Effects of temperature and irradiance on vegetative

Journal of Experimental Botany, Vol. 48, No. 313, pp. 1591-1598, August 1997

Journal of Experimental Botany

Effects of temperature and irradiance on vegetative growth of cauliflower (Brassica oleracea L. botrytis) and broccoli (Brassica oleracea L. italica) Jorgen E. Olesen 1 - 3 and Kai Grevsen 2 1

Department of Soil Science, Research Centre Foulum, PO Box 23, DK-8830 Tjele, Denmark Department of Fruit and Vegetables, Kirstinebjergvej 6, DK-5792 Aarslev, Denmark

2

Received 22 October 1996; Accepted 18 April 1997

linear relationship between yield and intercepted radiation has been found for many crops (Monteith, 1977). The Cauliflower {Brassica oleracea L. botrytis) and broccoli dry matter production of a plant stand (W) integrated (Brassica oleracea L. italica) plants were grown in large over time (t) is thus given by

pots in growth chambers for a range of temperatures (mean air temperatures from 7.0-25.3 °C) and irradiances (from 9.3-50.8 mol m~2 d" 1 or 4.7-25.4 MJ m~2 d ~ 1 ). The extinction coefficient for PAR decreased with plant size reaching a value of 0.55 in cauliflower and 0.45 in broccoli at plant leaf areas of 0.235 m2 and 0.227 m2, respectively. The leaf area expansion rate was unaffected by irradiance when compared at identical leaf surface temperatures. The response of expansion rate to surface temperature was fitted to a broken stick model with a base temperature of — 0.7 C and an optimum temperature of 21.0 C. The radiation conversion coefficient increased with air temperature below 13.8°C and remained constant above this. The estimated radiation conversion coefficient above 13.8°C and for a PPFD of 20 mol m"2 d~ 1 was 0.77 g m o l 1 in cauliflower and 0.87 g mol 1 in broccoli. The radiation conversion coefficient declined with increasing irradiance level from a maximum of 1.89 g mol" 1 at near nil irradiance in cauliflower. Key words: Leaf area, dry matter, radiation use efficiency, extinction coefficient. Introduction

Crop growth rate under conditions where nutrients and water are not limiting can be largely explained by the ability of the crop to intercept and utilize radiation (Gosse et al., 1986; Arkebauer et al., 1994). An approximately

(1)

where Qd is daily incident radiation, / is the fraction of radiation intercepted, and e is the radiation conversion coefficient (Russell et al., 1989). In many cases only the above-ground biomass is included in estimates of «, which may be expressed on the basis of either absorbed or intercepted radiation using either global solar radiation or photosynthetically active radiation (PAR). The fraction of intercepted radiation may be calculated from Beer's law as /=l-exp(-A:L)

where k is the extinction coefficient and L is the green leaf area index [m2 leaf m~2 ground area]. The dry matter production of crops under non-limiting conditions can therefore be calculated from observations of Q and L, and from quantified response functions of k and e to external conditions or intrinsic crop characteristics. The dry matter production functions in many crop models are basically identical to equations (1) and (2). Dry matter production may be calculated directly from equations similar to equation (1) (Steer et al., 1993; Amir and Sinclair, 1991) or by submodels of photosynthesis and respiration (Weir et al., 1984; Graf et al., 1990; Olesen and Grevsen, 1993). Under conditions of ample supply of water and nutrients, e depends on temperature and radiation level and on crop characteristics. Most

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(2)

