PROBLEMS IN SOILLESS GREENHOUSE CULTIVATION OF PROTEACEAE IN FRENCH MEDITERRANEAN REGION

P. ALLEMAND*, M. MONTARONE, R. BRUN Institut National de la Recherche Agronomique. Unité de Recherches Intégrées en Horticulture. Route des Colles, Sophia-Antipolis. 06410 BIOT FRANCE

Summary:

The adaptation of Proteaceae culture to soilless technique requires the definition of proper nutrient solutions and the finding of adequate substrates for this type of plants. However, it can bring considerable changes in quantitative aspects of their development, flowering and vegetative cycles.

During this preliminary stage, the following questions had to be solved: - relative N.P.K. concentrations and K/(Ca+Mg) ratio at equilibrium conditions; - chemical compatibility of the substrate with respect to excretions from proteoid roots;

these, being acide, can attack natural substrates and modify the medium around the rhizosphere;

- respect of proper growth cycle of the plant; vegetative and floral stages imply different water and mineral requirements.

- pruning and harvesting modes most adapted to architectural features of the species. - level of physiological maturity for flowering.

This paper gives the main results of different trials realised in the south of France for the cultivation of Proteaceae and explains the problems which are unsolved.

1. Introduction

Production of Proteaceae can be part of horticultural diversification in south of France since climatic analogies exist between this region and the native areas of this family. Unfortunately, Proteaceae used in floriculture (mainly native of Southern Africa) are not sufficiently frostresistant to accept winter climatic conditions in south of France. The acclimatation trials, realized in botanical garden, confirm this.

Moreover, Proteaceae do not stand limestone and most soils in south of France are calcareous clay soils.

So, if we want to grow Proteaceae in this region, we must grow them in soilless greenhouse conditions. This is an expensive way for growing them, but the higher cost induced by soilless greenhouse cultivation must be redeemed with an increase in production and quality as it is generally observed for the other plants.

The increase in production requires a good knowledge of the growth and the development of these plants. They are not modified by the soilless cultivation in their principles but only in their potentialities of expression. Thus, pruning should be modified compared to those of plants grown in the field conditions.

Lastly, some species or cultivars having their flowering induced by external factors (thermo or photoperiod), it is necessary to adjust the fertirrigation (composition, frequency) to have a synchronism between the physiologic or morphologic maturity and the inductive conditions. The internal climatic conditions of the greenhouse should satisfy so the thermic requirements of the different species.

Acta Horticulturae 408, 1995 Soilless Cultivation Technology for Protected Crops

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The objective of this paper is to show the studies now realised in the south of France to give to the growers the possibility to include these new species in their cultures and so to diversify their production. The results come from different trials begun in 1986. We don't give the detail of each trial but give the most interesting points we have obtained.

2. Definition of a nutrient solution

This work was thus intended, first, to develop a nutrient solution fitted to soilless culture, under cold greenhouse, of most Proteaceae species. The starting point of this trial was derived from the available litterature. In fact, several works stress phosphorus as being an essential element for the success of a culture. CLAASSENS (1980) reported that nutritional needs of Proteaceae species grown for cut flowers were ill defined, but that the proteoid roots were particularly considered for their role in the growth and development of plants (BRITS, 1984 LAMONT, 1983 - VOGTS, 1989); CLAASSENS reports also that phosphorus can reduce, in some cases, the formation of roots of Leucospermum grown on sand. ELLYARD et al. (1978) show that phosphorus is a factor for the mortality rate of Banksias in West Australia. According to BARROW (1977), a weak phosphorus concentration in the leaves of some of the species is necessary to achieve maximum photosynthesis. NICHOLS et al. (1979) could evidence some phosphorus toxicity effects on a few Australian Proteaceae; they also showed that a high calcium content worsened leaves necroses induced by high phosphorus contents, whereas high nitrogen and potassium levels reduced them (NICHOLS et al. 1981). Finally, it is generally admitted that a high phosphorus content can be noxious for Proteaceae in some cultural conditions

With these comments in mind, we paid more special attention to the relative ratios of N, P, K, the main mineral elements responsible for the nutrition equilibrium

For each of the 3 mineral elements, a factorial test involving Fisher blocs with 2 factors was designed. Each element was varied in 3 ways and applied to 5 species: Protea cynaroides, P. eximia, P. lacticolor, P. neriifolia and P. obtusifolia. All of them derived from seeds originating from South Africa and were grown in containers on chemically inert siliceous sand. The greenhouse temperature was maintened above 12°C. The nutrient solution was brought by drippers, at a rate which varied according to the season and the vegetative stage of the plant. As ordinary tap water was used, the Ca and Mg contents were respectively 5 me and 1 me per liter and could not be decreased further. Oligo-elements were brought by means of a commercial solution at the rate of 200|j.g.l~l for Zn and Mn,100|ig.H for B, and 50 ng 1"' for Cu and Mo. Iron was brought as chelate at a concentration of 800 (J-g-H. Supplies in others elements, N, P, and K, were varied dependinq on the test.

