Increase of leaf tipburn in chervil and lettuce by restricting volume of growing medium

Abstract

The objective of this investigation was to examine the effect of restriction of growing medium volume on the level of tipburn in chervil and leaf lettuce. Chervil: The plants were grown in different sizes of rock wool cubes: a. 31 cm³, b. 64 cm³, c. 135 cm³. The incidence of tipburned leaves decreased by increasing the size of cubes. Leaf lettuce: Plants were grown in different sizes of pots: 12 cm pots with capacity of 0.6 L and 8 cm pots with capacity of 0.25 L. The incidence of tipburned leaves increased by decreasing pot size.

Keywords:

• Calcium content
• chervil
• growth
• lettuce
• root volume
• tipburn

Introduction

Although scientists have been working to protect plants against physiological calcium deficiency for more than 100 years, the problem still exists and there has been no good protection method until now. Physiological calcium deficiency is usually related to the inability of the plant to translocate adequate calcium to the affected plant part, rather than insufficient Ca levels in soils. Physiological calcium deficiency is not generally prevented by calcium fertilization. Therefore it is quite difficult to protect plants against it. Many scientists have investigated ways to protect lettuce against tipburn (Murdoch et al. 2002, Frantz et al. 2004, Vita Serman et al. 2004, Wang et al. 2005, He et al. 2006, Choi and Lee 2008, Ferriol et al. 2009, Olle and Bender 2009, Torres et al. 2009, Jenni and Hayes 2010).

The unpredictability of tipburn occurrence and the absence of totally effective control procedures make the problem very serious (Olle and Bender 2009). Losses to the grower from tipburn on lettuce can reach up to 50% of production (Benoit and Ceustermans 1986).

Sufficient root growth is essential for uptake of calcium. Aloni (1986) found that restriction of Chinese cabbage root growth by limiting root volume impairs Ca uptake and translocation into the young leaves, thereby causing tipburn. Restriction of root volume is one factor that favours the development of Ca deficiency in leafy vegetables (Kleemann 1999, Olle and Bender 2009). The objective here was to examine the effect of restriction of growing medium volume on the level of tipburn and calcium uptake in chervil and leaf lettuce.

Materials and methods

Experiments with chervil

The experiments in the greenhouse were carried out in the wintertime at the Agricultural University of Norway. Chervil (Anthriscus cerefolium (L.) Hoffm.) seeds were sown in different sizes of rock wool cubes on 4 February 1998. The sizes of rock wool cubes (treatments) were: a. 4×2.8×2.8 cm = 31 cm³, b. 4×4×4 cm = 64 cm³, c. 4×5.8×5.8 cm=135 cm³. The seedlings were grown in greenhouse and watered with a complete nutrient solution until transplanting. The seedlings were transplanted on 25 February 1998. One drip emitter was placed in each rock wool cube and a timer turned on the watering of nutrient solution two times per day for 5 minutes. The excess of solution drained through the cubes into a holding tank. The plants were harvested on 14 March 1998. Each treatment consisted of 54 plants. The experiment had three replicates. The experiment was repeated twice.

The basic nutrient solution at 2 mS cm⁻¹ contained the following nutrients (mg L⁻¹): N 166, P 47, K 280, Ca 100, Mg 24, S 32, Fe 2.8, Mn 0.8, B 0.4, Zn 0.3, Cu 0.15, Mo 0.05 and Co 0.01. The illumination of supplementary light at plant level was approximately 12000 lux from fluorescent (Philips) lamps. The plants were illuminated in the period of 16 hours (04.00–20.00). All plants were grown with a minimum day and night temperature of 20 °C and 15 °C, respectively.

