2024..01.01.51
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Root-shoot ratio and its relationships with physiological characteristics, growth and biomass yield of Gynura procumbens under different shade levels and plant density
Omar Ali Ahmed1,2, Martini Mohammad Yusoff1*, Azizah Misran1, Puteri Edaroyati Megat Wahab1and Qusay Abdualhamza Muttaleb1,3*
1Department of Crop Science, Faculty of Agriculture, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia.
2Department of Crop Science, Faculty of Agriculture, University of Diyala, Iraq. ORCID
3Technical Institute of Babylon, Al-Furat Al-Awsat Technical University (ATU), Iraq. ORCID
* Correspondence: [email protected]
Available from. http://dx.doi.org/10.21931/BJ/2024.01.01.52
ABSTRACT
Gynura procumbens is one of the most common medicinal plants in the Asteraceae family and has extensive pharmacological properties. The experiment was conducted to evaluate the effects of different shade levels (0 and 30% shade) and plant density (9, 15, and 25 plants m-2) on root-shoot ratio and it is a relationship with physiology, growth, and biomass yield using split-plot design with three replications. Increasing shade level to 30% shade significantly decreased root-shoot ratio (RSR) by22.54%, while total leaf dry weight per plant (TLDW) and total leaf dry weight per square meter (TLDW m-2) increased by 35.64, 11.58, and 32.18%, respectively due to negative correlation with RSR. Increasing plant density from 9 to 25 plants m-2 significantly increased RSR and TLDW m-2 by 67.71 and 18.54%, respectively, while TLDW decreased by 57.31%. There was a negative correlation between RSR and biomass yield per plant. Under stressed conditions (full sunlight and high plant density), G. procumbent plants appeared to change their strategy to absorb limited resources, allocate more biomass to the root system, and reduce the size of the aboveground parts to survive, resulting in high RSR.
Keywords: Gynura procumbens, shade, plant density, root-shoot ratio, physiology, growth, biomass.
INTRODUCTION
The Gynura procumbens is an evergreen medicinal shrub belonging to the Asteraceae family. It is widely distributed in Africa 1,2 and tropical regions of Southeast Asia and China 3,4. Non-toxic leaves have extensive pharmacological properties, including anti-diabetic, anti-hypertensive, antioxidant, and vasorelaxant activity 5. The medicinal benefits of G. procumbens are related to its bioactive compounds, such as saponins, flavonoids, and terpenoids4,6.
The root-shoot ratio (RSR) is the ratio of the belowground biomass to the biomass of the aerial part reflected in plant biomass allocation. The differences in biomass allocation between the shoot and root systems would result in variations in the RSR ratio. The decrease in biomass under biotic and abiotic stress conditions is associated with the availability of resources above and belowground, thereby resulting in trade-offs in biomass allocation between aboveground and belowground biomasses, suggesting that plants allocate biomass to acquire the most limiting resources 7,8,9,10. Many plants would change their RSR to respond to shading and low nutrient availability 11,12. Therefore, RSR is vital in studying competitive interactions between growth conditions 13.
Biomass allocation between above and belowground parts results from long-term environmental modification, significantly affecting plant growth and reproduction 14,15,16. The root biomass was less affected by biomass allocation under stress conditions than shoot biomass, reflected in the plant RSR ratio 17. The differences in RSR due to the differences in above and belowground biomass indicate that different biomass exhibits various biomass allocation strategies. RSR for grass (4.23) was significantly higher than that of shrubs (1.68) and trees (0.40), and evergreen plants had higher RSR than deciduous plants in tropical rainforests 17.
Generally, the response of plants to light intensity depends on the type of plant, resulting in different concentrations of plant secondary metabolites 18. Consequently, the medicinal properties of the target plants are affected, which also brings about changes in the morphology and physiology of the plants in question 19,20. Changes in microclimate and light intensity due to shading of the canopy resulting from different plant densities also affected above and belowground biomass 21.