The effect of each trial nutrient solution tested was appreciated in terms of stem elongation but also by the observation of any symptom that could indicate a disfunctioning of the plant growth. Results were treated with a variance analysis, and means difference was treated by the Newman and Keulz test.

Five successive tests were enough to obtain some accurate estimate of the adequate phosphorous content, the N.NH4 / N.N03, P/N, K/N ratios and, finally, the global salt concentration. This study enabled us to make considerable progress defining a nutrient solution suitable for growing soilless Proteaceae. A general fertilization proportion emerges from the whole set of observations, with N, P and K contents being proportional to 1-0.1-0.7. The N.NH4 N.N03 ratio is almost one to one and the global salt content amounts to 0.8 me.H.

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As this first attempt appeared satisfactory, a whole collection of 20 species was grown under soilless culture conditions.

3. Nutrition and vegetative rvthms

A trial made on Protea neriifolia had displayed the importance of mineral supply on the development as well as the needs in function of the growth periode. It was bedded in porphyry from the Esterel, a local and cheap substrate with a granulometric size of 5 to 8 mm. In this trial, 2 irriqation frequencies were applied to specify the importance of water supply and mineral elements. In the lower frequency rate (Fl), a mean value of 3 liters of nutrient solution is delivered in a day; the higher frequency rate (F2), the mean delivered volume is doubled. These inputs of course vary with the vegetative stage and the season. On this trial, it was kept track of the leachings and the results are reported in Table 1.

Table 1 : Leaching composition according the seasons and the frequencies pH EC(RS) N.N03- P04H2- N.NH4+ K+ Ca++ Mg++

Winter*4) input sol(1> freq. K2> freq. 2<3>

6,6 7,8 7,5

876 861 859

0,43 0,50 0,52

0,07 0,06 0,06

0,34 0 0

0,43 0 ,36 0,42

5 ,76 6,44 6,29

3,82 3,41 3,58

input sol. Springe) freq. 1

input sol. Summer freq. 1

input sol. Autumn freq. 1

4,2 6,9 5,8 4,4 6,7 ,6

296 3 9 7 299 195 340 2 1 2

0,51 0 , 2 2 0,36 0,41 0,06 0,34

0,08 0,02 0.03 0,07 0,03 0,03

0,34 0

J> 0,31 0 0

0,47 0,33

J) ,31 0 ,46 0,48 0,40

0 2 , 0 0 1,20_ 0 2,02 0,90

0 0,74

J), 54 o

0,67 0 , 2 8

4,8 248 7,7 359 7,5 330

0,61 0,52 0,64

0,07 0,03 0.04

0,37 0 0

0,47 0,40 0,39

0,63 1,35 0,98

0,34 0,55 0,46

(1) input sol. = solution supplied to the plant. (2) freq. 1 = leaching values corresponding to the low frequency irrigation F1. (3) freq. 2 = leaching values corresponding to the high frequency irrigation F2. (4) = during winter, the nutrient solution was made from tap water, which

explains the pH value and the high observed contents in Ca and Mg; at spring time the substrate was rinsed with pure (deionized) water, and the solution used later on was then free from Ca and Mg.

Even if the present nutrient solution is not yet perfectly suited to actual plant requirements, this table shows, first, distinct leaching compositions, depending on the irrigation frequency and, second, definited periods during which absorption becomes more important.

In winter and whatever the irrigation frequency, plants absorb 85% of the offered nitrogen and 80% of the offered potassium. In spring, the figure for N rises to 94% for the (low irrigation) Fl frequency and 90% for the (high irrigation) F2 frequency. The figures for K are around 85% for Fl and F2. In summer, the leachings are completely depleted from nitrogen for the Fl and 90% depleted for F2, while figures for K are 75% and 81%. At fall, N and K absorption rates get back to the winter values.

For calcium and magnesium found in the leachings during spring and summer, when their supplies were nil, we can explain this by a precipitation of nutrients in the substrate before these periodes, or by a degradation of the substrate by the input solution or the proteoid roots (see §4). If we compare the offered quantities and the leached quantities in autumn, the increase is 56% for Fl and 20% for F2. When the plants are supposedly at rest, this increase is still 36% for Fl and 23% for F2.

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4. Substrate-proteoid roots interaction

This interaction had been displayed in a trial initialy planned for a development study on Protea eximia.