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Abstract

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Olesen and Grevsen HQI T400/D lamps placed in a separately cooled section above the plants. This section was separated from the chamber by an insulating acrylic plate with high light transmissivity. Two of the growth chambers had five possible set-points of PPFD (photosynthetically active photon flux density) and one had six set-points. The PPFD on a horizontal plane was measured at six positions in each chamber using a LICOR point quantum sensor. The measurements were carried out at a height of 50 cm above the cabinet floor. At the highest set-point a measurement was also made at 100 cm above the floor. The mean PPFD at individual set-points was almost identical between chambers. The coefficient of variation in PPFD between measurements within a chamber was about 5%. PPFD was 17% higher at 100 cm compared with 50 cm above the floor. Another series of measurements showed that this increase in PPFD with height was almost linear. The PPFD on a horizontal plane at 75 cm above the floor may thus be estimated as 109% of the measurements at 50 cm giving the following levels of PPFD in all chambers: 152, 307, 468, 649, 1041, and 1430/xmol m " 2 s" 1 . Similar horizontal PPFD values at midday in June in Denmark ranges from an average of 1300/xmol m " 2 s'1 to a maximum of 1900^mol m " 2 s" 1 (Hansen et al, 1981). The reflection from the wall onto a vertically mounted sensor shielded from radiation from the lamps was found to be 40% of the PPFD on a horizontal surface irrespective of height of measurement. Three growth chambers were used in each experiment. Each growth chamber had a different treatment of temperature or irradiance. Seeds of summer cauliflower (Brassica oleracea L. botrytis cv. Plana Fl) and broccoli (Brassica oleracea L. italica cv. Shogun) were sown in nutrient enriched peat soil (Pinstrup spaghnum plus) in rectangular pots with a cross-sectional area of 30 x 40 cm and a depth of 40 cm. Ten seeds per pot were sown in experiments 1 and 2, and 3 x 1 0 seeds per pot were sown in experiment 3. More plants were sown in the last experiments to allow more frequent samplings. The top of the pots were placed 65 cm above the cabinet floor. The soil was watered to field capacity at time of sowing and subsequently watered to field capacity every second day. The initial nutrients in the peat soil were considered sufficient to ensure optimal growth in experiments 1 and 2. The higher initial plant density and the higher irradiance treatments in experiment 3 increased the nitrogen demand in this experiment, so nitrogen fertilizer was added in the irrigation water in experiment 3. After sowing, all pots were kept in identical conditions until about 10 leaves had been initiated. During this period, air temperature was kept at 14 °C during an 8 h night and at 16°C during a 16 h day, and with a daily PPFD of 12.1 mol m" 2 d" 1 . The diurnal variation in PAR was simulated by varying the PPFD during the day using two of the light levels. The air humidity was kept at 80%. Experimental treatments

Materials and methods Three experiments were conducted in growth chambers at Research Centre Foulum with cauliflower and broccoli. The growth chambers allowed control of air temperature, air humidity and irradiance. The internal dimensions of the growth chambers were width = 238 cm, depth = 197 cm and height = 206 cm. Air temperature was varied between chambers in the two first experiments and irradiance was varied in the third experiment. All experiments were carried out at present day atmospheric CO 2 concentration. The growth chambers were illuminated using sets of Osram

When about 10 leaves had been initiated on most plants, the plants were thinned to a maximum density of six plants per pot in experiments 1 and 2 and 10 plants per pot in experiment 3. Small and atypical plants were also removed on this occasion. The pots were then distributed randomly among the three growth chambers (five pots of each crop in each chamber). The pots were placed in four rows of 3, 2, 2, and 3 pots in each row. Each pot thus faced a growth chamber wall at the same distance. The pots were randomly rearranged within the chamber every second day. The plant size at the onset of the experimental treatments corresponded to the size of transplants normally used in Denmark (Grevsen and Olesen, 1994). The experimental treatments comprised different air temperatures in


