Relation between photosynthesis and nitrate content of lettuce cultivars

Scientia Horticulturae, 49 ( 1992 ) 175-179

175

Elsevier Science Publishers B.V., Amsterdam

Relation between photosynthesis and nitrate content of lettuce culfivars U. Behr and H.-J. Wiebe Institute of Vegetable Crops. University of Hannover, Herrenhiiuser Str. 2, D-3000 Hannover 21, German), (Accepted 23 July 1991 )

ABSTRACT Behr, U. and Wiebe, H.-J., 1992. Relation between photosynthesis and nitrate content of lettuce cultivars. Scientia ttortic.. 49:175-179. Experiments with three lettuce cultivars (Lactuca sativa L. vat. capitata L. ), significantly differing in nitrate content, were carried out to investigate the reason for differences in nitrate accumulation. Significant differences in photosynthesis and sugar c,mcentration bctv,cen ~tdtivars were found at 6/ 6°C and 14/6°C. A c!ese ncgative correlation existed between photosynthetic activity and nitrate content. Nitrate as an osmoticum is partly replaced by sugars. Photosynthetic rate and sugar content were highest at low temperature (6/6°C) whereas nitrate content was lowest. Keywords: Lactuca sativa L. vat. capitata L; lettuce; nitrate; photosynthesis.

INTRODUCTION

Apart from the variation in nitrate accumulation due to environmental factors, large and significant genetic differences in nitrate accumulation of lettuce cultivars have been found (Eenink et al., 1984). Differences in nitrate accumulation may be an indirect effect of the morphology of lettuce heads or may be related directly to the uptake or assimilation of nitrate. However, no direct relation has been observed between morphological characteristics and the nitrate content. This result is confirmed by the close correlation between the nitrate content of marketable heads and of young plants in a large number of cultivars (Eenink et al., 1984; Behr, 1988). The activity of nitrate reductase as a cause of differences in nitrate content may also be excluded, since differences in the conter, t of organic nitrogen are small and there is no significant correlation between nitrate and organic nitrogen (Blom-Zandstra and Eenink, 1986; Behr, 1988 ). It may therefore be concluded that differences in nitrate content of butterhead lettuce cultivars are due to differences in nitrate uptake. © 1992 Elsevier Science Publishers B.V. All rights reserved 0304-4238/92/$05.00

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U. BEHR AND H.-J. WIEBE

Nitrate has a function as an osmoticum, and may interact with other components in the vacuole. In response to ecological conditions nitrate replaces organic compounds like carbohydrates and organic acids in the cell sap (BlomZandstra and Lampe, 1985; SteingriSver, 1986). An inverse correlation between nitrate content and that of other osmotica was also found in 19 lettuce cultivars by Behr and Wiebe (1988). They showed that cultivars low in nitrate had a higher content of malate, chloride, fructose, and glucose. The objective of this study was to investigate why genotypes differ in nitrate uptake. Differences in the ratio of nitrate to other osmotic components may be caused by differences in photosynthetic capacity because a higher synthesis of sugars might result in a reduced nitrate uptake. MATERIALS AND METHODS

The experiment was carried out with the butterhead cultivars 'Bellona' (low in nitrate ), 'Panvit' (high in nitrate ), and 'Ravel' (intermediate in nitrate )o The plants were grown in growth chambers at a day/night temperature of 14/10 ° C, 1.36 hPa saturation deficit and 400 ppm CO2. An irradiance of 40 W m - 2 photosynthetic active radiation (PAR) was given with fluorescent lamps with 20% incandescent lamps for 12 h daily. Each tray (30× 30 cm) contained eight plants and was filled with an aerated nutrient solution with the following composition (mol m-3~ • NO3 - - N 10.7; P 1.0; K 4.7; Ca 2.5; Mg 1.5 and trace elements. Photosynthesis was measured in a gas exchange chamber which controlled the CO2-uptake according to the compensation technique (Krug and Fink, 1987). When measuring the CO2-uptake of the eight plants irradiance was 16.5 W m - 2 PAR, temperature 6/6 ° C or 14/6 ° C, the CO2-concentration 400 ppm, and the saturation deficit 2.5 hPa. The plants were transferred to the gas exchange chamber at the end of the dark period and analysed 24 h later (i.e. after 12 h light and 12 h dark). The mean shoot weight of the used plants was 4 g and leaf area was 1.5 dm 2. For each cultivar the measurement of the photosynthetic rate was replicated on 4 different days. The shoot material was freeze-dried, ground, and analyzed. The nitrate concentration was determined with an ion-sensitive electrode (Temperli, 1983 ). The content of glucose, fructose, and sucrose was measured enzymatically (Bergmeyer, 1974). RESULTS

Significant differences in nitrate content between cultivars were found (Table 1 ). The nitrate concentration in the shoot of'Bellona' was at least 1500 ppm lower, or 30%, than the corresponding concentration of 'Panvit'. 'Ravel' had an intermediate content, but was not significantly differNitrate

content.

