DINAMICS OF NECTAR: NEW INSIGHTS FROM Cucurbita pepo

Transcription

1 DINAMICS OF NECTAR: NEW INSIGHTS FROM Cucurbita pepo Massimo Nepi, Massimo Guarnieri, Daniele Artese, Laura Cresti, Ettore Pacini Department of Environmental Sciences G. Sarfatti, University of Siena, Italy Malgorzata Stpiczyńska Department of Botany, Lublin Agricultural University, Poland

2 THE FLOWER Cucurbita pepo is an entomophilous, monecious species with unisexual flowers S A s n n n O

3 THE NECTARY before secretion after secretion A S n s O n n parenchyma epidermis

4 THE NECTAR Sucrose dominant S/G+F = Sucrose 78-80% of total sugars Nectar feature Male flower Female flower Volume (µl)( (±SD,( n=6) 94.6± ± Mean secretion rate (µl/h)( Sucrose conc. (mg/ml) (±SD,( n=6) 258.5± ± Ann. Bot : Unconsumed nectar is resorbed by the flower in the hours soon after closure Nepi et al. 1996; Nepi et al 2001

5 Unconsumed nectar is resorbed by the flower Nepi et al Nepi et al. 2001

6 Ovule of umpollinated flower WHERE IS REABSORBED NECTAR CONVEYED? Use of radiolabelled sucrose ( 14 C) and autoradiography Nepi and Stpiczynska 2007 Ovule of pollinated flower Fecundated ovules represent a strong sink of reabsorbed sucrose Nectar is removed more quickly in female flower

7 RESOURCE RECOVERY FUNCTION OF NECTAR SUGAR RESORPTION Nectar production requires a considerable expenditure of energy. Southwick (1984) estimated that in Asclepias syriaca 4-37% of daily photosynthate assimilated during blooming is secreted as nectar sugar. He reported also that in Medicago sativa the energy invested in the nectar production is twice the energy invested in seeds. Therefore, when nectar is not collected by pollinators plants can re-utilize this important source of carbohydrates through nectar resorption. This mechanism was demonstrated in several plants and generally in old stage flowers.

8 SUGAR RESORPTION IN NECTAR HOMEOSTASIS It is to be considered that the relationship between the equilibrium concentration of a sucrose solution (as most of the nectars are) and the relative humidity of the air is such that for a 20% sucrose solution to gain water from the air, the relative humidity must be over 98% (Corbet et al., 1979). Such high humidity is not most likely to occur when diurnal pollinators are active so that in the daytime nectars will usually be losing water to the air. Nectar homeostatic mechanism, by nectar resorption, enables regulation of nectar volume, concentration and thus viscosity by reducing the effect of water loss due to evaporation.

9 The two functions were demonstrated separatley in different species Does the two mechanisms operate in the same species? Is there the possibility of nectar homeostasis in Cucurbita pepo?

10 Detailed information about pattern of nectar production Repeatedly sampled flowers (n=6) during anthesis h 00 6 h 00 7 h 00 8 h 00 9 h h h h 00 the quantity of sugar produced is already determined before secretion by the quantity of starch accumulated in the nectary parenchyma: the nectary produces a fixed amount of sugars. vol (µl) tot. Sug. (mg) conc (%) no effect of nectar removal on total nectar production Cumulative production Standing crop (n=6) vol. (µl) conc tot. sug. (mg) 67.6± ± ± ± ±1.5 32±4.7 ns ns ns

11 experiment I NECTAR SUBSTITUTION The nectaries was emptied at 5.30 a.m with a microcapillary The natural nectar was replaced with a low volume (20 µl) of concentrated nectar (60%) or with a high volume (100 µl) of diluted nectar (10%). For each treatment n=6 (each one from a different plant) h 00 6 h 00 7 h 00 8 h 00 9 h h h h 00 MALE FLOWERS vol (µl) tot. Sug. (mg) conc (%) volume and concentration were measured again after 3 hours Total sugars were calculated according to Bolten (1979): mg=c(%)*vol(µl)*d/100