The experimental set-up comprised 36 benches with 6 plants/bench. The dimensions of the benches were: L: 3.00 x 1: 0.90 x H: 0.18 m. The substrate was the porphyry of Esterel. Fertirrigation was brought by drippers. Each plot received a minimum of 10 liters of nutrient solution per day and square meter, inputs being increased as plants develop and according to the ReaRon

The composition of the nutrient solution, expressed in milliequivalent, is shown in Table2.

Table 2 : Composition of the nutrient solution tor P.eximia

m e i • N 0 3 " P O i H z - S O r ; I

K+ 0,313" 0 ,189 0 , 5 0 2 "

NH4+ 0 ,125 0 , 1 8 8 0 ,313

H+ I 0 ,313 0 ,125 0 . 3 7 7 0 ,815 Trace elements : Fe, 800 M~g.|-\ Zn and Mn, 200 ug.l'1, B, 100 ng.M, Cu and Mo, 50 ng.M. Tape water brings 7 me calcium, 3 me magnesium.

As a whole, all plants have had a satisfactory growth during the early 18 months of the trials.

Important chlorosis symptoms appeared later, as well as leaf reddening, which could lead to complete drying of the plant. Most treatments deviced to correct those effects were useless, except, though, the increase of irrigation frequencies. The trial was carried as follows:

1/3 of the containers were offered 131/day/m2of solution 1/3 of the containers were offered 261/day/m2 1/3 of the containers were offered 391/day/m2.

The percentage of plants which had not chlorosis symptoms, observed as a function of offered volume is shown in Figure 1. It clearly indicates that this percentage increases whith the increasinq of irrigation volume.

/ / /

-

./\i

. ¿ I t 13 26 39

Liters/day/m2

Figure 1 : Percentage of non chlorotic plants according to fertirrigation volume

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In order to explain this observation, substrate samples were extracted, either in the vicinity of the roots, or away from roots, between two plants. The results are shown in Table 3. This one indicates a very high salinity level in the whole medium, and underlines the higher salt concentrations in regions close to the roots, compared with regions away from the roots. Such high concentrations are most probably unacceptable to the plants.

Table 3: Substrate analysis

E C

mS.cnr1

Around roots 128.5 3.8

Total solubl e salts g.i"1 g.M 119.5 16.6 87.2 13.3

Soluble salts without S0 4 N.NOa N.NbU P Ca Mg

1000 600

18.0 16.1

21.8 10.9

1240 800

24400 1600

2800 2000

After GARNER's observations (1981) on the behavior of proteoid roots, one can propose some explanation to the present symptoms; if, as it is claimed, proteoid roots can strongly acidify their immediate surrounding, porphyry, a rhyolitic rock containing 65% alkaline feldspar, can undergo in such regions a chemical decay and free several kinds of minerals that alter the composition of the local solution. A sharp rise of pH will induce the precipitation of most metal cations and induce deficiencies and toxicity. Given the very high salt concentrations observed in the medium, plants were rinsed with pure water during a few weeks, until one couid detect a lack of N, P, and K in the leachings.

5. Development of plants

The trials, mainly realised on Protea eximia and P. neriifolia, had for objective to determine the construction and flowering ways resulting from the soilless protected cultivation. In these trials, we have included different growth ways (pinching on varied development stages) to determine their influence on the later development and floribundity as well as the productivity.

The treatments applied to P. eximia were the following (Figure 2):

- T1 = the plant is grown on one axis untill the first flowering and afterward pruneU at the top of the 5th growth unit. The following 2nd order axes are grown as T2 and T3.

- T2 = pinching of the seedlings at the top of the 3rd growth unit and disbudding of the 2nd order axes untill their flowering. Then, these axes are pruned at the top of their 2nd growth unit to obtain the 3rd order axes.

- T3 = pinching of the seedlings at the top of the 5th growth unit and disbudding of the 2nd order axes untill their flowering. The following growth is the same as for T2.

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c

6 Pruning

7

4

3

2

8

9

10 5

Pinching

3

2

T 1 T 2 T 3

Figure 2: Pinching and pruning systems according the treatments The treatments applied to P. neriifolia differed only for the T1 which was a free form, without any pruning. More, 2 fertirrigation frequencies were applied for this species.

These first results allow us to think, for the considered species, that: * the potentialities of expression of the ramification are on two levels which can act in

opposition:

- one the first order axes, the more developped the plant is at the time of pinching or pruning, the more branches it produces. - beyond the 2nd order, the higher the order of ramification is, the smaller the number of branches per axis becomes.

