Photosynthetic activity of green nectaries can explain a significant part, if not all, of the sugar secreted in nectar.
The importance of green chlorophyll to photosynthesis has been established for many years (so much so I’m not even giving a link to support this contention). But just because a tissue is green does that mean it must be photosynthetic? And if so, does it follow that the degree of photosynthesis undertaken must be ‘significant’? If one answers yes to the first of those, one could argue that experience of other green, photosynthetic situations supports the view that others will be too – the predictive power of science. If one answers yes to the latter, one is not really being that scientific, which ought to be evidence-based (at the very least; botany is after all an ‘ology’!).
In true scientific tradition, the importance of photosynthesis in green nectaries has been examined by Ulrich Lüttge. Nectaries are glandular structures that secrete nectar (‘a sugar-rich liquid’ with major ecological and economic importance and relevance), and which may be located on flowers – floral nectaries – or on other plant parts – extrafloral nectaries. Although there has been a general assumption that photosynthesis by such structures can supply the carbohydrates secreted in the nectar, to date quantitative data on their photosynthetic capacity has been largely missing. Cue Lüttge’s work, which examined 20 floral and six extrafloral nectaries from a range of plant taxa. Whilst the photosynthetic parameters measured were lower for nectaries than their corresponding leaves, the accompanying quantitative analysis supported the contention that photosynthetic activity of green nectaries can explain a significant part, if not all, of the sugar secreted. A nice example of not taking anything for granted (even if the original assumption is not overturned, it is at least now substantiated!).
[This work chimes timely with recent studies devoted to aspects of the ecology and evolution of extrafloral nectaries in Brigitte Marazzi et al.’s Viewpoint article entitled ‘The diversity, ecology and evolution of extrafloral nectaries: current perspectives and future challenges’. For more on non-foliar photosynthesis generally, check out this review by Guido Aschan and Hardy Pfanz – Ed.]