The Western Cape region of South Africa, where Proteaceae have traditionally been cultivated as cut flower products, has a Mediterranean-type climate with warm, dry summers with minimum and maximum temperatures of 9.3 and 41.3 °C in January–February and cool, wet winters from June to August with a minimum of 0.8 °C and maximum of 28.9 °C. Elevated mean annual temperatures of up to +4.5 °C by midcentury relative to the base period (1981–2010), with both higher minimum and maximum temperatures, as well as more frequent and more intense heat waves are projected in climate change models (Midgley et al., 2016). Already, the Western Cape has experienced a drastic rate of increase in daily maximum temperature of around 0.2 °C per decade, as recorded over the 1960–2010 period (Midgley et al., 2016). As expected, changes in rainfall are far more complex; nonetheless, climate change is likely to result in a reduction in surface water availability and cause potential shifts in seasonality of rainfall and an increase in the magnitude and frequency of flood events (Midgley et al., 2016).
The potential impact of increasing temperatures due to climate change on cultivated Protea inflorescence production is unknown. A study by Louw et al. (2015) investigated gas exchange and growth of Protea ‘Pink Ice’ under increased temperatures and found increased leaf dry weight per unit area with increasing temperature, which indicated leaf structural changes. Leaf area–based gas exchange [net CO2 assimilation rate, stomatal conductance (gS), and dark respiration rate] did not differ across the temperature gradient, but leaf weight–based CO2 assimilation rate and dark respiration rate decreased significantly toward the upper end of the temperature range. Under warming, spring budbreak occurred earlier, but inflorescence initiation extended from the spring flush to the summer flush, leading to delayed flowering. In addition, with warming, aboveground biomass allocation patterns were altered whereby less carbon is invested into the inflorescences and more carbon is invested in the leaves and, to a lesser degree, stems. The study by Louw et al. (2015) suggests that warming may prolong the vegetative growth period in some Protea cultivars, at the expense of flower production. However, large gaps still remain in our understanding of responses within such systems under enhanced environmental stress. In addition, cut flower production of Protea and other genera within the Proteaceae is increasingly viewed as potential alternative crops in South Africa, particularly in areas regarded as too warm for most temperate horticultural crops such as apples and pears. This further justifies the need to understand the response of the vegetative and reproductive growth of Protea under warmer conditions than those prevalent in current production areas.
The demand for Protea cut flowers in Europe peaks during the European autumn and winter, from September to February (Gerber et al., 2001a; Hettasch et al., 1997). Unlike Leucospermum (Malan and Jacobs, 1990) and Leucadendron (Hettasch and Jacobs, 2006), which are short-day plants, the underlying mechanism for floral induction in Protea has not been established. In the southern hemisphere, Protea ‘Pink Ice’ (Protea compacta R. Br × Protea susannae Phill.) generally initiates inflorescences on the spring flush (September–October), and the harvest stage is reached from January to May (Gerber, 2000), falling mostly outside of the optimum export period. Together with the harvest time, a minimum stem length and an unblemished bloom determine the quality and price of these niche market cut flowers (Hettasch et al., 1997). The combined vegetative and reproductive cycles of ‘Pink Ice’ extend over 14–16 months. Commercially, flowering time in Protea can be manipulated by pruning (Greenfield et al., 1994; Nieuwoudt and Jacobs, 2010). A biennial bearing cycle with two blocks in alternating phases is recommended for commercial production (Gerber et al., 1995; Hettasch et al., 1997). This biennial management system ensures that inflorescences borne on long stems can be harvested annually, albeit from different blocks, and provides for a sustained income.
In a South African biennial bearing cycle, Protea ‘Pink Ice’ is pruned back to a basal bearer in winter (June/July). New growth initiates during spring in late August, and vegetative growth continues by means of several growth flushes through the summer and autumn of the first year. Flower initiation only commences during the spring of the second year, with the subsequent harvest in the summer/autumn of that season. It was observed that, in a biennial cycle, a limited number of ‘Pink Ice’ shoots initiate inflorescences terminally on the autumn flush, ≈9–10 months after pruning (E.-L. Louw, personal communication; Nieuwoudt and Jacobs, 2010). These long-stemmed autumn-initiated inflorescences reach the European market from December to February, within the period of high demand, thus optimizing the price per stem.
As little information is available on how temperature affects commercial Protea cultivation, especially floral induction and initiation, the different inflorescence initiation systems within ‘Pink Ice’ provide the ideal opportunity to study the effect of post-initiation temperature on leaf gas exchange, vegetative growth, and inflorescence growth patterns in comparable phenological stages.
The aim of this study was to compare the two distinctly different inflorescence initiation systems in Protea ‘Pink Ice’ as managed within a biennial bearing regime, with respect to the temperature sensitivity of inflorescence development and quality. The more common spring-initiated system was compared with the more preferred out of season autumn-initiated system, in the context of different seasonal temperature regimes. This would provide an indication of future shifts in inflorescence production and profitability that would be expected under projected climate change.
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