When foraging, a floral visitor actively collects or passively touches pollen grains that then attach to its body, it can then be delivered to flower receptive stigmas of the same species resulting in pollination. Sometimes the pollinators move from one species to another and therefore move pollen to “wrong” stigmas.
For the plants it is very important to offer highly attractive resource, such as floral nectar, that may attract floral visitors and manipulate floral visitors moving among individuals of the same plant species within the communities.
Nectar is mainly made of water and sugars, including sucrose, glucose and fructose, but also includes some minor components, i.e. amino acids, lipids and secondary metabolites.
In general, floral nectar production is a dynamic process involving production and reabsorption. After visitors remove nectar from the flowers, it is usually replenished. However when the attractive function is ended, the nectar left in the flower is usually recovered by the mother plants, saving all the sugar and/or water.
Given floral nectar is essentially just water and sugar, it is a valuable energetic resource with plants experiencing costs/trade-offs associated with its production or absorption, and adopting mechanisms to regulate nectar in flowers.
Prof. Graham H. Pyke, a winner of a Chinese Academy of Sciences President’s International Fellowship Initiative (PIFI) and a professor in the School of Natural Sciences, Macquarie University, Australia, works with Associate Professor REN Zong Xin of Kunming Institute of Botany, Chinese Academy of Sciences on floral nectar production. They constructed a bibliographic database containing 320 articles considered to have some connections to nectar cost. By investigation and analysis of those references, they ask if it is actually a cost for the plant to produce floral nectar?
Previous publications often reach agreement that costs of floral nectar production are significant, but relevant evidence is scant and difficult to interpret. They found that cconvincing direct evidence comes from three types of experimental studies. Firstly, enhanced nectar sugar production (through repeated nectar removal) associated with reduced ability to produce seeds; Secondly, increased sugar availability (through absorption of additional artificial nectar) associated with increased seed production; Thirdly, high proportion of available photosynthates allocated by plants to nectar production.
In one of the convincing experiments, Pyke (1991) removed nectar of all flowers of each plant of Blandfordia nobilis (Blandfordiaceae; Figure 1) repeatedly and hand cross-pollinated the treated flowers with and without repeated nectar removal. He found that seed production was reduced for plants with repeated nectar removal demonstrating a significant cost to a plant for nectar production.
Southwick (1984) estimated the plant energy allocated to nectar production in the milkweed Asclepias syriaca and found that it was between 4 and 37%. However, experiments similar as Pyke (1991) and Southwick (1984) are rare.
Some studies do not include treatments of all flowers per plant and all lack quantitative cost-benefit comparisons for nectar production.
Other circumstantial evidence to support the cost of floral nectar production are listed as follows. First of all, temporal mismatch between nectar production and pollinator feeding activity. If floral nectar production is costly, then planst should invest to produce nectar temporally matching the time of pollinator feeding activity. Temporal mismatching between nectar production and pollination feeding activity would suggest no significant cost of nectar production.
Secondly. trade-offs between nectar production and plant growth, size or reproduction. Negative correlations between nectar production rate per plant and measures of plant size, such as above-ground biomass, root biomass, and number of flowers, or measures of plant reproduction, such as number of seeds per flower or seed mass, or measures of plant growth suggest a cost of nectar production.
Thirdly, nectar reabsorption and floral deceit. Reabsorption of floral nectar occurs after a flower has been pollinated or as a flower ages, provides good but circumstantial evidence that nectar production entails a significant cost and reinforces the view of nectar as valuable resources that can be saved. Deceiving flowers without producing nectar also provide evidence that nectar is costly, as resources of nectarless flowers can be instead allocated to floral signal display to attract floral visitors.
Lastly, possible resource movement within plants, number of treatments per plant, and number of treated flowers per plant. It is important, in experimental studies related to possible nectar costs, that any effects are not confounded by an ability of plants to move resources between flowers on the same plant. Therefore, flowers within a plant should be subject to a single experimental treatment and all or almost all flowers on each plant should be included in treatments.
Much of this circumstantial evidence of nectar cost is difficult to interpret and largely equivocal. For example, nectar reabsorption by flowers suggests that nectar is a valuable and costly resource, but a lack of such reabsorption is equivocal as plants may lack the opportunity or the process may be overly expensive.
In addition, the observation that some plant species do not produce floral nectar suggests significant nectar cost, but other explanations are possible, such as promoting outcrossing.
Pyke and Ren propose that these following aspects of research should be pursued in the future as follow”
Repeat direct experimental approaches that relate reduced or enhanced nectar sugar availability for a plant with consequent ability to produce seeds. To avoid confounding effects of inter-flower resource transfer, each plant should experience a single treatment, with treatment of all or almost all flowers per plant.
Resource allocation by plants, pathways used for resource transfer, and the locations of resource sources and sinks should also be investigated.
Extension of nectar cost into other areas of biology, including evolutionary models of nectar production and pollination syndromes involving floral nectar.
Assessment of the potential economic benefits if relationships between nectar production and crop value are evaluated.
The review titled: “Floral nectar production: what cost to a plant?” hass been published in Biological Reviews. This work is supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB31000000) and the Chinese National Natural Science Foundation (Grant No. 32271594 and 31971570). Graham Pyke work is supported by Macquarie University and the Chinese Academy of Sciences President’s International Fellowship Initiative (Grant No. 2020VBA0004).
Figure 1. Blandfordia nobilis (Blandfordiaceae) is a model system used by Graham Pyke to study floral nectar production (Photo by Graham Pyke).