Why is apple blossom pink when it's stated bees cant see red?
Apples require cross-pollination for a sizeable crop, ie the transfer of pollen from one tree variety to another. Apple trees have evolved to use insects for this so there is good genetic mixing despite human efforts at cloning them. No two wild apple trees are the same genetically similar to humans. The pollen grains are too heavy and too sparse for effective wind pollination. Bees are the main insect used to pollinate apple trees in orchards. In New Zealand native solitary bees such as lasioglossum sordidum, some flies and bumblebees are the most effective pollinators.
Lasioglossum sordidum
Bumblebee on pear blossom
Honey bees are used commercially however prefer other flowers.
The bees brush pollen off the antlers onto the hairs of their bodies whilst dipping past them into the base of the flower to reach the nectar it has at its base. The pollen is held on the hair by static electricity generated by the bee’s flight.
The pollen is then smeared onto the stigma of the next blossoms the bee visits. At Beau Vista we have established self sustaining bumblebee population in the area.
This got me thinking as apple blossom is pink on its outer petals and white on its inner petals. And there has been a long standing and pervasive dogma that bees do not see reds (but can see into the UV instead, that we as humans cannot). Bees like many insects see colour but their eye receptors are tuned to different wavelengths to human and primates the other tricolour animals. Humans see light in wavelengths from approximately 390 to 750 nanometers (nm). These wavelengths represent the spectrum of colours we can see. Bees, like many insects, see from approximately 300 to 650 nm. Red is at 700nm.
The earliest experiments showing bees did not respond to red stimuli was by Nobel Prize-winner Karl von Frisch and first published in 1914.
Karl von Frisch
Often present on bee pollinated flowers including many “white” ones are ultraviolet “nectar guides” which act much like airport runway lights that guide the pilot to a safe landing, the ultraviolet markers guide the bee directly to where the flowers nectar reserves are in the centre. Nectar is a reward for any pollinator who visits the flower and ensures that pollen is transported to other flowers. Red flowers are alluring to many butterflies and/or hummingbirds. Temperate region flowers such as apple have co evolved to attract insects inc bees with UV to green range of colours and patterns; tropical flowers are often red to attract butterflies and humming birds. with the nectar deeply hidden in long narrow tubes reachable with long tongues or proboscises. Sadly I have not been able to find any evidence that apple blossom has evolved in this manner. The only Rosacea species I can find so far examined, Rosa dumalis or dog rose show no UV colouration.
Rosa dumalis under natural light
and under UV light. Photo by Bjorn Rorslett
However it is not only colour but iridescence from polarised light that attracts insects. Insects have compound eyes which gives them the ability to see this, humans cannot directly.
Flowers also come in a variety of scents. Apple nectar is sweet smelling and attractive to bees.
Although honeybees are the insects most used in commercial orchards, they are not as efficient pollinators as some solitary bees and bumble bees. A lot of honeybees take nectar from the flower without even touching the anthers and so do not contribute to pollination. It has been shown adding bumblebees has multiple effects on pollination: not only the number of pollinating insects increased, they were more efficient at carrying pollen. Bumblebees work in harsher weather conditions and even the foraging behaviour of the honeybees was enhanced!
Interestingly for us at BeauVista orchards, were we have pear and apple trees mixed in the orchard, scientific studies show that honeybees working pear orchards carried only 2‑11% of pollen other than pear, whilst for apple the values were 30%-43%. Ie honey bees are attracted away from apple flowers by the plants in surrounding fields of rape, plum (if still in flower) and weed species such as dandelion and groundsel.
So why are apple blossoms red?