At harvest the plants were weighed. Total number of leaves and the numbers of fresh and injured leaves were counted at harvest. After harvest the content of calcium was determined in young and old leaves. Preparation of sample was made after 922.02(a) (Helrich 1990). Determination of total Ca was made after 985.01(c) (Helrich 1990). Determination of total Ca was accomplished by an atomic absorption spectrophotometer. Analyses of variance were carried out on the data obtained using Minitab for Windows. Newman–Keuls test was carried out on the means obtained (Montgomery 1997).

Experiments with leaf lettuce

Leaf lettuce (Lactuca sativa var. crispa L.) seeds were sown in different sizes of plastic pots filled with limed and fertilized peat on 6 March 2001. The sizes of pots (treatments) were: 12 cm pots with capacity of 0.6 L and 8 cm pots with capacity of 0.25 L. The seedlings (height 4–5 cm) were grown in a Grüne Fee Eesti Ltd. greenhouse and watered with a complete nutrient solution until transferral. The seedlings were transferred to the greenhouses of University of Tartu on 19 March 2001, and plants were harvested on 11 April 2001. Each treatment consisted of 54 plants. The experiment had three replicates. The experiment was repeated twice.

The basic nutrient solution at 2 mS cm⁻¹ contained the following nutrients (mg L⁻¹): N 166, P 47, K 280, Ca 100, Mg 24, S 32, Fe 2.8, Mn 0.8, B 0.4, Zn 0.3, Cu 0.15, Mo 0.05 and Co 0.01. The illumination in greenhouses at plant level was 242 µmol m⁻² s⁻¹ from high pressure mercury (HPI/T) lamps. All plants were grown with a minimum day and night temperature of 20 °C and 15 °C, respectively. The plants were illuminated in the period of 14 hours (07.00–21.00). The relative humidity was 75 ± 1%.

At harvest the weight and height of plants were measured. Total numbers of leaves, the numbers of fresh and injured leaves were counted at harvest. After harvest the content of dry matter, chlorophyll and calcium were determined. Dry matter content was determined by the loss of weight of dried leaves at 105 °C until they reached a constant weight. Chlorophyll content was determined according to the method of Bukatsch (1964). The Ca content of chervil young leaves was determined. Preparation of sample was made after 922.02(a) (Helrich 1990). Determination of total Ca was made after 985.01(c) (Helrich 1990). Determination of total Ca was accomplished by an atomic absorption spectrophotometer. Analyses of variance were carried out on the data obtained using Minitab for Windows. Newman–Keuls test was carried out on the means obtained (Montgomery 1997).

Results

Experiments with chervil

Chervil develops quite quickly. Restriction of cube volume caused inhibition of fresh weight. The mean fresh weight of plants from 135 cm³ cubes was 15.1 g at the end of the experiment, whereas those from the 64 cm³ cubes were higher than 31 cm³ cubes (Table I). The number of fresh leaves was highest in plants grown in 135 cm³ cubes (Figure 1). The mean number of fresh leaves per plant of chervil was higher when the size of cube increased (Figure 1).

Figure 1.  Total number of leaves, the number of fresh and injured leaves per plant of chervil depending on the sizes of rock wool cubes. Y – Numbers of leaves. X – the size of rock wool cube (4×2.8×2.8 cm = 31 cm³, 4×4×4 cm = 64 cm³, 4×5.8×5.8 cm = 135 cm³); (fresh leaves p=0.015, injured leaves p<0.001).

Table I. The weight (g) per plant of chervil according to the sizes of rock wool cubes (a. 4×2.8×2.8 cm = 31 cm³, b. 4×4×4 cm = 64 cm³, c. 4×5.8×5.8 cm = 135 cm³).

Size of rock wool cubes (cm^3)	Weight (g)
a. 4×2.8×2.8 = 31	11.2 c
b. 4×4×4 = 64	13.4 b
c. 4×5.8×5.8 = 135	15.1 a
Statistical differences	p=0.032

The first leaves with injury symptoms appeared 32 days after sowing on plants grown in 31 cm³ cubes. If chervil plants were grown in 135 cm³ cubes leaf tipburn appeared two days before harvesting and at the time of harvest injury concerned few leaves (Figure 1). The incidence of tipburned leaves was highest in the plants grown in 31 cm³ cubes. The number of injured leaves per plant of chervil was 54.5% higher in plants grown in 31 cm³ cubes compared with plants in 64 cm³ cubes (Figure 1).

