Whole-plant gas exchange measurements of mycorrhizal 'Iceberg' roses exposed to cyclic drought

Nursery stock is purported to benefit from arbuscular mycorrhizal (AM) symbioses when subjected to drought, low fertility, or transplant stress. Yet these benefits have not been well defined. Whole-plant gas exchange measurements describe plant performance under environmental strain more reliably than individual leaf measurements. Understanding the whole-plant response to drought stress will yield decision-making tools for ornamental plant producers about benefits from mycorrhizal symbiosis. Container-grown Rosa x hybrida 'Iceberg' rose plants were subjected to repeated drought episodes intended to simulate missed irrigation cycles during commercial production or retail sales periods. Whole-plant gas exchange parameters of mycorrhizal, low phosphorus (ML) and non-mycorrhizal, low (NML) and high (NMH) phosphorus treated roses were compared using 14-day continuous measurements. The NMH plants, which were provided supplemental KH2PO4 fertilization, had larger plant canopies and initially had higher whole-plant photosynthesis (Pnet) rates than similar-sized NML and ML plants. Gas exchange, carbon, and water use efficiencies of 'Iceberg' roses were not significantly improved by colonization with the AM fungus Glomus intraradices. All plants had similar water and carbon use efficiencies at the end of the third drought episode. Photosynthetic capacity decreased after 'Iceberg' roses were rewatered, following water deficit stress, regardless of mycorrhizal status. During the second drought cycle, maximum Pnet approximated 70% of pre-drought levels and continued to decline. Improved shoot hydration, and thus aesthetic appearance during drought strain episodes, was not achieved by G. intraradices colonization of 'Iceberg' roses.