pH Affects Seed Germination of Eight Bedding Plant Species

Seeds of eight commonly grown bedding plant species [Ageratum houstonianum Mill., Begonia × semperflorens Hort., Impatiens wallerana Hook., Lobularia maritima (L.) Desv., Petunia × hybrida Hort., Pelargonium hortorum L.H. Bailey, Salvia splendens F. Sellow, Tagetes patula] were germinated at pH values from 4.5 to 7.5 at 0.5 increments. Seeds were germinated in petri dishes on filter paper saturated with buffer solutions or in petri dishes containing a 50 sphagnum peat: 50 coarse vermiculite (peatlite) medium moistened with buffer solutions. Germination on filter paper was affected by pH for all species tested. Peatlite medium pH affected germination of all species tested, except Salvia splendens. Species response to similar pH values differed between the two germination procedures. Total percent germination of seeds germinated was less in peatlite medium than on filter paper. . The influence of hydrogen ion concentration (pH) on germination has received little attention in horticulture. Most information reported on the effects of pH on germination is for field crops, weed seeds, conifers, and broad-leaved trees. pH values lower than 3 and higher than 8 have been reported to inhibit germination (Jansen and Cronin, 1953; Malhotra, 1930; Salter and McIlvaine, 1920), while other reports suggest that germination of many species is unaffected by pH 3 to 7 (Chan, 1937; Schmidt, 1927). An optimum pH is reported for some species (Jansen and Cronin, 1953; Malhotra, 1930; Peel, 1949). An optimum pH of 6.0 is reported for sunflower (Helianthus annus) and tomato (Lycopersicon esculentum) when germinated between layers of filter paper in petri dishes (Malhotra, 1930; Peel, 1949). Germination of tomato and petunia (Petunia × hybrids) was not affected when seeds were irrigated with water at various acidities (Bailey and Hammer, 1986). Heather (Calluna vulgaris) germinated best at pH 4.0 on an agar medium containing Knop’s solution (Peel, 1949). A slightly acidic condition favored germination of several forage crops (Promsy, 191 1). Alfalfa, corn, red clover, soybeans, and wheat seeds placed on filter paper in petri dishes showed no statistical differences in germination for pH 3 through 8, although a slightly Received for publication 12 Dec. 1988. Michigan Agr. Expt. Sta. J. Article no. 12920. This study was partially funded with a grant from the Western Michigan Bedding Plant Growers Assn. We express our appreciation to the Ball Seed Co. for the donation of seed. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. lResearch Associate. Current address: Dept. of Horticulture, Virginia Polytechnic Inst. & State Univ., Blacksburg, VA 24061-0327. 2Professor. 762 acid condition tended to be favorable (Salter and McIlvaine, 1920). A growth medium pH of 5.5 to 7.0 is recommended for producing bedding plants (Carlson and Rowley, 1980). The optimum pH for bedding plant seed germination is not known, although a pH of 5.2 to 6.0 is recommended (Koranski, 1985). The small volume of growth medium used in plug production can cause wide fluctuations in moisture content, aeration, soluble salt levels, nutrient levels, and pH (Koranski, 1985). The objective of this study was to determine if an optimum pH range exists for the germination of eight plant species commonly grown as bedding plants. Species and cultivars tested were: A . houstonianum ‘Adriatic’ (ageratum), B. × semperflorens ‘Scarlanda’ (begonia), L wallerana ‘Fantasia Red’ (impatient), L. maritima ‘New Carpet of Snow’ (alyssum), P. × hybrida ‘Old Glory Red’ (petunia), P. hortorum ‘Red Elite’ (seed geranium), S. splendens ‘Red Hot Sally’ (salvia), and T. patula ‘Queen Sophia’ (marigold). Experiments with two germination media were conducted twice in reach-in controlledenvironment growth chambers that were maintained day and night at 24C with continuous irradiance. Photosynthetic photon flux (PPF) was 50 μmol·s-1·m-2 provided by coolwhite flourescent lamps. PPF was measured with a LI-COR LI-185B meter and LI-190SB quantum sensor (LI-COR, Lincoln, Neb.). Filter paper as medium. Seeds were germinated in 100 × 15-mm petri dishes on saturated filter paper at pH 4.5 to 7.5 in 0.5 increments. Buffered pH solutions (1 ml) containing potassium phosphate (0.2 M) and calcium phosphate (0.2 M or 0.1 M) were applied daily to maintain saturation. One hundred seeds were used per petri dish for each species, except T. patula, for which we used 50. The experiment was conducted twice, with three samples per treatment each time it was conducted. The experiment was established using a randomized block design. Peatlite as medium. Seeds were germinated in 100 × 15-mm petri dishes containing a 50 sphagnum peat : 50 coarse vermiculite medium (peatlite) (v/v). Four-liter aliquots of medium were adjusted to the various pH levels by adding up to 200 g calcium carbonate. After the calcium carbonate was added, the pH was monitored to determine when the reaction stabilized. The pH was determined with a 1 medium : 1 distilled water mixture (v/v). After 48 hr, three consecutive pH measurements of similar readings indicated the reaction had stabilized and the experiments were initiated. Each petri dish contained 200 ml of medium. Medium pH was within ± 0.2 increments of their desired level at the beginning of the experiment. The pH of each petri dish was determined at the conclusion of the experiment. Medium pH was within ± 0.1 unit of the desired level at the conclusion of the experiment, except the target pH 5.5 medium, where actual pH was 5.9. The results from this treatment were excluded from analysis. At the onset of the experiment, 4 ml of deionized water was applied to the medium in each petri dish. Subsequent waterings were made by using 2 ml of solution buffered at the respective pH levels. Ten seeds were placed, uncovered, in each petri dish for each species. The experiment was conducted twice, with three samples per treatment each time, for a total of 60 seeds per species. The experiment was established using a randomized block design. Germination was counted when the cotyledons were fully expanded. Number of seeds germinated was recorded 5 days after sowing for ageratum, 8 days for marigold, 12 days for petunia and seed geranium, 14 days for alyssum, 18 days for impatiens, and 21 days for begonia and salvia. Observation of daily counts of germinated seeds on filter paper indicated that germination was not delayed by pH in the range evaluated, so analysis was conducted using final percent germination. Data were transformed using arcsin transformation and then subjected to analysis of variance and trend analysis. Low ( < 20%) or no germination was found in the following treatments: pH 4.5 and 5.0 for ageratum, begonia, alyssum, petunia, salvia, and marigold; pH 4.5 to 5.5 for impatiens; pH 4.5 for seed geranium placed on filter paper; and pH 4.5 for petunia and impatiens on peatlite. Trend analysis excluded data from these treatments to remove bias. Filter paper. Germination on saturated filter paper was dramatically affected by pH for all species tested (Table 1). Begonia, impatiens, alyssum, petunia, and salvia did not germinate at or below pH 5.0. Germination was very low for ageratum and marigold in this range. In the range where germination did occur, it was linearly related to pH for all species evaluated, except begonia and marigold (Table 1). For ageratum, begonia, impatiens, HORTSCIENCE , VOL. 25(7), JULY 1990 zGermination test is percent germination provided by seed supplier. yThe results from the peatlite experiment for PH 5.5 were excluded from analysis due to the +0.4 unit increase in PH during the experiment. x Missing data. NS,*,**,***Nonsignificant or significant at P = 0.01, 0.05 , or 0.001 , respectively, for the main effect