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PHOTOSYNTHESIS AND NITRATE CONTENT OF LETTUCE CULTIVARS

TABLE 1

Nitrate content, sugar concentration, and photosynthesis of three lettuce cultivars at two temperatures (24 h ) (plants grown before measurements at 14/10 ° C) Cultivars

'Panvit' 'Ravel' "Bellona'

Mean LSD 0.05

"Panvit' "Ravel' "Bellona'

Mean LSD 0.05

"Panvit' 'Ravel' 'Bellona'

Temperature ° C (day/nigh!) 6/6 °

14/6 °

NO3(ppm) 4149 4103 2728

5275 4885 3842

3659

Mean

LSD 0.051

4712 4494 3285

477

427 381 701

156

4667 316

Sugars(mg 100g -I dry wt.) 526 449 872 615

358 313 530 390

103 CO2-uptake (mg d m - 2 h- t ) 3.30 3.65 3.75

Mean LSD 0.05

3.57

2.67 3. I 1 3.49

2.98 3.38 3.62

0.518

3.09 0.343

ILSD, least significant differences between mean values at 5% level by Tukey test.

ent from 'Panvit'. At 14/6 °C the nitrate content was significantly higher than at 6/6°C. c o n t e n t . - The different demand for NO3 might be caused by a cultivarspecific accumulation of sugars which could replace nitrate as an osmoticum. Cultivars low in nitrate had the highest sugar content. The sugar content of 'Bellona' was about 60% higher than that of'Panvit' (Table 1 ). 'Ravel' showed a slightly different reaction. According to the intermediate nitrate concentration a higher sugar content would have been expected. At 14/6°C the sugar content was significantly lower than at 6/6 ° C.

Sugar

In agreement with the high sugar content of 'Bellona' this cultivar also had the highest photosynthetic activity. 'Panvit' had a significantly lower CO2-uptake. It seems likely that cultivar-specific sugar concentrations were caused by different photosynthetic rates. Photosynthesis.

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U. BEHR AND H.-J. WIEBE

The correlations between sugars and nitrate (r 2=0.99) and photosynthetic rate and nitrate content (r 2=0.79) were very close. Both, photosynthetic rate and sugar content were lower at 14/6 °C than at 6/6 ° C, whereas nitratecontent increased at the higher temperature. The behavior of the tested cultivars at 6/6 ° C, with respect to nitrate, sugar and CO2 uptake, was parallel to that at 14/6°C. DISCUSSION

Butterhead lettuce grown under protected cultivation has a rather high nitrate content especially in winter months. Searching for cultivars low in nitrate and breeding for low nitrate concentrations might be an efficient way to decrease nitrate intake of consumers. Significant differences in nitrate concentration between commercially used cultivars have been found. This variation in nitrate accumulation is attributed due to differences in nitrate uptake. Nitrate reduction and morphology of heads seem to play a minor role. Nitrate uptake is an active transport; the ion is accumulated against an electrochemical gradient (Mengel and Kirkby, 1979 ). As this requires energy, the N-uptake may be promoted by a sufficient supply of carbohydrates. On the other hand it was observed that nitrate accumulation decreases with increasing carbohydrate concentration in the vacuoles (Blom-Zandstra and Lampe, 1983 ). The negative correlation between the concentration of sugars and nitrate, and photosynthesis and nitrate, respectively, was also demonstrated when testing different cultivars. The present experiments support the hypothesis of Maynard et al. (1976) that differences in nitrate accumulation between cultivars can be caused by differences in photosynthetic capacity. In spite of extensive investigations on the physiological role of nitrate by SteingRiver (1986), however, it is still unclear, how carbohydrates in the vacuole regulate the nitrate uptake. The data on the photosynthetic activity were not biased by morphological differences between cultivars (van Holsteijn, 1981 ), since plants in rosette habitus were used. Between 'Panvit' and 'Bellona' no significant differences in fresh weight, leaf area per plant, and absolute growth rate were found (data not shown). The decrease of the photosynthesis with increasing temperature from 6 °C to 14 °C confirms results of Lorenz and Wiebe (1980). Hence nitrate accumulation increases with temperature. The conclusion can be that differences in nitrate content between lettuce cultivars may be the result of different photosynthetic rates through which nitrate as an osmoticum is replaced by sugars. Before taking advantage of this result for breeding aspects, however, it should be tested with further cultivars.

PHOTOSYNTHESIS AND NITRATE CONTENT OF LETTUCE CULTIVARS

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