models include the effect of temperature on « by applying functional responses such as those proposed by WarrenWilson (1971) or Versteeg and van Keulen (1986). The increase in leaf area index with time has been modelled in several ways: by describing the response of the expansion of individual leaves to temperature (Porter, 1984; Steer et al., 1993), by using empirical temperaturedependent functions for the development of leaf area index (Amir and Sinclair, 1991; Jamieson and Wilson, 1988), by partitioning daily assimilate assuming a constant specific leaf area (leaf area per leaf dry weight) or a constant leaf area ratio (leaf area per top dry weight) (Graf et al, 1990; Olesen and Grevsen, 1993; Aikman and Scaife, 1993) or by a combination of these methods (Spitters et al, 1989; Hansen et al., 1991). The extinction coefficient is usually assumed to depend on plant type but otherwise to be constant (Weir et al., 1984; Graf et al., 1990; Steer et al., 1993). There are, however, indications that the extinction coefficient changes with plant age in some crops (Yunusa et al., 1993; Imai et al., 1994; Kubota et al, 1994). The quantification of the responses of the parameters in equations (1) and (2) to temperature and radiation are important for assessing the performance of crops under changing environmental conditions such as those implied by the enhanced greenhouse effect. Crop models should adequately describe the effect of interactions between the various environmental factors on these parameters. Cauliflower and broccoli are short rotation crops that are grown throughout the season under varying temperature and radiation regimes. Therefore, models used for the analysis and prediction of current crop production in these crops need to incorporate appropriate responses to temperature and radiation. Information on the response functions for the parameters in equations (1) and (2) can be obtained from growth analysis experiments (Hughes and Freeman, 1967; Warren-Wilson 1981). The aim of the present work was to quantify the effects of temperature and irradiance on leaf area expansion, extinction coefficient and radiation conversion coefficient for use in simple models of vegetative growth of cauliflower and broccoli.

Temperature effects in Brassica Table 1. Summary of experimental treatments: night and day air temperatures, calculated mean daily air temperature, estimated mean leaf surface temperature, and daily photosynthetically active radiation The duration of the experiment is the number of days from onset of experimental treatment until the last sampling. Growth chamber 2

5/8 7.0/5.6 20.4 43

11/16 17/24 14.3/12.9 21.7/20.3 19.1 19 1 35 20

8/12 10.7/9.2 20.4 27

14/20 18.0/16.5 19.1 27

11/16 14.3/15.0 50.8 38

11/16 11/16 14.3/13.7 14.3/12.1 32.7 9.3 38 38

3

20/28 25.3/23.9 19.1 27

experiments 1 and 2 and different irradiances in experiment 3 (Table 1). In all treatments an 8 h night and a 16 h day was used. The natural diurnal variation in light intensity was simulated as far as possible using the available set-points. The relative air humidity was kept at 80% for PPFDs below 500 ^mol m " 2 s~' and at 70% for higher PPFDs. Measurements

Samples of one plant per pot were taken weekly or bi-weekly. Thus plant population densities in the pots were successively reduced. The first sample was taken at the onset of the experimental treatments. One plant per pot was cut at the soil surface. The sampled plants were chosen as to minimize gaps in the canopy. Each plant was dissected into stem, leaf blades and midribs or stalks. The weight of the individual organs was determined before and after oven-drying at 80 CC for 24 h. Areas of leaf blades and of midribs and stalks were determined before oven-drying by passing the material through a LiCor LI-3100 area meter. All measurements were recorded for individual plants. There was a linear relationship between leaf blade area and area of stalks and midribs. The fraction of stalks and midribs of the total leaf area was estimated as 8.4%

(^ = 0.97, df=73) for cauliflower and 9.4% (^ = 0.93, df=71) for broccoli. Table 2 shows the experimental treatments and the size of the plants at the onset of the experiment. The number of plants was lower than planned for broccoli in experiment 1 due to the low emergence rate, and for cauliflower and broccoli in experiment 2 due to the variation in emergence time. The plants were slightly larger in experiment 2 compared to experiment 1. In experiment 3 the broccoli plants were larger than for the other experiments. The later plant samples in experiment 3 in chambers 1 and 2 showed discolouring of the leaves indicating that other factors were limiting the growth of leaf area and dry matter. These data were therefore excluded from further analysis including parameter estimation, except for the analysis of the extinction coefficient which showed no statistically significant influence of irradiance. For cauliflower all data in chambers 1 and 2 after 28 d of experimental treatment were excluded. For broccoli in chamber 1 data after 28 d and in chamber 2 after 20 d of experimentation were excluded. In experiment 3, the surface temperature of the leaves was measured using an AGA Thermopoint 80 infrared thermometer. Measurements (25 replicates for each crop and light set-point) were taken on day 28 after the onset of the experimental treatments. The air temperature was controlled at 16 °C. The difference between surface and air temperature increased linearly with PPFD. The mean surface temperature of cauliflower plants varied from 13.6 °C at the lowest light level to 18.5 °C at the highest light level. The broccoli plants had slightly higher surface temperatures than the cauliflowers, especially at higher PPFDs. This may be associated with the discolouring of the leaves, which was more pronounced in broccoli than cauliflower at the time of measurement. The linear relationship between PPFD and the difference between surface and air temperature for cauliflower was used for both crops to convert the air temperatures to a mean daily surface temperature taking the diurnal variation in PPFD into account and assuming no temperature difference during dark. The calculated mean surface temperatures are shown in Table 1. The light interception of the crops was determined in experiment 3. A LiCor line quantum sensor was modified to an effective measurement length of 40 cm by covering the remainder of the sensor with black tape with no light transmission. All measurements were taken at the same height within the growth chambers. For each pot two measurements were taken without the crop cover, and then six measurements at evenly spaced points across the length of the pot by inserting the line quantum sensor from the wider pot side. These measurements were

