Expt. 1.

Rooted ‘Designer Dark Red’ (zonal) and ‘Galleria Bright Violet’ (ivy) geranium cuttings were fumigated for insects overnight with 0.5% Bifenthrin (Attain TR; Whitmire Micro-Gen Research Laboratories, St. Louis), planted the following day into 11-cm-diameter (606 cm³) azalea pots, and grown for 63 d at the North Carolina State University Phytotron. The substrate consisted of 3:1 (by volume) peat:perlite amended with MgSO₄, Micromax micronutrient mix (The Scotts Co., Marysville, OH), triple-superphosphate (19.7% P) at 0.59, 0.89, and 0.26 g·L⁻¹ substrate, respectively, and flowable dolomitic limestone (Cleary Chemical Corporation, Dayton, NJ) at 8.3 mL·L⁻¹ substrate. Treatments were arranged in a factorial design with four light treatments (105, 210, 575, and 1020 ± 25 μmol·m⁻²·s⁻¹) selected to range from sub- to superoptimal and two cultivars. There were three replications and two single plant pots per plot, resulting in a total of 48 plants. Plants in the three lowest light treatments were grown in growth chambers with 9 m² of growing area and 2.13 m vertical clearance. Light was provided by cool white fluorescent and incandescent lamps separated from the growing area by a polycarbonate barrier. The 210 and 105 μmol·m⁻²·s⁻¹ light treatments were achieved by using one and two layers of green shadecloth (army surplus mosquito netting), respectively. In the highest light treatment, plants were grown in chambers with 1.1 m² of growing area and 1.2 m vertical clearance. Light was provided by incandescent lamps and an equal ratio of metal halide-to-high pressure sodium lamps separated from the growing area by a polycarbonate barrier. The standard long-day photoperiod provided by the Phytotron was used. This light regime for all light treatments consisted of a 9-h photosynthetic light period, 6 h of darkness, a 3-h low-light night interruption, and 6 h of darkness. The 3-h low-light night interruption was provided by incandescent lamps and supplied a photosynthetic photon flux (PPF) of 25 μmol·m⁻²·s⁻¹. The day/night temperature in all chambers was 22/18 °C. Carbon dioxide concentrations were maintained between 300 and 400 μL·L⁻¹ by controlled injection of commercial grade gas. Plants were fertilized with each irrigation with a solution made from deionized water consisting of 1.4 mm NH₄NO₃, 3.6 mm Ca(NO₃)₂, and 4 mm KNO₃ (N, K, and Ca at 196, 156, and 144 mg·L⁻¹, respectively).

Expt. 2.

This experiment was similar to Expt. 1 except that it had one cultivar (‘Designer Dark Red’), three light treatments (210, 575, and 1020 ± 25 μmol·m⁻²·s⁻¹), five P treatments (0, 0.065, 0.13, 0.26, or 0.52 g·L⁻¹ substrate), and two single plant pots per plot. There were three replications, giving a total of 90 plants. Lime in the substrate was CaCO₃ powder (Fisher Scientific, Pittsburgh) supplied at 3.5 g·L⁻¹ because the flowable dolomitic limestone caused substrate pH to rise for the first 5 weeks during Expt. 1 (Fig. 1).

Substrate pH of geraniums measured every 7 d after transplanting (DAT) for ‘Designer Dark Red’ (A) and ‘Galleria Bright Violet’ (B) geraniums at very low (105 μmol·m⁻²·s⁻¹), low (210 μmol·m⁻²·s⁻¹), medium (575 μmol·m⁻²·s⁻¹), and high (1020 μmol·m⁻²·s⁻¹) light in Expt. 1. Error bars represent se (n = 3). On the right side of each light curve is the tissue dry weight percentage of P 63 DAT. Mean separation by Duncan's multiple range test at P ≤ 0.05.

Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 133, 4; 10.21273/JASHS.133.4.515

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Substrate pH of geraniums measured every 7 d after transplanting (DAT) for ‘Designer Dark Red’ (A) and ‘Galleria Bright Violet’ (B) geraniums at very low (105 μmol·m⁻²·s⁻¹), low (210 μmol·m⁻²·s⁻¹), medium (575 μmol·m⁻²·s⁻¹), and high (1020 μmol·m⁻²·s⁻¹) light in Expt. 1. Error bars represent se (n = 3). On the right side of each light curve is the tissue dry weight percentage of P 63 DAT. Mean separation by Duncan's multiple range test at P ≤ 0.05.

Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 133, 4; 10.21273/JASHS.133.4.515

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Substrate pH of geraniums measured every 7 d after transplanting (DAT) for ‘Designer Dark Red’ (A) and ‘Galleria Bright Violet’ (B) geraniums at very low (105 μmol·m⁻²·s⁻¹), low (210 μmol·m⁻²·s⁻¹), medium (575 μmol·m⁻²·s⁻¹), and high (1020 μmol·m⁻²·s⁻¹) light in Expt. 1. Error bars represent se (n = 3). On the right side of each light curve is the tissue dry weight percentage of P 63 DAT. Mean separation by Duncan's multiple range test at P ≤ 0.05.

Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 133, 4; 10.21273/JASHS.133.4.515

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Expts. 1 and 2 measurements.

Substrate pH was measured via the pour-through technique (Wright, 1986) every 7 d for the duration of both experiments, and immediately after each pour-through, plants were fertilized. In Expt. 2, pour-through extracts taken at 14, 28, 42, and 56 d after transplanting (DAT) were also analyzed for P. At 63 DAT, four to six recently mature leaves in both experiments and the oldest leaves in Expt. 2 were removed from plants, rinsed with 0.1 N HCl, and rinsed again with deionized water. Plant samples from all three experiments were dried in a forced-air oven at 60 °C for 48 h, weighed, ground in a Wiley mill through a 25-mesh screen (1-mm particle size), and the P content of both leaf samples and pour-through extracts were determined with inductively coupled plasma optical emission spectroscopy (IRIS-Intrepid II; Thermo Fisher Scientific, Waltham, MA). Pour-though P concentrations were not detectable (ND) below the lower limit of 11 μg·L⁻¹. All tissue concentrations are expressed on a dry weight basis. In Expt. 2, total shoot P was determined by multiplying the dry weight of the shoots by the average of the dry weight percentage of P of recently mature leaves and the oldest leaves. Experimental data were statistically analyzed as a complete block design with mean separation by Duncan's multiple range test at P = 0.05 using SAS (version 9.1; SAS Institute, Cary, NC).

Expt. 3.

‘Designer Dark Red’ geranium cuttings, partially rooted in Oasis® Wedge® growing foam (Smithers-Oasis North America, OH), were fumigated overnight, as in Expts. 1 and 2. The following day, they were placed in the same type of growth chambers used for the low- and medium-light treatments in Expts. 1 and 2. The temperature was 22 °C day/18 °C night. Plants were fertilized daily with the standard nutrient solution provided by the Phytotron. This solution contained N, P, K, Ca, Mg, Fe, S, Mn, B, Zn, Cu, Mo, and Na at 106, 10, 111, 54, 12, 5, 13, 0.1, 0.2, 0.01, 0.005, 0.005, and 11 mg·L⁻¹, respectively. After 12 d, 48 rooted cuttings were transplanted into 12 opaque plastic buckets containing 4 L of nutrient solution. Buckets were covered with plastic lids with four evenly spaced 2.5-cm-diameter holes to hold plants and were placed into two chambers that were the same type as the high-light chambers used in Expts. 1 and 2. The day/night temperature was 22 °C day/18 °C night, and one of the chambers was shaded with standard window screen with a mesh size of 7.1 × 6.3 per cm², giving ≈50% light exclusion for the control light treatment. The hydroponic containers were aerated and filled with solutions consisting of 1.5 mm NH₄NO₃, 5 mm Ca(NO₃)₂, 2 mm KNO₃, 1 mm KH₂PO₄, 1.5 mm KCl, 2 mm MgSO₄, 69 μm Fe from Fe-DTPA, 9.1 μm MnSO₄·H₂O, 0.76 μm ZnSO₄·7H₂O, 46.3 μm H₃BO₃, 1.57 μm CuSO₄·5H₂O, and 0.10 μm Na₂MoO₄·2H₂O. The volume of the containers was maintained by the addition of deionized water every 2 to 3 d.

