Study of the interaction between greenhouse cover type and plant growth retardants in zonal geranium

The effects of two covering materials (polycarbonate+aluminized shade cloth [P+AS] and plastic shade cloth [PS]) and three plant growth retardants (chlormequat at 600 ppm, paclobutrazol at 40 ppm and ethephon at 400 ppm) on the development of potted zonal geraniums (Pelargonium x hortorum) were investigated. A two-way ANOVA was established to study the statistical significance of the two factors and their interaction. Generally, all the plants cultivated under PS (25% shading) showed less growth than those grown under P+AS (70% shading). All retardants, particularly paclobutrazol, reduced the size of plants compared with the control. Leaf volume was smaller under PS than under P+AS. PS reduced leaf area compared with P+AS (29%), but had no significant effect on the number of leaves. PS and paclobutrazol resulted in plants with small leaves. PS and all the retardants reduced the fresh and dry weight of the aerial part. The chlorophyll content of leaves was higher when the plants were cultivated under P+AS. The use of PS led to a loss of quality in leaf colour, while paclobutrazol had the contrary effect. The response of geraniums to ethephon depended on the covering used. It seems that the conditions produced by each type of cover interacted with ethephon, which was less effective under P+AS (less light and higher temperature compared with the PS), especially as regards fresh and dry weight of the aerial part.