm) and using elevated CO₂ environment (700 ppm). The eggplant (Solanum melongena L. cv. Senryo No. 2) was used as plant material in the study. Each of selected seedlings was 18 to 20 cm in height and was transplanted one in each pot on May 12 in 2004; documented as 0 days after transplant (DAT). The pot used for this study was 90 cm in height and 15 cm in inner diameter in which volcanic ash soil was used up to 0.80 m height for the eggplants growing purposes. The soil was previously well incorporated with mixed granular fertilizer of 1:1:1 for N, P, and K @ 50 g/20 L soil as maintenance dose along with 10 g lime/20 L soil. The experiments lasted for 63 days after transplanting (DAT) in the pot. The experiment was conducted in the environment controlled growth chamber for crop culture. Environment controlled growth chambers were used separately for the ambient and elevated CO₂ condition. The chamber was controlled at 25⁰C from 8.00 AM to 18.00 PM and at 20⁰C from 20.00 PM to 6.00 AM, and artificial lighting of 40 klux was provided by lamps with a photoperiod of 12 h. The relative humidity was controlled at 70% all day long and the wind speed fluctuated between 0.4 to 0.8 m s^-1. Ambient CO₂ concentration containing air was supplied in one growth chamber while 700 ppm CO₂ concentration was maintained in another chamber during study period by supplying CO₂ gas with a regulator from cylinder. A completely randomized design with three replications was followed for the study purposes. The experiment was started on May 13, 2004 as the day 1st of experiment and was continued for 63 days period. Thirty six pots (18 for ambient and 18 for elevated CO₂ condition) with test plants were used for plant parameter (like root weight, surface area, length and volume) measurement. Three replicated plants from each treatment were harvested at 21 DAT and 42 DAT and 63 DAT, respectively. Note is that four pots (two from ambient and two from elevated CO₂ condition) of the last twelve were equipped with soil moisture measuring sensors and used for soil parameter measurements. In the present study, the irrigation schedule was sequenced as T₁ (control, well-watered)- irrigating the eggplant pots once at every two to three days interval and T2 (water stress)- irrigating the eggplant pots once every 21 day interval. The Amb_T₁ and Ele_T₁ were used for well-water condition under ambient and elevated CO2 concentrations, while Amb_T₂ and Ele_T₂ were used for water stress condition under ambient and elevated CO₂ concentrations. Note that the pot was irrigated to raise its soil moisture status up to pot capacity level by just replenishing the total amount of water lost by ET after each wetting and drying cycles. The soil moisture content at pot capacity level was tested in this study on the basis of watering the pot at saturation level and then the moisture was allowed for depletion to the pot soil up to its water holding capacity level against gravitational force. All the holes of the pot were completely sealed with silicon grease before starting the experiment. Growth stage of eggplants for this study was assigned by 0-21 DAT as vegetative stage;1st irrigation-drying cycle, 21-42 DAT as early reproductive stage; 2nd irrigation-drying cycle and 42-63 DAT as peak reproductive stage; 3rd irrigation-drying cycle, respectively. Volumetric water content: The four pots with test plants applying four different treatments were used for soil volumetric water content (VWC) measurement with Time Domain Reflectrometry (TDR) sensor (Model: Campbell Scientific Inc., USA). The measurement was monitored in each pot during whole growing period at every 30 minutes interval at three different layers (upper 0-30 cm, mid 20-50 cm and bottom 50-80 cm pot soil depth to monitor VWC during growth time. The data was collected instantly at 7-d interval for this study through data logger CR10X (Model: Campbell Scientific Inc., USA). A statistical analysis following completely randomized design (CRD) with three replications was performed. The three factorial (Time, Water and CO₂) analysis of variance (ANOVA) test was done separately for the total root dry biomass and surface area. This analysis was carried out using statistical software MSTAT-C computer program to compare the treatments effect and differences between two CO2 levels and water treatments for root dry biomass and root surface area collected at different growth stages in the study. When (Time×CO₂×Water) interaction and (CO₂×Water) interaction were insignificant, the individual effect of elevated CO₂ and soil water was analyzed and used for interpretation.

Source of Information: J. Agrofor. Environ. 4 (2): 7-14, 2010 ;ISSN 1995-6983

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Achievement/Findings:

In conclusion, elevated CO₂ environment increased root growth in relation to biomass and surface area expansion in soil leading to acclimation, with increased root growth at top soil layer; water stress increased root biomass and surface area at deeper bottom soil layers. Indeed, the root dry biomass is as larger in water stressed eggplants under elevated CO₂(700 ppm) as in well-watered eggplants in ambient CO₂ environment. Stimulated RDW and RSA at both soil water conditions under elevated CO₂ concentration resulted in increased net organic C sequestration in soil and may enhance more soil water and nutrient acquisition. A complementary root growth in water stressed eggplants under elevated CO₂ indicated better survival ability and might be beneficial during future global environment change.

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