Abstract
Reliance on carbohydrates during flower forcing was investigated in one early and one late flowering cultivar of azalea (Rhododendron simsii hybrids). Carbohydrate accumulation, invertase activity, and expression of a purported sucrose synthase gene (RsSUS) was monitored during flower forcing under suboptimal (natural) and optimal (supplemental light) light conditions, after a cold treatment (7°C + dark) to break flower bud dormancy. Post-production sucrose metabolism and flowering quality was also assessed. Glucose and fructose concentrations and invertase activity increased in petals during flowering, while sucrose decreased. In suboptimal light conditions RsSUS expression in leaves increased as compared to optimal light conditions, indicating that plants in suboptimal light conditions have a strong demand for carbohydrates. However, carbohydrates in leaves were markedly lower in suboptimal light conditions compared to optimal light conditions. This resulted in poor flowering of plants in suboptimal light conditions. Post-production flowering relied on the stored leaf carbon, which could be accumulated under optimal light conditions in the greenhouse. These results show that flower opening in azalea relies on carbohydrates imported from leaves and is source-limiting under suboptimal light conditions.
Highlights
Azalea hybrids in the genus Rhododendron are well known for their beautiful flowers
Invertase cleavage of sucrose is predominantly done by the acid invertase (AI) in ‘Nordlicht’ but neutral invertase (NI) activity is present
Increasing sucrose synthase (SUS) activity was detected in rose petals, though its activity was weak compared to the invertase activity (Yamada et al, 2007; Kumar et al, 2008a; Horibe et al, 2013)
Summary
Complete flower opening is dependent upon petal growth, which in turn requires cell division and cell expansion. Cell division in petals is the primary mechanism during the first stages of petal growth, but cell expansion becomes the dominant process responsible for full flower opening (Reale et al, 2002). Cell expansion is a combined process of cell wall weakening, carbohydrate allocation and water uptake. Expansins are extracellular proteins involved in cell wall modifications that contribute to cell expansion. Large amounts of soluble carbohydrates are known to accumulate, especially in vacuoles (Yamada et al, 2009). Carbohydrate flux into petal cells is necessary for biosynthesis and maintenance respiration, Abbreviations: AI, acid invertase; CWAI, cell wall bound AI; DLI, daily light integral; NI, neutral invertase; SPS, sucrose phosphate synthase, SUS, sucrose synthase
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