The effect of different types of culture substrate on some morphological and physiological indices of African violet (Saintpaulia ionantha H. Wendl)

The function of a growth substrate is to provide an ideal medium for plantlet emergence which allows for the optimal development of plants during the time they stay in the container. A high-water storage capacity is necessary due to the limited volume available in growth substrate. There is a large variety of materials available for substrate elaboration, and their selection depends on the plant species to be propagated, season, propagation system, price, substrate availability and proper features. Compost seems to be an excellent alternative to peat. Also, the fresh material presents a high carbon: nitrogen ratio (C: N) and it may contain substances that are toxic to the plants, such as phenols, resins and tannins. Choosing the suitable planting bed for ornamental plants is one of the problems for most greenhouse managers. In this regard, in order to investigate the possibility of using liquorice residue compost as a by-product of the medicinal plant industry, in growth medium, the effect of peat moss, coir and liquorice root residue compost on the growth of African violet tested in an experiment.  
 
The experiment was done with 7 treatments and 5 replications in a completely randomized design. Experimental treatments included: 60% (volumetric) peat moss, 60% coir, 60% liquorice root residue compost, 30% peat moss + 30% coir, 30% peat moss + 30% liquorice root residue compost, 30% coir + 30% liquorice root residue compost. Each treatment was mixed with 40% perlite. According to the results of a pre-test on the low water retention capacity of liquorice compost, a treatment was also performed using liquorice compost with superabsorbent polymer (Aquasorb made by SNF) at a rate of 5g/kg mixed medium.
 
Experiment results in African violet showed that in shoot fresh weight, leaf number, root fresh weight, leaf area and relative leaf water content of 30% peat + 30% coir treatment and shoot dry weight, flower number and chlorophyll a were the best in 30% peat moss + 30% liquorice root residue compost treatment. The highest amount of chlorophyll b and total chlorophyll were observed in 60% liquorice root residue compost + superabsorbent polymer, the highest carotenoid content in 60% peat treatment, and the highest root dry weight and proline content in 60% liquorice root residue compost treatment. In general, according to the obtained results, by using licorice root residue compost with peat, it is possible to achieve favorable growth results, and it is recommended to use it up to 50% of the required organic matter in the bed of African violets. Based on the results of this study, licorice root resigue compost with peat moss can achieve favorable growth results and it can be used up to 50% of the required organic matter in African violet bed. The effects of licorice compost on morphological and physiological parameters of plants may be due to its effects on increased water content and soil aeration. Because there is a strong relationship between RWC and plant biomass. That also suggests that plants having greater biomass can maintain higher water content in their leaves.
 
Presence of nutrient elements increased the overall fertility, and functionality of the tested media, leading to maintain better plant growth. RWC is a measure of plant water status, which reflects metabolic activity in plant tissues and is one of the most important indicators to identify differences in substrate conditions.