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Institut für Kulturpflanzenwissenschaften

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    Growth regulation of ornamental and vegetable plants under greenhouse conditions by air stream-based mechanical stimulation
    (2022) Sparke, Marc-André; Müller, Joachim
    Plant growth regulation is an integral part within the production chain of ornamentals and vegetable seedlings. In protected ornamental horticulture, chemical-synthetic plant growth regulators (PGR) are used to reduce plant size. In vegetable production, the use of these substances is prohibited by law in most counties, which is why non-chemical growth regulation methods must be applied. In this respect, a production method for non-chemical growth control of ornamentals and vegetable seedlings under greenhouse conditions has been developed that is based on the application of air streams, inducing thigmomorphogenesis, the morphological and structural shaping of a plant organism during its development phase as influenced by touch-like stimuli. In own experiments jointly performed at the State Horticulture College and Research Station in Heidelberg, Germany, the application of a regularly applied air stimuli significantly reduced plant height by 24% in bellflower (Campanula ‘Merrybell’) compared to the control. In a subsequent practical trial at a local horticulture company (Fleischle GbR, Vaihingen Ensingen, Germany) plant height of creeping inchplant (Callisia repens) was significantly reduced by 20% on average compared to the control. In both experiments, a compressor generated the air stream which was then applied to the plant stand through custom-built stainless-steel nozzles (air pressure module). In tomato (Solanum lycopersicum ‘Romello’), air streams applied by the ‘air knife’ module, the ‘360° rotor’ module, or the ‘air pressure’ module resulted in a reduction in plant height of 26%, 33%, and 36% compared to the control, respectively. The air stream guided into the air knife module was applied by an aperture slot, which could be adjusted between 1 and 5 mm, while the air stream guided into the 360° rotor module was applied via two 360° rotating PVC tubes that were inserted on the bottom of a rectangular aluminium box. It turned out, that the air outlet velocity along the aperture slot of the air knife module was highly variable. Consequently, the stimulus intensity perceived by individual experimental plants was unequal. A multiple regression analysis clearly showed that the maximum air velocity explained the variability in plant height reduction by air streams generated with the air knife module best, while the stimulus duration and the cumulative air velocity were less relevant. Plant height reduction by air stream generated with the 360° rotor module was most homogenous compared to the other prototypes. Therefore, a subsequent series of experiments at the University of Hohenheim, Stuttgart, Germany, was carried out with the most promising prototype, the 360° rotor. No systematic dose-response relationship related to increasing application frequencies of 8, 24, 40, 56, 72, and 80 d-1 was found, confirming previous findings that the plants do not integrate the mechanical stimulus over time. In contrast, plant height reduction was significantly influenced by the air stream velocity. A sigmoidal dose-response relationship was fitted to the data and showed negligible effects on tomato plant height reduction between 0.7 m s-1 and 2.0 m s-1, followed by a steep increase in the reduction effect up to 4.7 m s-1 and a fading of the effect at 36 % reduction for air velocities beyond that. With the optimised settings for daily application frequency and air velocity, another experiment was conducted focusing on the effect of air stream application on phenotypic and physiological responses in tomato. Air stream application resulted in a gradual reduction of total leaf area by 14% on day 14 after treatment start, and radial growth was promoted relative to internode elongation compared to the untreated control, resulting in a more compact and stable plant phenotype. Air stream-treated plants translocated proportionally more assimilates to leaves and stems, at the expense of dry matter accumulation to petioles. The reduction in total leaf area was compensated by an increased leaf density, accompanied by a higher leaf green intensity and consequently by an average 8% increase in net CO2 assimilation rates compared to the control. Thus, air stream-treated plants partially sustained total biomass accumulation at the same level as compared to the control.