Biology Reference
In-Depth Information
and Corbet (1991) stated that the supply of material for nectar sugar may
depend on immediate photosynthesis or carbohydrate reserves, the latter be-
ing essential in the case of plants producing nectar at night. Nectary
parenchyma cells may serve as a storage site for starch that will be hydro-
lysed at the moment of secretion or they may photosynthesize because of the
presence of chloroplasts. The source of nectar components, especially sug-
ars, is closely related to the rate of nectar production: a high rate requires
storage of reserve material in nectary parenchyma cells. The rate of nectar
production and its total quantity is, in turn, linked to the type of foraging
animals, their behaviour and food requirements. These features also influ-
ence the manner and site of nectar exposure (nectar presentation).
This chapter closes with a look at the enormous variability in nectar fea-
tures (volume, concentration, and composition) existing at flower, plant,
species, and population levels. Nectar variability within or among plants
may result from the interaction between the pattern of nectar secretion by the
plant and pollinator foraging strategies. Nectar secretion patterns may be
genetically determined (Mitchell, 2004; Leiss and Klinkhamer, 2005b) and
also influenced by macro- and micro-environmental conditions (Nicolson,
1994; Nicolson & Nepi, 2005; Petanidou, 2007). It is fundamental to identify
the plant effects before studying other potential sources of variability
(Galetto & Bernardello, 2005).
2
NECTAR SECRETION MECHANISM
AND MODELS OF NECTARY FUNCTION
Two main types of secretion can be recognized in animals and plants: the
holocrine type, in which the process involves cell death at the moment of
secretion, and the merocrine type, in which the secreting cells survive and
continue their secretory activity.
In most cases nectar secretion is merocrine, but in a few cases it is holocrine,
implying the death of the cells (Elias et al., 1975; Vesprini et al., 1999;
Horner et al., 2003; Nepi, 2007) (Fig. 1).
There is a general consensus in considering phloem sap the “raw” mate-
rial of nectar. The pre-nectar is unloaded from the sieve elements to the
adjacent phloem parenchyma cells and sometimes even in the intercellular
spaces. Pre-nectar unloading is favoured by phloem companion cells that often
have wall ingrowths of the transfer cell type as observed in
Vicia faba
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