Restriction of cube volume affected the concentration of Ca in the young and old leaves. In young leaves from plants grown in 135 cm³ cubes, Ca concentration was higher than in leaves from the 64 and 31 cm³ cubes (Figure 2). Restriction of cube volume reduced Ca content in old leaves (Figure 2). The content of calcium in old leaves was 19.0% higher in plants grown in 135 cm³ cubes compared with plants in 31 cm³ cubes, on average old leaves contained more Ca than the young, almost twice as much (Figure 2).

Figure 2.  Calcium content (in young and old leaves g kg⁻¹ dry weight) in chervil plants according to the sizes of rock wool cubes. Y – Ca content in leaves (g kg⁻¹ dry weight). X – the size of rock wool cube (a. 4×2.8×2.8 cm = 31 cm³, b. 4×4×4 cm = 64 cm³, c. 4×5.8×5.8 cm = 135 cm³); (young leaves p=0.041, old leaves p=0.028).

Experiments with leaf lettuce

Leaf lettuce as chervil develops quite quickly. Restriction of pot volume decreased the height of leaf lettuce plants (Table II). The mean height of plants grown in 0.25 L pots was 23.5% less than those grown in 0.6 L pots. Growing leaf lettuce plants in smaller pots caused inhibition of fresh weight. The mean fresh weight of plants in 0.25 L pots was at the end of the experiment 22.7% less than from the 0.6 L pots (Table II). The number of fresh leaves was highest in plants grown in 0.6 L pots (Table II). The mean numbers of fresh leaves per plant of leaf lettuce from the 0.25 L pots were 24.8% less than those from 0.6 L pots.

Table II. The height (cm), the fresh weight (g), total number of leaves, the number of fresh and injured leaves per plant of leaf lettuce depending on the capacities of peat pots (8 cm pots with capacity of 0.25 L; 12 cm pots with capacity of 0.6 L).

			Number of leaves
Pot size (L)	Height (cm)	Fresh weight (g)	Total	Fresh	Tipburned
0.25	14.25 b	30.66 b	22.75 b	11.37 b	11.38 a
0.6	18.63 a	39.66 a	24.5 a	15.12 a	9.38 b
Statistical differences	p=0.027	p<0.001	p=0.042	p=0.027	p=0.034

The first leaves with injury symptoms appeared 22 days after sowing on plants grown in 0.25 L pots. If leaf lettuce plants were grown in 0.6 L pots leaf tipburn appeared 27 days after sowing and at the time of harvest injury was on 9.38 leaves (Table II). The incidence of tipburned leaves was highest in the plants grown in 0.25 L pots. The number of injured leaves per plant of leaf lettuce was 17.7% higher in plants grown in 0.25 L pots compared with plants in 0.6 L pots (Table II).

The content of dry matter did not show statistically different results (Table III). The content of chlorophyll was 5% lower in leaf lettuce plants grown in small pots (Table III). Restriction of pot volume affected the concentration of Ca in young leaves. The content of calcium in young leaves was higher in plants grown in 0.6 L pots compared with plants in 0.25 L pots (Table III).

Table III. The content of dry matter (%), chlorophyll (mg g⁻¹ fresh weight) and calcium (in young leaves g kg ⁻¹ dry weight) in leaf lettuce plants according to the capacities of peat pots (8 cm pots with capacity of 0.25 L; 12 cm pots with capacity of 0.6 L).