This experiment was a factorial design with two light treatments (500 and 1100 ± 25 μmol·m⁻²·s⁻¹), two sampling dates (11 and 15 DAT), and three replications with four plants per plot. On each sampling date, plants were removed and roots were rinsed with deionized water so that all the solution remaining on the roots was washed back into the bucket. Plants were then separated into roots and shoots. Roots were dipped in 1 mm CaSO₄ for 30 s followed by deionized water to remove ions from the apoplast (Naegle et al., 2005), patted dry, and weighed. Shoots were weighed, rinsed with 0.1 N HCl, rinsed again with deionized water, and then both roots and shoots were dried and weighed again. All leaves with petioles were removed from the stems for tissue analysis as described previously. Roots were also analyzed. Solutions in buckets were brought back to volume with deionized water and were analyzed for P content to determine P uptake by roots. Phosphorus concentration of hydroponic solution was determined by the colorimetric method of Murphy and Riley (1962).

Expt. 1.

For all light treatments and both cultivars, pH increased from less than 5.3 to greater than 6.1 during the first 35 d (Fig. 1). This unforeseen increase in substrate pH could be a consequence of an unexpectedly slow dissolution of the preplant flowable dolomitic limestone. All interactions were significant for substrate pH and tissue percentage of P. Starting 49 DAT, substrate pH of plants grown at high light (1020 μmol·m⁻²·s⁻¹) were lower than plants from all other light treatments (Fig. 1). Substrate pH decreased to 4.78 for ‘Designer Dark Red’ and 4.80 for ‘Galleria Bright Violet’ by 63 DAT. At this point, substrate pH of ‘Designer Dark Red’ plants grown at very low light (106 μmol·m⁻²·s⁻¹) was 5.96 and greater than plants at the low (210 μmol·m⁻²·s⁻¹) and medium (575 μmol·m⁻²·s⁻¹) light levels, which had statistically similar pH values of 5.61 and 5.49, respectively. ‘Galleria Bright Violet’ plants at the very low, low, and medium light levels had statistically similar substrate pH levels of 6.12, 5.99, and 5.77, respectively.

Krug et al. (2006) determined that the lower end of the acceptable range for tissue dry weight P concentration in recently mature leaves for mature zonal geraniums is 0.17% to 0.19%. Although substrate pH decreased with increasing light, tissue P was deficient in both cultivars grown at medium and high light (Fig. 1). Acidification due to P deficiency is a widespread phenomenon among dicotyledonous species (Marschner, 1995). Phosphorus deficiency was also visually apparent in this experiment and was expressed as interveinal and marginal yellowing and reddening of the lower leaves.

It is unclear why the increasing light levels caused the observed drop in substrate pH. In addition to high light intensity and P deficiency, larger root systems producing more acidity is possible because dry weights of shoots increased with increasing light intensity for both cultivars (Table 1). A fourth possibility could be the increased number of irrigations (Table 2). As the irrigation rate increases, lime may have been leached more quickly, causing a more rapid drop in substrate pH.

Table 1.

Main effects of light and cultivar on dry weights of shoots of geranium plants 63 d after transplanting from Expt. 1.

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Table 2.

Number of irrigations applied to geranium plants in the very low, low, medium, and high light treatments in Expts. 1 and 2.

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Expt. 2.

All interactions were significant for substrate pH. At 7 DAT, substrate pH decreased with each increase in P rate for all light treatments (Fig. 2). Triple superphosphate was the P source and is composed primarily of monocalcium-phosphate hydrate. A saturated solution of monocalcium-phosphate-hydrate has a pH of 1.0 to 1.5, indicating higher rates will cause lower initial substrate pH (Khasawneh et al., 1978). The effect of P rate on substrate pH diminished after 7 DAT and was gone by 35 DAT. At all light intensities, substrate pH of plants grown without P began to decrease immediately and pH largely continued to decline for the length of the study. Plants that did not receive P had the lowest percentage of P in all cases and would be expected to have the highest acidification rate due to the severity of the P deficiency. This effect did not occur and is most likely due to reduced plant growth (Table 3). Small, severely stunted plants would have less effect on rhizosphere acidification than larger plants with mild to moderate P deficiency. Plants grown with zero P were small in overall size, the younger leaves were small and dark green, and the older leaves were reddish, chlorotic, and necrotic, and eventually turned completely necrotic and senesced.

