Physiological and phenological responses of Protea ‘Pink Ice’ to elevated temperatures

• Leaf weight-based photosynthesis and respiration rate decreased with increasing temperature.
• Strong warming advanced budbreak but extended flower initiation into summer.
• Aboveground biomass allocation shifted from reproductive to vegetative growth.

Rising temperatures associated with global climate change may alter the physiology and phenology of Protea species and cultivars. Protea species are assumed to be well adapted to warm summers characteristic of their natural Mediterranean-type habitat, but their plasticity in responding to higher growth temperatures is not known. Using infrared lamps, a greenhouse-based temperature gradient was constructed, with temperatures ranging from ambient to ambient + 3.1 °C. Potted plants of Protea ‘Pink Ice’ (P. compacta R. Br × P. susannae Phill.) were grown at five positions along this gradient for 12 months under irrigation. Simultaneously, a field verification experiment in a nearby commercial ‘Pink Ice’ orchard was conducted under ambient temperature and ambient + 2.9 °C. Increased sclerophylly (leaf dry weight per unit area) with increasing temperature indicated leaf structural changes. While leaf area based gas exchange (net CO2 assimilation rate, stomatal conductance and dark respiration rate) did not differ across the temperature gradient, leaf weight based CO2 assimilation rate and dark respiration rate decreased significantly towards the upper end of the temperature range. The optimum temperature for net CO2 assimilation rate (Topt) showed seasonal adjustments, but increased in response to experimental warming only in the field experiment. Significant temperature elevation resulted in an earlier onset of spring bud break, but warming extended inflorescence initiation from the spring flush to the summer flush, leading to delayed flowering. Aboveground biomass allocation shifted from inflorescences to leaves and to a lesser degree to stems, with elevated temperatures, whereas root growth was stimulated in the middle of the warming range. The results of this study suggest that elevated temperature may prolong the vegetative growth period in some Protea cultivars where water is not limiting, at the expense of flower production. This could have significant economic and marketing consequences for commercial cut flower production systems. The findings are also of significance to ecologists studying the responses of Proteaceae to climate change.