Pot size (L)	Content of dry matter (%)	Content of chlorophyll (mg g^−1 fresh weight)	Content of calcium (g kg ^−1 dry weight)
0.25	9.45	4.24	0.98
0.6	8.48	4.46	1.05
Statistical differences	p=0.595	p<0.001	p=0.013

Discussion

Under greenhouse conditions chervil (Kleemann 1999) and leaf lettuce are extremely susceptible to physiological calcium deficiency. The experiments were carried out in the middle of the winter with short days and supplementary light. Those conditions, including rather high temperature, favour tipburn development on the leaves of chervil and leaf lettuce.

Cox et al. (1976) and Saure (1998) postulated that reduction of growth might reduce tipburn incidence. Plants grown in intensive growing conditions (greenhouses) might have larger needs for nutrients. Ca can be diluted in the tissue by growth. High growth rates increase the risk that tissue content of calcium falls below the critical level resulting in calcium deficiency related disorders. Farmers need as short a production period as possible because of lower costs of production therefore intensive production systems are in use. A vegetable grower needs to have a compromise between the shortest growing period and the quality of a product. Therefore it was important to conduct experiments which can give information about the optimal growing medium volume (both cube size and pot size) to avoid calcium deficiency injury by growing chervil and leaf lettuce in greenhouses.

The results of the present investigation showed that growing chervil and leaf lettuce plants in restricted growing medium volume caused reduced fresh weight. Cox et al. (1976) in experiments with lettuce found that conditions giving rapid growth (like increasing growing medium volume) give higher fresh weight of plants, as was investigated in present experiments.

Sufficient root growth, as well as sufficient growing medium volume, is essential for uptake of calcium. In experiments with Chinese cabbage, Aloni (1986) grew plants in 10.0, 3.0 and 0.5 L pots. He found that root growth was not restricted and leaf tipburn did not occur on plants growing in 10 L pots. Other plants showed the symptoms of tipburn and restriction of root and shoot growth. Similar results were found in the present investigation by growing chervil and leaf lettuce in different sizes of growing medium. Restriction of growing medium volume is one factor that favours development of calcium deficiency in plants. Palzkill et al. (1976) postulated that tipburn occurs when root development is limited and the shoot/root ratio increases, causing the shoot's demand for Ca to exceed the supply. Root growth is limited in conditions of restriction of growing medium volume, like in present investigation. Root growth can also be inhibited by several other factors, for example low temperature, inadequate aeration, poor nutrient status or high H⁺ concentration. By avoiding those factors we can delay the development of calcium deficiency symptoms on plants.

Root growth is reduced some weeks before maturity. On the other hand during the same period the accumulation of fresh weight by the shoot is very rapid. Plants require many nutrients in this period, but root growth has almost ceased. Kleemann (2000) found in experiments with chervil that the number of tipburned leaves developed quickest (0.58 leaf per 24 hours) some days just before harvesting, which can be the reason of reduced root growth. Cox (1980) indicated that root development of lettuce some weeks before maturity, might be implicated in tipburn resistance.

Old chervil leaves contained higher concentration of Ca compared with young leaves. Calcium flows through the plants in the xylem, mostly passively by water flow of transpiration. Ca is not recycled when deposited in leaf tissue. Therefore old leaves always contain more Ca than young leaves. Cube size restriction reduced the concentration of Ca in the young and old chervil leaves. Restriction of pot volume reduced Ca content in young leaf lettuce leaves. The results in the present investigation are in agreement with results obtained by Aloni (1986) in experiments with Chinese cabbage. Results from the present investigation showed that chervil plants grown in 31 cm³ cubes and leaf lettuce plants grown in 0.25 L pots suffered most under tipburn injury and at the same time Ca content of leaves was lowest.

Optimal size of rock wool cubes for chervil plants in present investigation was 135 cm³. The optimal pot volume for leaf lettuce plants was 0.6 L. The plants grown in those kinds of conditions had reduced amount of tipburned leaves, highest fresh weight and Ca content in leaves. The unpredictability of tipburn in every situation makes the problem very serious.