Growth and photosynthesis of lettuce

Butterhead lettuce is an important glass-house crop in the poor light period in The Netherlands. Fundamental data about the influence of temperature, light and CO 2 on growth and photosynthesis are important e.g. to facilitate selection criteria for new cultivars. In this study on lettuce emphasis has been given to light interception in the poor light period, the relationship of growth rate and relative growth rate with time, dry weight and soil cover, and to photosynthesis properties of the cultivar 'Amanda Plus' and other cultivars.The soil area which is covered by a lettuce plant determines to a certain extent the light interception and growth of a plant. Therefore, the process of soil covering was studied in two experiments, the first one in spring with 8 cultivars and the second one in autumn with 5 lettuce cultivars and one endive cultivar ( chapter 2 ). The cultivar 'Amanda Plus' was used in both experiments. Three plant densities (20 cm x 20 cm; 25 cm x 25 cm; 35 cm x 35 cm) and 3 day/night temperatures were applied. The soil cover was determined according to the dot counting method. The process of soil covering related with time was described by a four parameter sigmoid curve with the parameters t (time in days from planting), S (amount of soil cover at time t), S max (maximal covered area) and p (the position of the inflexion point (t i , S i ). of the curve). Derived parameters are r (the initial soil cover rate), L i (S i /S max ) and R i (the soil cover rate in the inflexion point). W max is the fresh weight of the head at t max (time from transplanting until no visible increase in soil cover occurred), and W end is the fresh weight at the end of the experiment.Sigmoid curves fitted from the obtained data were all asymmetrical. Problems with curve-fitting occurred for the data of the treatments with long growing periods (low temperatures, 35 cm x 35 cm spacing). The data of the endive cultivar also could be fitted according to the similar sigmoid curve. The standard errors for the parameters r and p were high and these parameters were less useful for further analysis. At higher temperatures t max is lower. Mutual shading shortens the period until t i . At lower temperatures t i became higher. Wider spacings resulted in higher t i - and S i -values. The soil covering process of 'Amanda Plus' is more rapid in autumn than in spring. For the 35 cm x 35 cm plant density S max tended to decrease at lower temperatures. For the other two densities the maximum available soil cover was reached in almost all treatments. When t max is low and the growth period is short or the plant density high, L i becomes high. Differences between the parameters of the S-curves of the cultivars existed in spring as well as in autumn.A favourable combination between some parameters e.g. low t i with high S i , or high r with high L i , is present for some treatments and cultivars, but no cultivar showed the optimal combination of all parameters (high r, high L i , low t i combined with a high S i , high R i and low t max ). for a fast soil covering process. The correlations of some max of the soil cover parameters (t i , L i or W max ). with W end were low, especially of L i with W end , and of t i with W end for a number of cultivars in spring. The low correlation was partly due to the late harvest dates in the experiments. High correlations, however, are not to be expected and indirect selection of a high W end based on parameters of the soil cover curve is doubtful.In chapter 3 a quantitative growth analysis for the butterhead-- cultivar 'Noran' grown in spring and 'Deciso' in autumn has been described. The plants were cultivated at similar day/night temperatures and plant densities as those described in chapter 2. For the quantitative analysis a good fit of the growth curves is essential. Polynomials between the third and ninth degree were needed for an adequate description of dry weight (W) and leaf area (A) versus time (t). The long growth period and the partly controlled conditions of the glasshouse complicated a good fit of some data.The growth rate (GR = dW/dt), being the derivative of the polynomial of dry weight with time, was also used for the calculation of other parameters. The relative growth rate (RGR = dW/dt.1/W) decreases with time as well as with an increase in dry weight for all treatments presented. Plants grown at wider spacings have a higher RGR than plants at narrow spacings.Attention was paid to the relationship of GR with soil cover. These GR-S curves indicate the growth stage during wich mutual and self shading occur and heading becomes visible. When head formation occurs (between 2 and 5 gram dry weight) GR reaches a maximum value and starts to decrease. Plants at 35 cm x 35 cm have higher maximal growth rates, whereas the decrease of GR starts at a higher dry weight. The relationship between GR and S for the growth period until 80% of S max is almost linear. After this period the rise of GR is larger and than followed by a decline of GR at maximal or increasing S. Except one situation, the linear relation of GR with S gave higher correlation coefficients than those with A and W. Multilinear regression showed that mainly S is related to the increase of GR over that period until 80% of S max .The plants grown at lower temperatures in spring had a lower GR and reached a certain soil cover at a later date in this season than plants grown at higher temperatures, which resulted in a higher interception of irradiance and/or better use of the intercepted irradiance. In the autumn experiment the plants grown at higher temperatures intercepted more irradiance than those grown at lower temperatures, because a high S was reached earlier during that period with a high level of irradiance and a longer day-length (Fig. 6). The growth rates of the