Establishment of a plastochron index for growth assessment of Phalaenopsis

Abstract

A plastochron index was established for four commercial cultivars of Phalaenopsis: Phalaenopsis 'Taisuco Carol' × (Doritaenopsis 'New Cinderella' × P. 'Isaribi') (Taiwan Sugar Corp., code No. S88-41); Phalaenopsis 'Taisuco Swan' × (P. 'Casablanca Dream' × (P. 'Hohoemi' × P. 'Grace Palm')) (code No. S88-89); Doritaenopsis 'I-Hsin Dreamer' × Phalaenopsis 'Taisuco Harmonylip' (code No. S88-104), and Phalaenopsis aphrodite. The plastochron was based on a 20-mm reference leaf length. Semilogarithmical plots of ln-transformed leaf lengths versus time and linear regression analysis were used to evaluate three criteria of the plastochron index. The results showed that leaves 3–9 of Phalaenopsis aphrodite and leaves 3–8 of the other cultivars grow exponentially (r2 values between 0.90 and 0.97). Therefore, leaves that fulfilled all three criteria for the plastochron index were leaves 3–5 of No. S88-41, leaves 3–4 of No. S88-89, leaves 5–6 of No. S88-104, and leaves 4–5 of Phalaenopsis aphrodite. However the plastochron index can not be used for the slow-growing Phalaenopsis, because two successive leaves could not be measured simultaneously during the exponential phase of leaf development. The average relative growth rate of 0.43 mm/week for Phalaenopsis aphrodite was significantly lower than that of the other three cultivars (0.52 mm/week); plastochron duration also showed significant variation among the four cultivars (P value = 0.0269). In addition, the linear relationship (r2 = 0.97–0.99) of the plastochron index plotted versus time indicates that the intervals between initiation of successive leaves was equal for each of four cultivars. Thus, for these cultivars of Phalaenopsis, leaves that emerge before the reproductive phase can not be fully assessed by the plastochron index.Key words: plastochron index, Phalaenopsis.