Green nectaries: the role of photosynthesis in secretion

Green nectaries have been frequently mentioned in the literature, leading to the assumption that photosynthesis of nectaries can supply the carbohydrates secreted in the nectar, especially when storage of starch is seen in the plastids in nectaries and this starch disappears during secretion. Photosynthesis in nectaries can also provide reduction equivalents for the nectar-redox cycle and energy for secretion. However, quantitative data on the photosynthetic capacity of nectaries are largely missing. Therefore, in the present study, the photosynthetic capacity of green nectaries from a range of plants was screened; 20 floral nectaries (including six septal nectaries) and six extrafloral nectaries were studied. For the screening, chlorophyll fluorescence parameters were measured as depending on photosynthetic photon flux density (PPFD). Parameters measured were basic ground fluorescence (F) and quantum yield (Y-0) of the dark adapted sample at 0PPFD. From the light saturation curves saturating PPFD (PPFDsat), quantum yield at saturation (Y-sat) and maximum apparent photosynthetic electron transport rates (ETRmax) were obtained. For comparison, leaves of the plants were also measured. In most cases, the performance of the nectaries was lower than that of the leaves. F was lower in 14 floral and four extrafloral nectaries (69% of total), ETRmax was lower in 18 floral and four extrafloral nectaries (85%), Y-sat was lower in 15 floral and three extrafloral nectaries (69%). In 18 floral and two extrafloral nectaries (77%) Y-0 was well below 0.8, indicating photoinhibition. In contrast, the range of ETRmax for green nectaries was 25-140molm(-2)s(-1) and overlaps well with that of green tissues in general. The lower end of the range of rates of photosynthetic carbon dioxide (CO2) uptake of sun leaves in the literature is 10mol CO(2)m(-2)s(-1). Taking this value for sun-adapted green nectaries, i.e. having a PPFDsat >1000molm(-2)s(-1), with an area of nectar tissue measured as 3-50mm(2) per flower, sugar secretion related to photosynthetic CO2 fixation in the green nectaries is estimated at approximately 0.2-3.0mol hexose units flower(-1)day(-1). This is compares well in order of magnitude with the range of secretion given in the literature and clearly suggests that photosynthetic activity of green nectaries can explain a significant part, if not all, of the sugar secreted. In some nectaries ETR did not saturate with PPFD. This could be attributable to spillover from photosystem II to photosystem I and cyclic photosynthetic electron transport. It is in agreement with observations in the literature and my preliminary findings that nectary plastids often lack grana thylakoids where photosytem II is located. Cyclic photophosphorylation could provide adenosine triphosphate (ATP) energy for the nectaries. This needs further investigation.(c) 2013 The Linnean Society of London.