Seasonal fluctuations in pigment chemistry of co-occurring plant hemi-parasites of distinct form and function

We investigated seasonal photoprotection in two species of rootless, climbing ‘dodders’ (Cassytha glabella and Cassytha pubescens), two species of root hemi-parasites (Leptomeria drupacea and Exocarpus cupressiformis) and the host, Leptospermum scoparium, that co-occur in Australian woodlands on north- and south-facing aspects. Concentrations of chlorophyll were at least two-fold lower in parasites than the host. Total chlorophyll and violaxanthin–antheraxanthin–zeaxanthin (VAZ)-cycle pigments per unit chlorophyll were generally lower in summer, and higher in winter, than in spring across all species. Except for L. drupacea, VAZ-cycle pigments per unit chlorophyll were greater in hemi-parasites than the host. VAZ-cycle accumulation was high in C. glabella relative to all other species, including C. pubescens, across seasons. Pre-dawn xanthophyll cycle engagement was generally greater in spring than winter or summer, inverse to trends in levels of total VAZ-cycle pigments per unit chlorophyll. Both Cassytha species contained lutein-5,6-epoxide (Lx), seasonal variation of which indicated near maximum engagement of the ‘Lx cycle’ in spring with relatively less engagement in winter and summer. Chlorophyll and carotenoid pigment levels were distinct between aspects and pre-dawn water potential was significantly lower on the north- than south-facing aspect. Maximum potential photochemical efficiency was generally higher in summer than winter.