* similarly, the higher the order of ramification is, the smaller the number of growth units needed to flowering becomes (ALLEMAND, 1993). Without confirming the effect of the addition of the number of growth units on the axes of successive orders (though we have relatively a constant value of 11 growth units for P. eximia and 9 growth units for P. neriifolia), it is possible to state that the plant will only flower if the total number of growth units is included within a more or less restraint interval (GRANIER.,1992). We can consider that from an order n (not reached in the trial) only one growth unit on the ramification will become necessary to the flowering. It would be advisable to prune back to a lower level when stem length of flowers starts to decrease. Another possibility would be that a reiteration brings back juvenility to the ramification processs

* from the third axis, the floribundity seems to be only partly influenced by the croping system, meaning that a stem has stable potentialities to flower. Therefore, the floribundity of a plant is a function of the number of branches produced since the ratio of flowering branches/vegetative branches is relatively constant.

The new element showed by P. neriifolia is the improvement of the floribundity brought about by the increase in the watering frequencies (Le BRIS, 1993). This can be explained by a more reaular and more important availability of water and nutrients.

The experiments conducted on P. eximia and P. neriifolia show that these two species react the same way to soilless cultivation and to the treatments.

The way their branch and flower follows the same principles. These parameters are not modified when plants are grown in soilless conditions. Their potentialities are even

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increase compared to those of open field cultivations.

The fact that the number of ramifications produced on a given axis decreases with the order of this axis, and that, for the first orders, this number increases with the age and perhaps the reserve nutrients of the plant, shouid be taken into account when pruning the plants. It will be necessary to consider these factors, apparently contradictory, in order to obtain the higher number of flowering stems.

In order to produce a flower, there must be a decrease in the number of growth units with the order of the axis. Indeed, the further away we are from the central axis, the smaller the number of growth units of the flowering stem is. Gradually, the market quality of the product decreases. And, if it is really possible to increase the length of the growth unit, and by doing so the length of the flowering stem by increasing the frequencies of fertirrigation, this increase remains, however, limited. It is, therefore, necessary to prune back the plants in order to keep an adequate number of growth units. The reiteration, or rebuilding of the tree on itself (HALLE, 197Û), which could be observed on a free form, could be induced by a traumatic pruning.

Finally, if it is interesting to increase the number of flowering shoots with regard to the number the vegetative axes in order to increase the productivity of the plant, it may become dangerous to have all the axes in flower because their harvesting can lead to the death of the plant, as we have sometime noticed in our studies. Therefore, it is necessary to keep some axes at a vegetative stage. Those will be used as permanent vegetative stock after the harvest of flowers.

6. Other problems

The following problems result from observations realised on species and cultivars grown in a collection but not included in trials. So, they are only hypotheses and it would be necessary to verify them in further trials to obtain a better cultural knowledge of the species or varieties.

Firstly, it seems that the soilless cultivation, by the highest induced growth speed, involves sometimes a dephasing between the morphologic maturity necessary to the flowering and the seasonal conditions inducing this flowering. WALLERSTEIN (1992), showed the importance of the natural long-short day sequences in the initiation and development of the floral buds of Banksia, Leucospermum and Leucadendron. Now, on some Leucospermum cultivars of our collection, we note the cessation of elongation, at the end of July, let two months before the coming out of short days inducing the différenciation of flower primordia. Thereby, the subterminal buds sprout immediately, giving branches which have not sufficient morphologic maturity when the short days appear. The plant remains consequently in vegetative stage. One observation made on some species of Banksia seems to confirme this hypothesis: all plants which flowered after one and half year of cultivation were grown with the low frequency of fertirrigation therefore with the lowest level of watering and nutrition. The other plants, with a double frequency remained in vegetative stage. The study and adaptation of fertirrigation rythms and composition of nutrient solutions might solve this problem.

Lastly, the study of the climatic conditions which should be respected under shelter, should be determined in function of the species as much for temperature as humidity.

As a matter of fact, it seems that the minimum of heating temperature used during the winter (5°C) is insufficient in relation to the zero growth temperature of some species (for exemple: 13°C for P. neriifolia).This can stop or reduce the vegetative development of the plants and so decrease the productivity. But we don't know if the production cost will be

69

balanced by the increase of productivity obtained from a more important heating.

Moreover, the higher water availability and possibly the lower ventilation of the greenhouse, compared with the open field one, induce a very important transpiration or exudation of water out of the young leaves of the buds. Sometimes, this water induces the rot of the terminal leaves and consequently a lower quality of the product.

7. Conclusion

We have tempted, in these pages, to take stock on the researchs made on the soilless protected cultivation of proteaceae in France.

The acquired results enable us to determine the composition of a nutrient solution adapted to the most part of species cultivated for cut flower or foliage. Other studies have shown that the water and nutrients requirements differ with the vegetative rythms, the development stages and the species. Though a standard definition of the fertirrigation could be considered, the right definition of the actual needs proper to each species or cultivar should be defined to have a perfect control of the production. This last point will require the adjustment of the pruning and harvest methodes according to the more importante growth induced by the soilless cultivation.

Many problems are unsolve because of a too short time of study. The continuation of the trials on hand and the implementation of new trials should unable us to gradually solve them.

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