12 140 diluted 120 concentrated ,76 20, ,5 18,6 5,30 8, ,68 15,46 32,4 5,30 8,30 volume (µl) tot. sug. (mg) con (%) volume (µl) tot. sug. (mg) conc (%) In each treatment all the final values are significantly different from the initial ones, except for the volume in diluted nectar treatment. vol conc. tot. sug. Final 100.7± ± ±4.4 Control 73±10 38± ±4.7 ns p<0.005 p<0.005 NSPR 2.8 mg/h Control NSPR 10.9 mg/h NVPR 18.2 µl/h vol conc tot. sug. Final 86± ± ±6 Control 73±10 38± ±4.7 ns p<0.05 ns NSPR 5.4 mg/h NVPR 21.6 µl/h

13 Conclusions from experiment I Secretion is sensitive to the nectar present on the nectary surface High volume and low concentration seem to inhibit sugar production Concentration of the secreted nectar can be adjusted during secretion In the case of Cucurbita pepo nectar homeostasis is able to adjust nectar features during secretion in concentrated nectar by sugar resorption

14 experiment II NECTAR SUBSTITUTION Nectaries was emptied at 10 a.m. with a microcapillary The natural nectar was replaced with a low volume (20 µl) of concentrated nectar (60%) or with a high volume (100 µl) of diluted nectar (10%). For each treatment n=5 (each one from a different plant) h 00 6 h 00 7 h 00 8 h 00 9 h h h h 00 MALE FLOWERS vol (µl) tot. Sug. (mg) conc (%) Volume and concentration were measured again after 2 hours Total sugars were calculated according to Bolten (1979): mg=c(%)*vol(µl)*d/100

15 Dilute nectar ,05 18,81 10, ,00 12,30 16, Concentrated nectar 60 28,47 34,8 15,46 11,72 10,00 12,30 volume (µl) tot. sug. (mg) conc (%) volume (µl) tot. sug. (mg) conc (%) In each treatment all the final values are significantly different from the initial ones vol. conc. tot. sug. Final 80± ± ±3.4 Control 73±10 38± ±4.7 ns p<0.005 p<0.005 Mean NSPR 2.5 mg/h vol. conc. tot. sug. Final 28.5± ± ±1.3 Control 73±10 38± ±4.7 p<0.005 ns p<0.005 Mean NSRR 1.6 mg/h After flower closure NSRR 2.1 mg/h (Nepi et al. 2001)

16 Conclusions from experiment II The nectary is sensitive to the treatments and respond differently There is the possibility to modify nectar features (vol. and conc.) after cessation of secretion (homeostasis) In the case of Cucurbita pepo nectar homeostasis adjust concentration in concentrated nectar by sugar resorption after cessation of secretion

17 GENERAL CONCLUSIONS As stated by Pedersen some decades ago (1958) nectar is not a static product remaining outside the plant once produced but it is in close contact with the plant system. It is evident now that secretion may occur concomitantly with resorption and that sometimes the latter process continues after secretion has ended. For this reason nectar production is best to be considered as a unified process comprising nectar secretion and resorption. The rate of the two opposite processes can be dynamically modified by the plant according to ecological (maintenance of a relatively constant nectar concentration to ensure visits by pollinators, this is nectar homeostasis) or physiological (reallocation of resources especially in post-fertilization development of ovules and ovary) constraints.

18 The topics covered vary widely: they include historical aspects, the structure and ultrastructure of nectaries and relationships to plant systematics, the dynamics of nectar secretion, nectar chemistry and the molecular biology of defense proteins, adaptations to insect and vertebrate nectar consumers and consequences for pollination ecology, and broad-scale studies of nectar resources at the community level. Written for: Scientists and students involved in the different approaches to plant reproductive biology and pollination biology Keywords: nectaries nectary plant reproduction plant-animal interactions pollination pollination biology pollination ecology