Why do flow­ers smell?

Jane Wrig­glesworth in­ves­ti­gates the re­la­tion­ship be­tween plants and scent.

NZ Gardener - - CONTENTS - Seen any moths on your flow­ers at night? Dr Jan­ice Lord is putting to­gether a list of plants for a night­time scented moth gar­den and in­vites you to con­tact her with your ob­ser­va­tions. Email jan­ice.lord@otago.ac.nz

The sci­ence be­hind a fra­grant bloom

Most gar­den­ers know that plants pro­duce scents to en­tice pol­li­na­tors, but there’s more to the com­plex re­la­tion­ship than meets the nose.

From the un­savoury stink of the skunk cab­bage to the se­duc­tive fra­grance of sweet peas, some flow­ers dis­charge a cor­nu­copia of scent mol­e­cules into the air.

But while many of these fra­grant com­pounds may be pleas­ing to the hu­man nose, they’re not ac­tu­ally for the ben­e­fit of hu­mans.

Flo­ral scent sig­nals have pre­dom­i­nantly evolved to at­tract pol­li­nat­ing an­i­mals, and the type of pol­li­na­tor re­lates to the type of fra­grance emit­ted.

Plants that am­plify their fra­grance out­put dur­ing the day are pri­mar­ily pol­li­nated by bees or but­ter­flies. Those that re­lease their scent at night are pol­li­nated by moths and bats.

In the bush short-tailed bats play a role in the pol­li­na­tion of matai, re­warewa, po­hutukawa, ko­romiko, nikau and hebe, but you’re un­likely to find them in your back­yard.

Moths, how­ever, are ram­pant. “The main flower-vis­it­ing moths peo­ple are likely to see are the noc­tu­ids, the chunky cream, gold or green moths that look like they are wear­ing a fur cape, and the ge­ometrids, the slen­der pale moths with darker geo­met­ric mark­ings,” says Uni­ver­sity of Otago plant evo­lu­tion­ary bi­ol­o­gist Dr Jan­ice Lord. “Moths lose their colour when they die, so you can only see how beau­ti­ful they are when they are alive.”

Moths are at­tracted to pale, nights­cented flow­ers with abun­dant nec­tar. “Gar­den plants such as nico­tiana, jas­mine, hon­ey­suckle, night-scented stock and evening prim­rose are vis­ited by moths overseas so should also be at­trac­tive to moths here,” rea­sons Jan­ice. “A stu­dent of mine has shown that na­tive pime­lea, manuka, cassinia (aka ozotham­nus) and er­i­caceae ( gaulthe­ria and dra­co­phyl­lum) are at­trac­tive to na­tive moths.”

Sweet smell of suc­cess

As gar­den­ers know, not all flow­ers smell alike… for good rea­son. “Hover­flies, but­ter­flies, moths and bees are most at­tracted to sweet smells,” says Jan­ice, “whereas bats and flower-vis­it­ing mam­mals are more at­tracted to musky smells.”

Flow­ers use these sen­sory sig­nals to ad­ver­tise their re­wards – nec­tar and pollen – en­sur­ing the trans­fer of pollen from flower to flower.

Bees then re­mem­ber the spe­cific plant scents as­so­ci­ated with re­cent ex­pe­ri­ences of good food re­wards and will re­turn to that same plant as a re­li­able food source.

Some flow­ers are gen­er­al­ists and re­lease scent sig­nals that at­tract a num­ber of dif­fer­ent pol­li­na­tor species; oth­ers are more spe­cific, tar­get­ing only cer­tain pol­li­na­tors. The soap­tree yucca ( Yucca elata) re­leases scent sig­nals that at­tract only the yucca moth. Hence, flo­ral scent sig­nals are es­sen­tial in al­low­ing pol­li­na­tors to dis­crim­i­nate among plant species, and even in­di­vid­ual flow­ers of a species.

The time of day scents are re­leased can vary too. “The blue chicory has nec­tar in the morn­ing. Red clover is best af­ter lunch. The four o’clock flower opens in late af­ter­noon. The evening prim­rose fol­lows the four o’clock,” writes Shar­man Apt Rus­sell in Anatomy of a Rose, an ab­sorb­ing book which un­veils the in­ner life of flow­ers.

But a flower must ‘want’ to smell. Newly opened flow­ers that are not yet ready to re­lease pollen have fewer scent sig­nals.