plants of the narrow spacings were lower than those of the plants of the wider spacings. The relationship between accumulated dry weight and total irradiance, intercepted per plant after correction for the covered area, is almost linear.The relationship between leaf area ratio (LAR A/W) and heading has also been studied. When LAR is lower than 550 cm 2 g -1 resp. 710 cm 2 g -1 for 'Noran' and 'Deciso' the quality of the head is good.Since equipment for photosynthesis measurements and determination of CO 2 compensation concentrations was not available at the Department of Horticulture of the Agricultural University of Wageningen, a closed system was built suitable for whole plants of lettuce and of other crops (sweet pepper, tomato). The system is described in chapter 4. The internal gaseous volume of the closed circuit as used for the lettuce measurements is 180 litres. The circuit consists of a cylindrical perspex plant chamber, a copper duct with a built-in fan, cooling coil, air-heating elements and connecting flexible tubes. The internal diameter of the chamber is 441 nun and the height is 340 mm, which can be enlarged to 690 mm. A cylindrical perspex pot chamber which has an internal diameter of 190 mm and a height of 190 mm is placed in the plant chamber.The equipment is placed on a metal trolley in a room, in which the temperature can be regulated between 10 and 34°C + 1° C. The temperature in the plant chamber can be kept constant between 5 and 32°C + 0.5°C and in the pot chamber between 15 and 35°C + 0.5°C. Temperatures are measured by thermocouples. The light equipment consists of 5 Philips high mercury vapour lamps (400 W) arranged above a waterbath with running water, which is constructed above the plant chamber. The maximum irradiance on plant level is 215 Wm -2 . Irradiance is measured by selenium photocells and the air humidity with thin film humidity sensors. Windspeed in the centre of the plant chamber is about 0.8 ms -1 . An infra-red gas-analyser determines the rate of CO 2 exchange. Injection of pure CO 2 or a mixture of air with CO 2 admits continuous monitoring of this exchange. During the relatively short periods of measurements leakage can be neglected. All measurements are recorded by a 24 channel mV-recorder or a data logger.Photosynthesis rates of whole lettuce shoots of butterhead cultivar 'Amanda Plus' were measured with this closed system ( chapter 5 ). In a first experiment the response of photosynthesis (P) to irradiance (I) was measured for plants of 3 different ages at 14°C and 26°C and in the second experiment the response to CO 2 -concentration (C) was measured at 15°C and 25°C, and the CO 2 compensation concentration was determined. In both experiments plants were cultivated at 2 different levels of irradiance and 2 different day/night temperatures.The photosynthesis data per plant were fitted with the use of a rectangular hyperbola, which related photosynthesis to both irradiance (I) and CO 2 -concentration (C), in which a represents the initial slope of the P-I-curve, i.e. the photochemical efficiency, and τthe initial slope of the P-C-curve, i.e. the plant conductance for CO 2 transfer. The carboxylation efficiency is included in this conductance. In the light series τdetermines to a great extent the gross maximal photosynthesis (P m,g = τ g C).Attention was paid to the basis of expression for the photosynthetic rates, obtained per plant. Since those rates expressed per unit leaf area, weight (or soil cover) were not adequate for comparison with other results, another basis, the effective leaf area (EL) was introduced. EL = α g .α -1g,con (m 2 Pl -1 ), with α g as the gross photochemical efficiency per plant and α g,con as the constant value of α g when all light quanta should be absorbed. For the calculations of the photosynthesis rates on EL-basis only α g -values have been used.A multilinear regression model of α g with S, A and W (in this order) gave high correlation coefficients, while addition of theheight of the plant, as included in the profile area, did not improve the model significantly. The linear relation of α g with the covered area by the plant gave higher correlation coefficients than of a with leaf area or weight, except for the group of younger plants.In experiment 1 the gross photochemical efficiency per plant (α g ). and per unit leaf area (α g1 ). the maximal gross photosynthesis per plant (P m,g ) and per unit leaf area (P 1m,g ), the dark respiration per unit dry weight (R d ) and the light compensation point (I c ) were calculated and listed. The values of α g and α g1 the net photosynthetic rates at 35 and 100 Wm -2 and at saturated level of irradiance, expressed on the basis of α g , the P 1m,g , I c and the corrected light compensation point (I cαg ) were used in a 3 way analysis of variance.The values of α g1 and P 1m,g decreased with ageing, but α g1 was almost not affected by the temperatures of the treatment and of the measurement. The net photosynthetic rates on α g -basis gave lower values for the group of young plants and similar values for the other age-groups. At a low irradiance level (35 Wm -2 ). the effect of the various cultivation treatments on net photosynthesis diminished, but at 100 Wm -2 the influence of the treatments on net photosynthesis increased, and this influencebecame much more distinct on the maximal net photosynthetic rates. At 35 Wm -2 the net photosynthesis on α g -basis is higher at 14°C than at 26°C. At saturated level of irradiance the opposite situation occurred, while at 100 Wm -2 this difference is absent. The light compensation point is strongly influenced by temperature during measurements and much less by treatment and age. The corrected I c was affected by age and measurement temperature and not by cultivation. The correlation coefficients (r) between specific leaf weight (SLW = W/A), as an average of the leaf area and leaf weight of the plant, and P m,n on α g -basis at 14°C is 0.73 and at 26°C 0.55.In experiment 2 the net conductance for CO 2 transfer per plant (τ n ) and per unit leaf area (τ n1 ), the maximal net photosynthesis per plant and per unit leaf area (P m,n and P 1m,n ) and the CO 2 compensation concentration (C c ) were calculated. The values of τ n and τ n1 decreased and the Pm,n increased with a rise in measurement temperature. C c is strongly influenced by temperature during measurement but not by temperature during cultivation.The use of the rectangular hyperbola and of αand τwas discussed in relation with the light interception and CO 2 transport of whole lettuce shoots. It was suggested that the boundery air layer resistance for CO 2 transport of the whole plant, which is considered to be low for most plants or crops in optimal conditions, can play a more important role for plants with a dense leaf orientation, such as lettuce. The use of α g as basis of expression did not completely solve the interpretation problems of photosynthesis data obtained per plant.In chapter 6 six experiments have been described in which the response of photosynthesis to irradiance of whole lettuce shoots of various cultivars was measured in an open system at 22°C. The butterhead lettuce cultivars 'Amanda Plus', 'Ostinata' and 'Hilde' were cultivated in 3 experiments (nrs. 2, 3, 5) in the glass-house, one in the phytotron (nr. 1) and one outdoor (nr. 6). Besides those 3 cultivars 4 other butterhead cultivars were used in the first spring experiment (nr. 3). Five butterhead, 2 cos- and 2 iceberg cultivars were used in the second spring experiment (nr. 4).According to a similar procedure as described in chapter 5 the gross photochemical efficiency per plant (α g ) and per unit leaf area (α g1 ), the maximal 1 net photosynthetic rates per plant (P m,n ) and per unit leaf area (P 1m,n ), the dark respiration per unit dry weight (R d ) and the light compensation point (I c ) were calculated. The SLW, stomatal (r s ) and residual (r m ) resistances were also calculated. The α g -values were used as basis of expression for photosynthetic rates according to the theory outlined in chapter 5. A multilinear regression of α g with S, A and W was carried out and the best fit of α g was obtained with S.In a two way analysis of variance α g , α g1 , the net photosynthetic rates at 30, 50, 100 and 150 Wm -2 and at saturated level of irradiance (all on basis of α g ), the P m,n per unit S, the I c and corrected (I c (I cαg ) were analysed for the 3 cultivars in the 5 experiments. For the plants of experiments 3 and 4 a one way analysis of variance for the same parameters except the net photosynthetic rates at 30, 100 and 150 Wm -2 was carried out. For 'Amanda Plus', 'Ostinata' and 'Hilde' α g1 is more influenced by treatment (thus experiments 1, 2, 3, 5 and 6) than by cultivar. The α g1 of 'Hilde' differs from those of 'Amanda Plus' and 'Ostinata'. A lower irradiance during growth resulted in a high α g and α g1 . In experiments 3 and 4 varietal differences for α g1 appeared to be higher. P m,n increased after a higher irradiance during growth. Differences between the values of P n (and also other parameters involved in photosynthesis) increased when differences between the cultivars regarding habitus, growth and genetical background were more pronounced (exp. 4). Results of the analysis of variance of P m,n perunit soil cover were identical to those of P m,n per unit α g for 'Amanda Plus', 'Ostinata' and 'Hilde'. When the photosynthesis measurements are carried out at a level of irradiance close to that during growth, no significant differences between the photosynthetic rates of the cultivars on α g -basis occur (exp. 3 and 4).For butterhead lettuce the influence of cultivar on the light compensation point is less pronounced than that of treatment. In two experiments the r m of 'Hilde' was larger than those of 'Amanda Plus' and 'Ostinata'. In the two spring experiments different r m -values between the cultivars were also present. A period of low irradiance resulted in a high r m for the plant. Differences between the r s -values existed only in experiments 3 and 4. The correlation coefficient (r) between the total plant resistance for CO 2 transfer (1/ τ n1 ) and r s + r m for all data is 0.81.For butterhead cultivars the specific leaf weight is more influenced by cultivation conditions than by genetic differences. However, significant differences between cultivars existed. A high negative correlation existed between SLW and r m when the differences were m caused by the various cultivation conditions. The two cos-lettuce cultivars, one selected for glass-house cultivation and one for outdoor growing, gave different results, while the two iceberg genotypes, both selected as glass-house crops, always gave similar results. The used cos- and iceberg lettuce cultivars were less adapted for growth during the winter season in The Netherlands.No clear criteria for indirect selection on higher yield have been found between the parameters, which describe the photosynthetic process. Success with indirect selection on higher yield based on parameters of the soil cover curve was also expected to be doubtful. When photosynthesis will be measured at irradiance and temperature conditions close to that during growth, no differences between the photosynthetic rates on α g -basis of the various genotypes can be expected. It is felt desirable that more research is carried out on the morphology of the lettuce plant in relation to growth, light interception and CO 2 transport and diffusion from the external air to the carboxylation sites. The introduction and use of non-heading cultivars would make the study of lettuce easier and facilitate cultivation of lettuce during the poor light period.