Table 3.

Dry weight percentage of P of recently mature leaves, shoot dry weight, and total shoot P determined from the average P concentration of recently mature and oldest leaves from geranium plants grown at low, medium, and high light 63 d after transplanting for Expt. 2.

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Substrate pH of geraniums measured initially and every 7 d after transplanting (DAT) for each preplant treatment rate of P (0, 0.065, 0.13, 0.26, and 0.52 g·L⁻¹ P in substrate) at low (210 μmol·m⁻²·s⁻¹), medium (575 μmol·m⁻²·s⁻¹), and high (1020 μmol·m⁻²·s⁻¹) light intensities in Expt. 2. Error bars represent se (n = 3).

Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 133, 4; 10.21273/JASHS.133.4.515

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Substrate pH of geraniums measured initially and every 7 d after transplanting (DAT) for each preplant treatment rate of P (0, 0.065, 0.13, 0.26, and 0.52 g·L⁻¹ P in substrate) at low (210 μmol·m⁻²·s⁻¹), medium (575 μmol·m⁻²·s⁻¹), and high (1020 μmol·m⁻²·s⁻¹) light intensities in Expt. 2. Error bars represent se (n = 3).

Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 133, 4; 10.21273/JASHS.133.4.515

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Substrate pH of geraniums measured initially and every 7 d after transplanting (DAT) for each preplant treatment rate of P (0, 0.065, 0.13, 0.26, and 0.52 g·L⁻¹ P in substrate) at low (210 μmol·m⁻²·s⁻¹), medium (575 μmol·m⁻²·s⁻¹), and high (1020 μmol·m⁻²·s⁻¹) light intensities in Expt. 2. Error bars represent se (n = 3).

Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 133, 4; 10.21273/JASHS.133.4.515

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At 63 DAT under low light, substrate pH of plants grown with 0.065 g·L⁻¹ P was 5.30 and lower than the substrate pH of all other plants (Fig. 2). Plants grown at low light and the 0.065 g·L⁻¹ P rate had recently mature leaf percentage of P of 0.14 at 63 DAT, which is below the accepted lower limit (0.17%–0.19%) (Table 3). All other P treatments at low light, except zero P, had recently mature leaf percentage of P above 0.19% and had the highest substrate pH, suggesting low pH was due to P deficiency. At 63 DAT under medium light intensity, substrate pH of plants that received the highest or lowest P rate was greater than the substrate pH of all intermediate P rates. All plants that received intermediate P rates had recently mature leaf percentage of P that was below the lower limit, which again indicates that P deficiency caused substrate acidification (Table 3). The effect of P rate on substrate pH at high light intensity again showed the same effect. At 63 DAT, when the 0 P rate was excluded, substrate pH decreased from 5.74 to 4.87 as the P rate decreased. Additionally, tissue P was deficient in all treatments except at the 0.52 g·L⁻¹ P rate.

The light treatment × P rate interaction was significant for dry weight percentage of P of recently mature leaves (P ≤ 0.0152), shoot dry weight (P ≤ 0.0001), and total shoot P (P ≤ 0.0001) (Table 3). Phosphorus concentration of recently mature leaves was significantly lower when plants were grown at high light compared with low light at all P rates and was lower than when plants were grown at medium light at the 0.065 and 0.52 g·L⁻¹ P rate. This suggests that high light intensity suppressed P uptake. The dry weight percentage of P of older leaves was unaffected by the light × P rate interaction and also decreased significantly as light intensity increased (Table 4), which again suggests that high light suppressed P uptake. Phosphorus content of older leaves also increased significantly with P rate.

Table 4.

Main effects of light and P rate on dry weight percentage of P of oldest geranium leaves 63 d after transplanting from Expt. 2. Plants from the 0 P rate were insufficient for sampling.

