Effect of air humidity on the growth and morphology of Hydrangea macrophylla L.
Introduction
Relative humidity in the greenhouse plays an important role on the growth and development of flowering ornamental plants. The increase of relative humidity leads to an increase in the height and the leaf area of many plants (Ford and Thorne, 1974, Schüssler, 1992). It results in an increase in the number of pairs of leaves in Saintpaulia ionantha, Euphorbia pulcherrima and Nephrolepis exaltata and increases the number of flowers in S. ionantha (Mortensen, 1986). Nevertheless, these experimental results show a high degree of variability between different genera and species. In the case of Begonia elatior (Schüssler, 1992) and Kalanchoë blossfeldiana ‘Sally’ (Mortensen, 2000), the increase in relative humidity leads to a decrease in plant height. The genus Rosa appears to be unaffected by this criterion (Mortensen, 1986).
In the case of Hydrangea macrophylla, the influence of relative humidity on plant development has not been studied. However, some observations show that this might be relevant. Hydrangea is a plant native to humid Japanese forests (Wilson, 1923) that very easily adapts itself to the climates of coastal areas with high relative humidity. It appears that relative humidity, along with the thermoperiod (Bailey, 1989) and the photoperiod (Lemattre, 1974), is a factor in initiating flowering (Guerin, 2002). Finally, during the transition from the vegetative phase to the floral phase, we can observe a variation in the length of the internodes and the leaf area (Guerin, 2002).
The study of the role of relative humidity on the development and the growth of plants is difficult. On the one hand, it interacts with other extraneous factors and, on the other, it has an effect on many physiological functions such as transpiration, photosynthesis, hydric functioning and the mineral uptake of the plant. (Gislerød and Mortensen, 1990).
The aim of this study was to evaluate the influence of relative humidity on plant growth during the vegetative phase. The experimental concept aims to follow the growth of the plants under the climatic conditions closest to the natural conditions. The experimental device must make it possible to integrate the climatic variations while controlling the studied factor: relative humidity. The experimental units are made up of small enclosures laid out in a greenhouse of large volume.
The experiment is based on the comparison of plant growth within two growth compartments with identical temperature but different relative humidity. This work required the design of specific experimental material that was built and tested during the experiments.
Section snippets
Plant material
Cuttings of two pairs of leaves were taken from mother micro-plants of H. macrophylla cv. Leuchtfeuer (Galopin et al., 1996). In order to avoid repotting stress, plants were directly planted in 5-cm plastic pots (TEKU®), filled with prewetted perlite, sifted between 1 and 3 mm. Sub-irrigation was applied six times a day for 10 min. Rooting took place in tunnels in the greenhouse, air humidity was maintained at 80% with a fine mist system (Fog Système®) and the temperature was regulated at
Measurement of experimental conditions
Experimental conditions were measured by recording climatic data. This made it possible to test the performance of experimental material and to measure the differences between pre-established values and the results for temperature and relative humidity (Table 2).
The pre-established relative humidity values of 50% and 80% correspond to a vpd of 1.6 and 0.6 kPa. For treatments 1 and 3, the day/night averages were 53% and 50% relative humidity, respectively. For treatments 2 and 4, the averages
Management of the different climatic factors in the greenhouse
Experiments were carried out in the greenhouse in order to be able to reproduce climatic conditions similar to those of H. macrophylla producers. The uniqueness of the experimental design was the availability of two compartments for climatic management. The first, that of the greenhouse itself, made it possible to control the temperature, the photoperiod and the radiation in a very large volume. The second, made up of growth compartments, made it possible to control the level of relative
Acknowledgements
The authors are grateful to A. Travers and M. Laffaire for their technical assistance. This work was supported by Florema France Production.
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