Why has this feature not been lost during evolution? Red must confer some advantage. So I looked further at bee colour vision. An important point is that flowers are not monochromatic like test cards previously used. Instead many of them, particularly those that appear red, orange, yellow, and white to humans, have reflectance patterns that are essentially step functions and bees should be able to discriminate such reflectance patterns over a range of 550–650 nm, since reflectance functions with steps at such wavelengths will occupy different loci in bee colour space and thus be distinguishable. In this sense, bees should distinguish between green-, yellow-, orange-, and red-reflecting objects as shades of grey before reaching a point when the longer wavelength opsin stops detecting all together, (similar to violet being the last colour humans can see at the shorter end of the visible spectrum). Behavioural experiments show that bumblebees can indeed perform this task.
This leads then into a dive into the molecular basis of of vision. Although a bee’s eye being compound is radically different to a humans, the detection of the photon of light energy is remarkably similar. Light enters through the eye and strikes the ‘retina’ at the back of it. The retina is composed of specialised cells, cones, which convert light energy into neural electrical activity. The 3 ( possibly 4) types of cones are for colour detection. This conversion is made possible by light-sensitive pigments located on the cones outer part.The photo responsive molecule is a chemical called 11 cis-retinal. ( cis and trans in organic chemistry refers to the way the molecule and its active radicals are arranged; trans has all the important moieties on one side cis refers to the opposite. 11 refers to the carbon atom on the backbone ie the 11th carbon in the chain is isomer bends). Rentinal is derived from Vitamin A, hence why carrors are good for vision. When visible light hits the 11 cis-retinal, the cis-retinal undergoes an isomerisation, or change in molecular arrangement, to all-trans-retinal.
The new form of all trans-retinal does not fit as well into the protein opsin it is embedded in. Opsins are G proteins based on a 623-amino-acid virion ( closely related to part of the rabies virus) transmembrane glycoprotein, an important group of proteins . A series of geometry changes in the protein begins. These changes make the photoreceptors' membranes less permeable to certain ions, such as sodium. This change in permeability alters each photoreceptor's membrane potential and allows it to send an electrical signal to cells in the next layer of the retina then by the optic nerve to the brain where the pattern of impulses are interpreted as colour vision. In accordance with the principle of invariance a photoreceptor's output signal is proportional only to the number of photons absorbed. The photoreceptors cannot measure the wavelength of light that it absorbs and therefore does not detect colour on its own. Rather, it is the ratios of responses of the three types of cone cells that can estimate wavelength, and therefore enable colour vision. Each of the 3 cones have different opsin amino acid structures. The differences are just a few amino acids over many hundreds but have been very stable over eons so the exact pigments in humans and bees are very closely related. The human genome its composed of up to 8% retrovirus fragments that evolution over eons has rendered useful to human cellular functions. All-trans retinal released from the protein and reduced to all-trans retinol. On the the retinal pigment epithelium it is converted to 11-cis retinol and then oxidised to 11-cis retinal before it is transported back to the photoreceptor to regenerate the opsin complex and complete the visual cycle. The 11 cis retinal is less stable than the all trans retinal isomer so one photon of energy can initiate the cascade. Reverting the more stable all-transform requires energy in the form of APT and NADH, so vision which is obviously of benefit is a net energy consuming function ie more energy is needed to restore the photopigment back to the cis form than it absorbed in the photon of light. The wavelength at which retinal absorbs a photon is measured to only 380 nm ( Violet) and it is the surrounding opsin protein structure that blue shifts it to to 360 nm in the S- cones and a larger red shift to 440 nm in the L cones. This is is referred to as the “opsin shift” and relates to just a few amino acid changes in the molecule. Opsins were not always vision related. This role began when retinal was incorporated. Prior to that and still existing today are roles in sensing light based circadian rhythms and even before that temperature, sound and chemicals in the environment.
Back to bees and apples. It seems bees can indeed detect the pink in apple blossom; (although not strongly as shown be the honey bee’s tendency to prefer bright yellow flowers), and this with iridescence and possible UV colouration, plus scent confers advantages to the apple by attracting the bee to pollinate the blossom. So the pink colour remains. However why it was pink to begin with still remains a mystery to me. Perhaps relating to the colour preferences of long lost insects.