Table 2. Date of sowing and emergence and mean plant characteristics at the onset of experimental treatments The leaf area comprises leaf, midrib and stalk area. The number of plants per pot does not include the initial sample. The values in brackets are standard errors of mean. Experiment

1 2 3

Crop

Cauliflower Broccoli Cauliflower Broccoli Cauliflower Broccoli

Sowing

1 Feb 94 30 Mar 94 12 Sep 94

Emergence

11 Feb 94 9 Feb 94 7 Apr 94 6 Apr 94 23 Sep 94 20 Sep 94

Start of experiment Date

Plants per pot

Large leaves

Leaf area per plant (cm2)

Dry weight per plant (g)

10 Mar 94

5.8 3.8 5.3 5.2 9.3 9.4

4.0 4.2 5.2 5.0 4.1 4.8

105(11) 127 (20) 214(14) 250 (26) 143 (7) 249(12)

0.87 (0.13) 0.72 (0.15) 1.23(0.11) 1.13(0.11) 0.92 (0.07) 1.62(0.11)

6 May 94 24 Oct 94


Experiment 1 Night/day air temperature [°C] Mean air/surface temperature [°C] Daily PAR [mol m ' 2 ] Duration of experiment [d] Experiment 2 Night/day air temperature [°C] Mean air/surface temperature [°C] Daily PAR [mol m~2] Duration of experiment [d] Experiment 3 Night/day air temperature [°C] Mean air/surface temperature [°C] Daily PAR [mol irT 2 ] Duration of experiment [d]

1

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carried out eight times during the experiment. The CV of individual measurements beneath the crop cover was 38%. The extinction coefficient was calculated from the mean PPFD without plants Qo and beneath plants Qp by applying the following equation:

Statistical analyses

Daily temperatures and irradiance were kept constant within each experimental treatment. This made it possible to calculate growth rates by fitting time-dependent functions to the appropriate variables. Mean sample values were used in the parameter estimation. A logistic equation (Thornley, 1990) was used to describe the development of leaf area with time: A=

+ exp(a-bt)

(4)

where A is leaf area per plant including stalks and midribs, / is number of days from onset of the experimental treatment, Ax is maximum leaf area, a is an offset parameter and b is the relative leaf area expansion rate. The a parameter is given by \n[(Ax—Ao)/Ao}, where Ao is leaf area at the start of the treatments. The logistic equation generally fitted leaf area data from all experiments well as illustrated in Fig. 1. It was not possible to estimate a proper value of Ax, as the data did not

=

bx(To-\T-To\-TbA)+/(To-TM)

(5)

where TbA is the base temperature, and bx is the maximum expansion rate. The suffix + denotes that only positive contributions are considered. The response of radiation conversion coefficient (e) to temperature was described as a linear increase to a maximum value (e,) above the threshold temperature (Tt) followed by a constant value: 0

(6)

where 7^ is the base temperature. An asymptotic exponential function was used to describe the effect of irradiance on radiation conversion coefficient: (7)

where Q is PPFD [mol m 2 d *], Px is production of aboveground dry matter at light saturation [g m~2 d" 1 ], and