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Shoot dry weight was unaffected by light treatment when plants received no added P. At all other P rates, plants grown at low light weighed less than plants grown at medium and high light (Table 3). Plants grown at medium light weighed less than plants grown at high light at the 0.065 and 0.26 g·L⁻¹ P rate. In contrast, the total amount of P in shoots was generally not significantly affected by light treatment (Table 3). Thus, with the exception of low light at 0.52 g·L⁻¹ P and high light at 0.13 g·L⁻¹ P, the rate of P uptake per gram dry weight plant decreased with increasing light intensity. Because increased P uptake did not correspond with increased growth, P concentration may have been lower due to growth dilution when plants increased in size due to higher light intensity.

An additional factor that could have caused lower tissue percentage of P at higher light intensities was the higher number of irrigations required with increasing light intensity (Table 2). A higher number of irrigations could have caused increased leaching of preplant P, making less P available for plant uptake. However, this does not appear to be true because the amount of P in pour-through substrate extracts averaged overall dates was unaffected by light treatment with the 0.065 and 0.52 g·L⁻¹ P treatments (Table 5). Also, at the 0.13 g·L⁻¹ P treatment, substrate extracts from plants grown at high and medium light intensity were statistically similar. At the 0.26 g·L⁻¹ P rate, pour-through extract P concentrations did decrease significantly with increasing light intensity. This trend is apparent with other P treatments when statistics are not applied. The possibility that increased irrigations was responsible for lower tissue P seems likely; however, the effect of light on tissue P is more assertable than the effect on soil P. Phosphorus in pour-through soil extracts was greatest at 14 DAT for all P rates except zero, which did not change. Because higher light intensity indirectly increases irrigation frequency and potentially causes greater lime leaching, this could also be a contributing factor to SPD in a commercial setting.

Table 5.

Main effects of light and days after transplanting (DAT) on P concentration in pour-through substrate extracts from geranium in Expt. 2.

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Expt. 3.

The data from Expts. 1 and 2 indicate that P deficiency causes substrate acidification by geranium and also indicates that high light intensity suppresses P uptake by roots. Expt. 3 verifies that high light intensity suppresses P uptake. There was no significant effect of light treatment on pH of hydroponic solution, shoot and whole plant dry weight, or root and shoot percentage of P. Mean values at high (1100 μmol·m⁻²·s⁻¹) and control (550 μmol·m⁻²·s⁻¹) light levels were 5.68 and 5.34 for pH, 7.75 and 6.61 g for shoot dry weight, 8.80 and 7.38 g for total dry weight, 1.48% and 1.48% for root P, and 0.60% and 0.62% for shoot P, respectively. When plants were grown at the control light intensity, root dry weight was significantly lower than plants grown at high light (Table 6). Cumulative P uptake per gram root and rate of P uptake per gram root per day were significantly lower when plants were grown at the high light intensity compared with the control (Table 6). In two studies with tomato and one with boston fern, increasing light from 250 to 750, 70 to 300, and 50 to 150 μmol·m⁻²·s⁻¹, respectively, caused P uptake to increase (De Groot et al., 2001; Magalhaes and Wilcox, 1983; Nowak et al., 2002). A study by Ming et al. (1999) showed that P uptake by Dendrobium nobile Lindl. increased 74% when light intensity increased from 85 to 335 μmol·m⁻²·s⁻¹ but decreased 29% when light intensity increased from 335 to 840 μmol·m⁻²·s⁻¹. These data indicate that increasing light intensity causes P uptake to increase, but there is a limit to this effect. Once this limit is reached, increasing light has a negative effect on P uptake. Expt. 3 verified this effect of supraoptimal light with geranium.

Table 6.

Main effect of light treatment and days after transplanting (DAT) on geranium root dry weight, the remaining P of 124 mg in 4 L of hydroponic solution, cumulative P uptake per gram dry weight of root, and P uptake per gram dry weight of root per day in Expt. 3.

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The main effects of DAT are also shown in Table 6 and as DAT increased, root dry weight increased and P remaining in the hydroponic tanks, cumulative P uptake per root weight, and P uptake per root weight per day decreased.