Evaluating Calibrachoa (Calibrachoa × hybrida Cerv.) Variety Sensitivity to Iron Deficiency at High Substrate-pH

Floriculture crop species that are inefficient at iron uptake are susceptible to developing iron deficiency symptoms in container production at high substrate-pH. The objective of this study was to compare varieties of iron-inefficient calibrachoa (Calibrachoa × hybrid Cerv.) in terms of their susceptibility to showing iron deficiency symptoms when grown at high versus low substrate-pH. In a greenhouse factorial experiment, 24 varieties of calibrachoa were grown in peat:perlite substrate at low pH (5.4) and high pH (7.1). Shoot dry mass, leaf SPAD chlorophyll index, flower index value, and shoot iron concentration were measured after 13 weeks at each substrate-pH level. Of the 24 varieties, ANOVA found that 19 varieties had lower SPAD and 18 varieties had reduced shoot dry mass at high substrate-pH compared with values at low substrate-pH. High substrate-pH had less effect on flower index and shoot iron concentration than its effect on SPAD or shoot dry mass. No visual symptoms of iron deficiency were observed at low substrate-pH. Varieties were separated into three groups using k-means cluster analysis, based on the four measured variables (SPAD, dry mass, flower index, and iron concentration). These four variables were each expressed as the percent reduction in measured responses at high versus low substrate-pH. Greater percent reduction values indicated increased susceptibility of varieties to high substrate-pH. The three clusters, which approximately represented high, medium, or low sensitivity to high substrate-pH, averaged 59.7%, 42.8%, and 25.2% reduction in SPAD, 47.7%, 51.0%, and 39.5% reduction in shoot dry mass, and 32.2%, 9.2%, and 27.7% reduction in shoot tissue iron, respectively. Flowering was not different between clusters when tested with ANOVA. The least pH-sensitive cluster included all four varieties in the breeding series ‘Calipetite,' which had low shoot dry mass at low substrate-pH, indicating low overall vigor. There were no differences between clusters in terms of their effect on substrate-pH, which is one potential plant response to affect iron availability. This experiment demonstrated an experimental and statistical approach for plant breeders to test sensitivity to substrate-pH for iron-inefficient floriculture species.