Calcium Can Improve Rooting of Rhododendron
(Azalea) 'Red Wing' Cuttings
A. T. Leiser and J. L. Paul
Department of Environmental Horticulture, University of California, Davis, California
Research in plant propagation in the past 35-40 years has been dominated by uses of auxins and mist. Little attention has been given to the chemical nature of the rooting medium. An early study by Hitchcock (1928) found that rooting was affected by pH of the medium.
All members of the family Ericaceae have been long reputed to be acidophiles (acid-lovers) or calciphobes (calcium haters). The literature has not been in agreement as to why this is so. Some workers attribute the cause to the effect of pH on the supposed obligate symbiotic mychorriza, some to the supposed preference of ericads for ammonia nitrogen over nitrate nitrogen (conversion of ammonium ion to nitrate ion increases with increasing pH), some to a supposed "poisoning" by calcium ion and some to supposed direct influence of the hydrogen ion.
The senior author (Leiser, 1949) demonstrated severe calcium deficiencies in a broad group of ericads grown in Canadian sphagnum peat and watered with distilled water. Later he (Leiser, 1959) demonstrated that high levels of calcium (4 times nutrient solution concentrations) did not seriously inhibit growth of azaleas if free carbonate was not present. The idea persists that rhododendrons respond best when calcium is at very low levels in the growing medium.
This paper reports on the effect of calcium on rooting of Rhododendron 'Red Wing'.
Preparation of peats
A British Columbia sphagnum peat was treated with Ca (OH2) to saturate the exchange sites with calcium. Following leaching, the peat was dried. Another peat sample leached with distilled water was prepared as a hydrogen-peat. These two peats were blended to give peats of approximately 0, 10, 20, 30, 40, 60, 80 and 100% calcium saturation. These peats were wetted and packed uniformly into flats. After cuttings were planted, moisture content was maintained by weighing to bring the flats to their initial weights. Cutting preparation and planting
Terminal cuttings, three to four inches in length, of Rhododendron (Azalea) 'Red Wing' were taken. Basal leaves were stripped, and cuttings were sorted into height uniform lots of 50. Cuttings were dipped in methyl mercury hydroxide ('Panogen', at 5 drops per gallon), dipped in 4000 ppm IBA (50-50 water-ethanol) solution and planted on October 12.
Flats were placed in an outdoor bottom-heated propagating bench covered with polyethylene film. Temperature of the peat was maintained at 65-70°F.
Cuttings were examined periodically and those with more than one cm. of total root length were harvested. Those with less than one cm. were replanted. This was done to separate effects of media on time of root initiation and growth. Data on root numbers and root length were taken on November 21 and December 14.
Properties of The Media
The hydrogen ion content of the untreated peat was found to be 139 milli-equivalents (meq.) per 100g by titration to pH 7. Total exchange capacity was 144 meq. per 1008 peat determined by the ammonium acetate method. The discrepancy was due to small amounts of Ca and Mg in the peat. Only trace levels of Na and K were present.
The calcium peat contained 130 meq. of exchangeable Ca and, therefore, complete Ca saturation was not obtained. Instead of the theoretical values of 0, 10, 20, 30, 40, 60, 80 and 100% Ca saturation actual values were. respectively, 2.7, 11.3, 19.9, 28.5, 36.0, 54.2, 71.4 and 88.5. (See Table 1).
TABLE 1. Exchangeable ion percentages, soluble calcium and pH of calcium peat-untreated peat blends. % Calcium
Ca2+ H+ Ca2+-me/l pH 0 2.7 94.6 0.124 4.38 10 11.3 86.0 0.48 4.40 20 19.9 77.4 4.67 30 28.5 68.8 4.75 40 36.0 61.3 0.80 5.09 60 54.2 43.1 0.93 5.60 80 71.4 25.9 5.91 100 88.5 11.5 1.49 7.00 *Determined in solution expressed from peats moistened to 528% gravimetric moisture content.
Air-filled porosity, density and moisture content of all peats were thought to be optimum.
Data taken were cumulative percentage rooting at each harvest, mean number of roots per cutting (R/C) and mean root length (RL). Results were similar at both harvests so for simplicity, data from the first harvest only are discussed here. Total root length per cutting derived from R/C x RL is used to make comparisons.
Rhododendron (Azalea) 'Red Wing'. Rooting at 40 days varied between 60 and 70% for the 2.7-54.2% Ca treatments and fell to 40% at the two highest Ca treatments. At 63 days rooting varied between 80 and 100% with lowest percentages at the three highest levels. Some browning of root tips occurred at the 2.7. and 11.31/. Ca levels and browning of stem tips was present in all treatments but was most severe at the 2.771 Ca.
Root length response was erratic (Fig. 28), though a downward trend beyond 54.2% Ca was indicated. R/C increased from 24 at 11.3% Ca to 40 at 71.4% Ca then decreased to 28 at the highest Ca level. Total root length per cutting (RL x R/C) was depressed at the two highest Ca levels, a result that might be expected from the acid-loving character Ericaceae. However, it is indeed interesting that this species was capable of rooting quite well over the whole range of Ca concentrations imposed, considering the widely held view that Ericaceae are calciphobes. And further, total root length reached a maximum at a Ca saturation of 54.2% Ca! These results though are in agreement with those of Leiser (1949) who found severe Ca deficiency symptoms in Ericaceae grown in pure peat with distilled water and who also found (Leiser, 1959) that Rhododendron (florists azaleas) made excellent growth at high Ca levels if free calcium carbonate (lime) was not present.
Fig. 28. Mean number of roots per cutting and mean root
length for azalea in relation to exchangeable Ca of sphagnum
The calcium requirements for rooting of Rhododendron 'Red Wing' are apparently much less (Paul and Leiser 1968) than for some other woody species. However, greatest total root length was obtained at calcium saturation of 54.2% (pH 5.60), and relatively good rooting occurred at a calcium saturation of 88.5% (pH 7.00).
Of the processes which take place during rooting, only root extension has been shown to be affected by pH and Ca concentrations of the media. Under acid conditions root growth seems to be particularly affected by Ca concentration, as has been shown in solution culture experiments by Arnon (1942) and Burstrom (1952). As pH is lowered, the Ca concentration required for normal root growth increases. The calcium requirement of R. 'Red Wing' is apparently low which supports the view of Leiser (1959) that Ericaceae are very efficient calcium absorbers.
At least four other processes take place during rooting, namely expansion and proliferation of cells, organization of primordia, growth of primordia, and emergence. Their dependence upon Ca concentration and pH is difficult to determine and has not been discussed in the literature. In this laboratory we found (unpublished data) that chrysanthemum cuttings, grown in peats of low Ca saturation, lost Ca from the stem in the zone of rooting. This loss was accompanied by an increase in numbers of root initials but an almost complete suppression of root growth.
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