Plant density or plant canopy is also a critical agronomic practice for effectively capturing environmental resources such as solar radiation, water, and nutrients 22, which can enhance optimum plant population 23 and influence the physiological and phytochemical characteristics of the plants. An increasing number of plants per unit area led to increased interplant competition for resources, resulting in the depletion of some limited resources. Plants allocated more biomass to roots to absorb more water and nutrients for survival and acclimatization under high competition or stress conditions 24,13. Moreover, the competition for resources led to a change in biomass allocation in the plant, which resulted in a variation in RSR 25,26.
Therefore, understanding the effects of different environmental conditions on the above- and below ground biomass of G. procumbent plants through RSR helps improve the quality and quantity of medicinal plants to meet the high demand. Therefore, the present study was conducted to determine the RSR and its relationship to growth, physiological attributes, and biomass yield under different shade levels and plant densities of G. procumbens.
MATERIALS AND METHODS
Experimental design and treatments
The experiment was conducted in November 2017 in a split-plot design (main plots presented by shade levels; subplots presented by plant densities) with three replicates using wooden planting boxes (2 m × 1 m × 0.5 m). The two levels of shade used were 0 and 30% using custom-made polyethylene shade netting with three plant densities (9, 15 and 25 plants m-2) using 9 plants m-2 = 33.3 × 33.3 cm, 15 plants m-2 = 20 × 33.3 cm and 25 plants m-2 = 20 × 20 cm, respectively were implemented.
Root: Shoot Ratio (RSR)
Root: shoot ratio (belowground biomass to aboveground biomass) for each treatment was determined according to the following formula:
RSR = Total root dry weight / Total shoot dry weight
Net Photosynthesis Rate and Total Chlorophyll Content
A portable photosynthesis system (LICOR-64001 LI-COR Inc., USA) measured the net photosynthesis rate between 0900 and 1100 hours.
Total leaf chlorophyl1 content was also measured using a modified method of Lichtentaler and Wellburn (1983). Leaf weighing 0.2 g were collected from plant samples and stored in small plastic vials containing 20 mL of 80% (v/v) acetone. Absorbance was measured using a scanning spectrometer Model UV 3101 PC, and the total chlorophyll was calculated as the sum of chlorophyll a and chlorophyll b using the following.
Chlorophyll a = 11.75 (Absorbance 662) − 2.350 (Absorbance 645)
Chlorophyll b = 18.61 (Absorbance 645) − 3.960 (Absorbance 662)
Leaf area and Leaf area index
A leaf area meter (LI-3100 Area Meter, USA) measured the total leaf area of all harvested leaves (TLA, cm2). The leaf area index (LAI) for each sample was computed after measuring the TLA and area of each plant using the following formula:
LAI = TLA (cm2) / Soil area (cm2)
Biomass dry weight
The plants were harvested to measure the biomass yield of leaves per plant and square meter. Three plants per treatment were harvested. Fresh samples were oven-dried at 45 °C until a constant weight of dry samples was achieved. An electronic balance (BP 2100, Sartorius, Germany) was used to determine the total dry leaf weight per plant and the dry leaf weight per square meter.
Crop growth rate (CGR)
It was measured using the formula, CGR = (W2 – W1) / (t2 – t1), where W1= total dry weight of plant at time 1 (t1) and W2= total dry weight of plant at time 2 (t2).
Statistical data analysis
Data collected were subjected to statistical analysis of variance (ANOVA) for split-plot design using the SAS program (SAS version 9.4, Car, NC) to determine the statistical significance of main and interaction effects. Significant main effects means were separated using Least Significant Differences (LSD) (P≤0.05). Only the main effect comparisons were performed if an interaction between shade levels and plant density was insignificant. The relationship between parameters was determined using correlation analysis.
RESULTS
Root-shoot ratio (RSR)
Shade and plant density significantly affected the root-shoot ratio (RSR). The RSR of G. procumbens decreased by 30.09% with increasing shade levels from 0 to 30% (Figures 1-A). The percentage of increment of RSR at high plant density was 67.71 and 33.33 % compared with 9 and 15 plant m-2, respectively (Figures 1-B).
