Many ornamental greenhouse crops are grown under some form of stress or growth regulation that limits the full potential for their shoot volume production. This is universally true for young plants produced in plug seedling trays and bedding plant flats where limited horizontal space tends to increase vertical growth. Commercial methods of shoot control have included restricted watering (Schnelle et al., 1993), low fertility, which mainly translates into N stress (Styer and Koranski, 1997), temperature manipulation (Berghage and Heins, 1991), and chemical growth retardants (Gaston et al., 2001; Tayama et al., 1992). The respective problems associated with these procedures in the order just presented are desiccation of tissue and possible leaf abscission, chlorotic foliage, and high cost of growth regulators and their very limited registration for use on vegetable seedlings.
In addition to the four forms of shoot control, greenhouse production managers often use the form of N to control shoot size (Styer and Koranski, 1997). The belief is that fertilizers with high proportions of NO3– produce compact shoots (smaller leaves and shorter internodes), whereas those with high proportions of NH4+ yield large shoots. This is achieved by using high NO3– fertilizers such as 15N–0P–12.5K, 14N–0P–11.6K, and 17N–0P–14.1K formulated from combinations of calcium nitrate [Ca(NO3)2], potassium nitrate (KNO3), and magnesium nitrate [Mg(NO3)2]. Our experience with forms of N has not shown a relationship with shoot size. Because high NO3– fertilizers typically contain little or no phosphate, we hypothesized that it is a low phosphorus (P) stress brought on by using these fertilizers that accounts for suppression of shoot growth rather than their high proportion of NO3–.
Plant species vary greatly in their response to NO3– and NH4+ (Haynes and Goh, 1978). There are reports of 100% NO3– limiting plant growth in some species, including Ageratum houstonianum Mill. (Jeong and Lee, 1992), carnation (Dianthus caryophyllus L.) (Green et al., 1973), and wheat (Triticum aestivum L.) (Cox and Reisenauer, 1973). In contrast, most species grow better with mixtures of NH4+ and NO3–, with up to 50% of N in the NH4+ form (Gaffney et al., 1982; Jeong and Lee, 1992; Schrock and Goldsberry, 1982). Minimal effect of the NO3– to NH4+ ratio on plant shoot growth had been observed on Impatiens wallerana Hook F. (Argo and Biernbaum, 1997) and Lobelia erinus L. ‘Cobalt’ (Jeong and Lee, 1992). These variable results could be the result of an unchecked impact of the fertilizer NH4+:NO3– ratio on root substrate pH and subsequent effects on nutrient availability.
Bedding plant growth can also be controlled by limiting the supply of phosphate in root media (Gibson et al., 2007; Huang and Nelson, 1994). Plants receiving a low level of phosphate resulted in compact shoots. It was found that growth inhibition by P deficiency is selective, affecting shoots much more than roots for many species, resulting in an often-desirable higher root-to-shoot ratio (Goldstein et al., 1988; Huang and Nelson, 1994; Loneragan and Asher, 1967; Trull et al., 1997).
The objective of this study was to assess the relative effects of NO3– to NH4+ ratio and phosphate supply, as found in high NO3– fertilizers, on vegetable and bedding plant seedling shoot growth.
Argo, W.R. & Biernbaum, J.A. 1997 Lime, water source, and fertilizer nitrogen form affect medium pH and nitrogen accumulation and uptake HortScience 32 71 74
Berghage, R.D. & Heins, R.D. 1991 Quantification of temperature effects on stem elongation in poinsettia J. Amer. Soc. Hort. Sci. 116 14 18
Cataldo, D.A., Haroon, M., Schrader, L.E. & Youngs, V.L. 1975 Rapid colorimetric determination of nitrate in plant tissue Commun. Soil Sci. Plant Anal. 6 71 80
Cox, W.J. & Reisenauer, H.M. 1973 Growth and ion uptake by wheat supplied nitrogen as nitrate, or ammonium, or both Plant Soil 38 363 380
Gaffney, J.M., Lindstrom, R.S., McDaniel, A.R. & Lewis, A.J. 1982 Effect of ammonium and nitrate nitrogen on growth of poinsettia HortScience 17 603 604
Gibson, J.L., Pitchay, D.S., Williams-Rhodes, A.L., Whipker, B.E., Nelson, P.V. & Dole, J.M. 2007 Nutrient deficiencies in bedding plants Ball Publishing Batavia, IL
Goldstein, A., Baertlein, D. & McDaniel, R. 1988 Phosphate starvation inducible metabolism in Lycopersicon esculentum. I. Excretion of acid phosphate by tomato plants and suspension-cultured cells Plant Physiol. 87 711 715
Green, J.L., Holley, W.D. & Thaden, B. 1973 Effects of the NH4 +:NO3 – ratio, chloride, N-serve, and simazine on carnation flower production and plant growth Proc. Fla. State Hort. Soc. 86 383 388
Loneragan, J. & Asher, C. 1967 Response of plants to phosphate concentration in solution culture: II. Rates of phosphate absorption and its relation to growth Soil Sci. 103 311 318
Murphy, J. & Riley, J.P. 1962 A modified single solution for the determination of phosphate in natural waters Anal. Chim. Acta 27 331 336
Schnelle, M.A., McCraw, B.D. & Dole, J.M. 1993 Height control for flowering and vegetable transplants Oklahoma State Univ. Extension Facts No. 7614.
Schrock, P.T. & Goldsberry, K.L. 1982 Growth responses of seed geranium and petunia to N sources and growing media J. Amer. Soc. Hort. Sci. 107 348 352
Tayama H.K., Larson R.A., Hammer, P.A. & Roll T.J. 1992 Tips on the use of chemical growth regulators on floriculture crops Ohio Florists’ Assoc. Columbus, OH
Trull, M.C., Guiltinan, M.J., Lynch, J.P. & Deikman, J. 1997 The responses of wild-type and ABA mutant Arabidopsis thaliana plants to phosphorus starvation Plant Cell Environ. 20 65 92