The effect of high temperature and water stress on the development of bract browning in Protea ‘Pink Ice’

Abstract

Protea 'Pink Ice' is a Cape Flora cut flower product that is widely planted in the Western Cape, South Africa, predominantly in the warmer and drier regions. Harvests coincide with mid-summer to early autumn, a period generally characterized by hot, dry conditions and high irradiance levels. 'Pink Ice' and other Protea cultivars harvested in this window are particularly prone to develop a preharvest disorder of the involucral bracts, described as 'Bract Browning' by Smith et al. (2015) or commonly referred to by the industry as 'dirty nail syndrome'. Bract browning can significantly reduce the market value and export potential of Protea ornamental stems by affecting the inflorescence appearance. The aim of this study was to obtain a better understanding of the effect of water and high temperature stress on the development of bract browning in Protea inflorescences. Flowering, two-year-old 'Pink Ice' potted plants were subjected to three irrigation and two temperature treatments in a factorial, randomized complete block design. An 'ambient' and a 'warmed/elevated' temperature (ambient temperature +1.6 degrees C) treatments were combined with three irrigation schedules consisting of an estimated optimal (x) irrigation application supplied throughout the season, and irrigation treatments which provided either half (1/2x) or double (2x) the amount of water provided at optimal irrigation. Results indicated no interaction between heat and water stress as main effects; however, both factors significant influenced the development of bract browning. Elevated temperatures only increased bract browning in Zone 1 (top third) of the inflorescence, with significantly less marketable flowers associated with the warmed treatment. Reduced irrigation impacted significantly on all zones of the inflorescence. With an expected rise in temperature and a possible increase in heat wave incidences as predicted with climate change, it is critical to develop effective strategies to prevent or ameliorate the development of bract browning in order to continue to deliver premium quality products to a highly competitive international market.