Keep me updated about it, sounds very interesting to look for information about it

At this time I lack the expertise to give you a good answer

Hi Andre,

Myself and colleagues are likewise very interested in this topic! Nectar as a manipulant, increasing pollen (or analogue) export has been shown, but little research done on whether presence of nectar results in decrease in pollen collection to my knowledge! See Young et al.  2007; Foraging behavior affects pollen removal and deposition in Impatiens capensis (Balsaminaceae)

Flower visitors expect a reward. Thus, if the flower offers only pollen, flower visitors have to eat/collect pollen. If the flowers offers also nectar, then the flower has the change to evolve nectar as the exclusive reward by concealing pollen in the flower (keel, upper lip, etc), producing pollen that flower visitors cannot use (pollinaria, bitter-tasting pollenkitt, pollen spines).

I not think that nectar presence (source of second metabolite) is in correlation with the pollen exportation to other flowers (cross pollination, reproductive function)

The both - pollen & nectar - can served as resource of food 

Thank you all for this interesting discussion. Dear Avery I'll take a look on the reference you cited. Great you are also interested in this topic. Dear Klaus thank you for the evolutionary comment, it lead us to think deeper about the meaning of nectar production. My specific point here is when a plant indeed have both nectar and pollen and visitors can decide what they are going to collect. Does pollen remain for longer? Is there an influence on the pollinator behaviour? e.g. some groups may collect only nectar first or maybe only pollen first. Maybe the landing on flower will be different when the expected resource is both or just one? Not sure, I'm clear here, but my question is more related to the ecological meaning or consequence of producing both resources at the same time for pollinators able to collect both...

Dear Andre, on my little experience in the area I can tell you that it depends on the needs of the flower visitors that interacting with the flowers as Klaus said. That is, if the floral visitor requires the "pollen resource" then it will visit the flowers for that resource, if it needs nectar then this one will visit the flowers for that resource. It should be mentioned that this does not always bring a benefit to the plant, since there are many visitors who can interact as system antagonists (such as nectar robbers) like ocurs with some Xylocopa species. In addition, the resources produced at the same time imply a cost-benefit balance for the plant. That is, through the production of nectar (cost for the plant) the plant guarantees subsequent visits by the visitors for this food resource, so this increases the possibility of pollination success for the plant (benefit).

What you can do is to see what flower visitors do during the interaction, that is, if it gets to feed on pollen, nectar or both and even more accurate observations like if visitors touch reproductive structures during the visit. In this topic I studied the flower visitors on endangered cactus (Astrophytum myriostigma) and I characterize the type of visit of each visitor species according to flower structure that visitors touched during the interaction (through direct observations).

There is the link to the mentioned paper: 

https://www.researchgate.net/publication/291385106_Floral_visitors_of_Astrophytum_myriostigma_in_La_Sierra_El_Sarnoso_Durango_Mexico?ev=prf_high

Best regards.

Cristian Martínez

Article Floral visitors of Astrophytum myriostigma in La Sierra El S...

Flowering plants face a pollen dilemma, that is they benefit if more pollen is for pollination of conspecific flowers than for attracting/rewarding flower visitors/pollinators. However, the competition for pollination is probably exclusively intraspecific and thus plays no role on the pollination market. But when some individual begin to use a mechanism which helps to allocate more pollen for pollination than for rewarding flower visitors, then the benefit will translate into reproductive success

In most cases, from the flower pollination point of view it is more effective to use pollen collecting pollinators then only nectar collectors. Therefore an excess pollen production, plus an effective mechanism for ensuring using a small fraction of the pollen for pollination, is the common solution for the dilemma. The interspecific competition for attracting the efficient pollen collecting pollinators doesn't make it possible to reduce the amount of collectable pollen. Therefore for most flower species a mechanism which allocates more pollen for pollination than for rewarding the flower visitors is not productive, and not needed. Well, competition always leads to waste. Of course, an opposite direction for that dilemma solution is to entirely prevent pollen collection as a reward, plus ensuring the pollination process using nectar collecting pollinators, or even deceived visitors.

Dera Gad, do have evidence for your claim that from the flower pollination point of view it is more effective to use pollen collecting pollinators then only nectar collectors?

Only nectar collecting bees frequently avoid contact with the pollen, and sometimes with the stigma as well. Few instances: While visiting an apple flowers (and some other Rosaceae flowers) only nectar collecting bees prefer to reach the nectaries entering between the filaments' bases ("bottom workers") instead of entering between the anthers ("top workers"), avoiding contact with both pollen and stigma. See: Mayer and Lunden, 1988: Foraging Behavior of Honey Bees on Manchurian Crabapple and Red Delicious Apple. Journal of the Entomological Society of British Columbia 85. Only nectar collecting bees visiting alfalfa flowers (and some other Fabaceae) also avoiding contact with pollen (and stigma) as "side workers". In avocado only nectar collector bees cleanup themselves from the pollen after every two-three visitations, while visiting male phase flowers. See: Ish-Am G., Eisikowitch D. (1993): The behaviour of honey bees (Apis mellifera) visiting avocado (Persea americana) flowers and their contribution to its pollination. Journal of Apicultural Research 32:175-186.

I'd think a increase in nectar production is unlikely to influence the amount of pollen collected by a pollen collecting bee during a single flower visit.

