1. Effect on oxalate and calcium levels of spinach and silver beet grown in ambient CO ppm and enhanced CO ppm conditions
Madhuri Kanala and Geoffrey Savage
Food Science
Worldwide CO2 levels are slowly rising, and this may have effects on the growth and composition of many food plants. The production of secondary metabolites such as oxalates has not been investigated.
Methods
A study was conducted where spinach and silver beet plants were grown in identical plant growth chambers with the same environmental conditions except that one chamber was supplied with ambient air (CO2 405 ppm) while the other chamber had the CO2 level increased to 650 ppm. The total and soluble oxalate content of the leaves of spinach and silver beet were determined following 6 weeks of growth
The total and soluble oxalate contents of the dried leaves of each set of plants were extracted and measured (Savage et al., 2000). The samples were analysed for oxalates using HPLC and the mineral contents were measured by ICP-AES spectroscopy.
Results
The oxalate contents of spinach and silver beet leaves were significantly reduced when the plants were grown in a high CO2 environment (Table 1). The proportion of soluble oxalate compared to the total was 71.5% for both the spinach treatments while the proportion of soluble oxalates was 72.3% for the silver beet grown in air this fell to 61.0% for silver beet grown in a CO2 enriched environment. In contrast, the total, soluble and insoluble oxalate contents of spinach grown in a high CO2 environment were significantly reduced compared to the fresh air treatment.
Table 1. Mean total, soluble and insoluble oxalate (g/100 g DM ± SE) in raw spinach and silver beet grown under ambient and elevated CO2 conditions.
The total calcium content of the silver beet leaves was significantly lower that the spinach leaves (Table 2). Overall calcium bound as insoluble oxalate made up an overall mean of 17.7%
of the total calcium in the leaves.
Table 2. Mean total calcium, calcium oxalate (mg/100 g DM ± SE) and % insoluble calcium in raw spinach and silver beet grown in ambient CO ppm and enriched CO ppm conditions.
Silver beet
Spinach
mg/100 g DM
Spinach leaves
Silver beet leaves
Analysis of variance
Air
CO2
Cultivars
Treatment
Cultivars x Treatment
Total calcium
805.6 ± 8.7
732.4 ± 8.4
705.9 ± 13.4
718.8 ± 4.4
*** *
Calcium oxalate
± 58.3
776.7 ± 140.6
957.9 ± 136.2
994.1 ± 41.3 ns
Insoluble calcium as a % of total calcium
17.4 ± 1.1
14.6 ± 2.9
19.4 ± 3.2
19.3 ± 1.0
The mean Ca and Mg were significantly reduced in the spinach grown in a high CO2 environment while
The Fe and Zn contents were increased (Table 3). Increased CO2 led to a decrease in Fe, Mg and Zn levels and a small increase in the Ca content in the silver beet leaves grown in a high CO2 environment.
Table 3. Mean mineral content (mg/100 g DM) of raw spinach and silver beet leaves grown in air or enriched CO2 conditions.
Oxalate
(mg/100 g DM)
Spinach leaves
Silver beet leaves
Analysis of variance
Air
CO2
Cultivars
Treatment
Cultivars x Treatment
Total
12.8 ± 0.2
8.3 ± 0.3
11.2 ± 0.3
8.2 ± 0.2 *
***
Soluble
9.2 ± 0.2
5.9 ± 0.2
8.1 ± 0.2
5.0 ± 0.2 ns
Insoluble
3.6 ± 0.2
2.5 ± 0.4
3.1 ± 0.4
3.2 ± 0.1
The total oxalate of spinach and silver beet grown in high CO2 conditions reduced by 36.6% and 26.8% and soluble oxalate reduced by 35.8% and 38.3% than in ambient conditions.
Conclusions
The reduction of the oxalate content of spinach leaves following an increase in CO2 content in air is an interesting positive response to an otherwise significant problem.