Additional nectar might however attract more visitors of other taxa. Whether or not these other taxa contribute to pollen export and pollination depends on their behaviour and on flower morphology. This means than under certain circumstances plants might be able to increase their reproductive success by increasing nectar volume, under other circumstances plants will be able to increase their reproductive success by decreasing nectar production. ...

If the simple rule "more nectar leads to better pollination" would apply, than all plant species should produce lots of nectar - which is obviously not the case.

I would like to turn your attention to 2 issues:

(1) pollen is not only for plants. It is also a reward for pollen eaters and is necessary and crucial for the development of their larvae (nectar is source of energy and pollen is source of the matter needed for growth and development);

(2) pollen-eating pollinators may visit different flowers for nectar and pollen collection (e.g. paper below).

This only refers to pollen-eating pollinators (like bees - their larval development depends on pollen) and may shape their interactions with flowers. This is not important considering pollinators that larvae feed on plant leaves (like butterflies). Therefore you may consider asking your question in the context of particular life history realized by a particular pollinator.

Kind regards!

Michał

Article Pollen Preferences of Three Osmia Species (Hymenoptera)

No, I am the avocat of plants and In judgment we must see the reality:

"pollen is  only for plants reproduction function" .

I agree with Marpha, in the first place pollen is for pollination, and flowers that use pollen as a reward have at minimum an allocation problem of how much pollen is for pollination and how much pollen is for reward. Hence attraction as many as flower visitors as possible is not a good option for flowers rather than selectively attracting good pollinators and selectively excluding nectar robbers, pollen thieves and pollen eaters(collectors and bad pollinators.

I'm not saying that pollen in the first place is a reward for pollen-eaters. I'm saying that it is also a reward for pollen-eaters, which are not able to growth and rich maturity without pollen. Their fitness is related to the nutritional quality of the matter eaten during larval development and non-sugar compounds of this nutritional quality originate from pollen. Some bee species depend on certain plant species and vice-versa. Why not consider this while studying plant-pollinator interactions? Are bees considered as bad pollinators?

Kind regards!

Michał

The primary role of pollen is obviously for propagation, acting as the male gamete. However, evolution wise pollen had act as the first attractant as well. The early pollinated plants used wind as the pollen vector. Later wind pollinated plants, like Cycas, used pollen eating beetles as a second order pollen vector. Beetle pollinated plants were the next evolutionary step. These plans' flowers, like Paraver, produced excess of pollen for the beetles, and ensured pollination fulfillment using a small pollen fraction. Nectar production as an attractant appeared only later, coevolving with nectar eating insect orders like flies, butterflies and lastly bees. Only then had appeared plants that could prevent pollen collection, and could use nectar eating animals as the pollination agent. Interestingly, using pollen as an attractant for pollen eaters had been a precondition for animal pollination (Zoophilly).

I'm asking to present one more idea about the evolution of pollen amounts and nectar. The early wind pollinated plants produced large numbers of pollen grains and carried only 1-2 ovules (a very high P/O ratio, pollen grain number per ovules number in a flower), since wind pollination is very hazardous. Moving to beetle pollination, which was much safer, enabled decreasing of P/O ratio, either by producing less pollen grains and/or by increasing ovule numbers. The nectar production, and the appearance of nectar eater animals enabled, for the first time, developing mechanisms for preventing pollen collection and consuming, and using pollen only for pollination. This diminished P/O ratio dramatically, mainly by increasing the ovule numbers. The overall evolutionary picture was a decrease of pollen grain numbers, and mainly decrease P/O ratio from very high values to much lower ones. However, as Michal Filipiak mentioned, for a single flower, or for a species, the "more nectar means less pollen grain numbers" assumption may not be valid.

Dear Andre,

Wonderful question!  I see from the comment thread that everybody has ideas, but no data.  I agree with you that there is likely to be a connection.  One thought I have relates to the mix of pollinators that are attracted to flowers with variable amounts of nectar, another to the behaviors that individual pollinator species engage in when they visit flowers with vs. without nectar.  If flowers with nectar attract more pollinators who do not collect pollen, and their visits are of greater value in distributing pollen, then indeed nectar production should influence pollen dispersal to other flowers.  Similarly, if the presence of nectar causes pollen-collecting visitors to switch behaviors so that they put less pollen in structures that do not contact the sex parts of subsequently visited flowers, then the presence of nectar would also enhance pollen dispersal.  And then, of course, is the trade-off issue:  perhaps investment in nectar decreases investment in pollen production.

Mary

Pedicularis should be a possible model to test this question. Some species of Pedicularis produce nectar, but most of species without nectar are buzz pollinated by bumblebees. Does buzz species produce more pollen than nectar producing species? All nectar producing species aren't beaked, pollen grains deposited on the back of bumblebee (head and/or thorax, these are relatively safe places), therefore nectar rewarding species produce large mount of pollen should be a waste. If bumblebee can discriminate the pollen amount of buzz flower, selection should favor species with more pollen grains in buzz flowers.

Hi there!

Indeed Mary this is the direction of my rationale. Thank you for joining and adding to this discussion!

Dear Zong-Xin, Yes I can see the point in your experimental design. In the Neotropics we have Melastomataceae where this approach will be feasible as wel. How about a cross continent cross family test? Thank you all for this incredible debate. Science cannot be other way than colaborativa!

Thank you all for the inputs in this question, all our debate was consider to discuss the results of the attached paper. Thank you!

Just to ponder. Pollen and honey sre complementing to each othrr as food...etc. That has to be addressed in your postulation that it is a zeri sum game between the two... thinking