Like­wise, when an older flower has been suf­fi­ciently pol­li­nated, its scent changes or shuts down to help di­rect pol­li­na­tors to un­pol­li­nated flow­ers. Snap­drag­ons lower scent pro­duc­tion about 48 hours af­ter pol­li­na­tion – it takes that long for the pollen tubes to reach the ovary, which trig­gers the shut­down. Petu­nias shut down scent pro­duc­tion within 36 hours of pol­li­na­tion.

Each flower varies, but the re­sult is the same. Scent sig­nals change or shut down af­ter pol­li­na­tion so that the plant doesn’t have to waste en­ergy pro­duc­ing them.

Why some plants smell bad

Beau­ti­ful fra­grances are not the only lure for pol­li­na­tors. Cer­tain plants spe­cialise in emitting odif­er­ous scents to dis­guise their true na­ture and in­tent.

“Some plants trick an­i­mals into pol­li­nat­ing them by mim­ick­ing the smell of some­thing com­pletely dif­fer­ent,” says Jan­ice. “For ex­am­ple, the dead horse arum, Heli­codiceros

mus­civorus, is pol­li­nated by car­rion flies which think it is rot­ting meat.”

Cer­tain plants spe­cialise in smells to en­cour­age pseu­do­cop­u­la­tion. “Some species of or­chids, for ex­am­ple

Ophrys apifera, pro­duce volatiles that mimic the sex pheromones of fe­male in­sects,” she adds.

These at­tract the males to land on the flower and in­ad­ver­tently pol­li­nate them.

But tim­ing is still ev­ery­thing

As in most mar­kets, sup­ply and de­mand fluc­tu­ates at cer­tain times of the year. A study pub­lished in Sci­en­tific Re­ports in 2013 re­ported that “scent ad­ver­tise­ment” was higher in plants that bloomed early in the sea­son when pol­li­na­tors are scarce, com­pared to flow­ers that bloomed later when there are more pol­li­na­tors. It was also found that plants with fewer flow­ers com­pete with dom­i­nant species for pol­li­na­tors early in the flow­er­ing pe­riod by send­ing out more of a chem­i­cal sig­nal called beta-ocimene.

For ex­am­ple, rose­mary and thyme were two dom­i­nant flower species in the study. They ac­counted for most of the nec­tar and pollen pro­duced. How­ever, less abun­dant flower species flow­er­ing at the same time and which shared pol­li­na­tors emit­ted a sim­i­lar scent but with a much higher pro­por­tion of beta-ocimene. These in­cluded Iris lutescens, Euphor­bia flav­i­coma, Mus­cari ne­glec­tum and Ra­nun­cu­lus gramineus.

Why some plants don’t smell of any­thing

Birds, with the ex­cep­tion of vul­tures and kiwi, have no sense of smell, so it fol­lows that flow­ers that are pol­li­nated by birds sel­dom have a scent, says Jan­ice.

Like­wise, wind-pol­li­nated flow­ers tend to be dull in colour and have no nec­tar or scent to at­tract pol­li­na­tors.

In the case of grasses, there are not even petals. Sweet corn, a grass, is one ex­am­ple. Its pollen is wind­blown, and thus is not sticky, which might oth­er­wise cause it to stick to leaves.

How pes­ti­cides affect scents

It’s also worth not­ing that spray­ing your plants can affect some pol­li­na­tors’ abil­ity to find those plants again, in par­tic­u­lar, bees. The global de­cline in bees has been linked, among other things to pes­ti­cide use.

“One of the main is­sues to do with mem­ory is that the use of neon­i­coti­noid pes­ti­cides – said to be safe for ben­e­fi­cial in­sects – have been shown to affect bees’ abil­ity to nav­i­gate,” Jan­ice ex­plains, “so they might not get back to the hive or might not re­mem­ber how to find the flow­ers again.”

A 2017 study pointed to neon­i­coti­noids caus­ing a re­duc­tion in bee num­bers. In the year fol­low­ing ex­po­sure to the seed coat­ings of win­ter-sown oilseed rape that con­tained neon­i­coti­noids, the bees es­tab­lished smaller colonies in the fol­low­ing spring (24 per cent de­cline).

“Avoid spray­ing bee plants with prod­ucts con­tain­ing thi­amethoxam, cloth­i­an­i­din, thi­a­clo­prid or im­i­da­clo­prid [all chem­i­cal names for neon­i­coti­noids which will be listed as an ac­tive in­gre­di­ent] or – my pref­er­ence – just not use these prod­ucts at all,” says Jan­ice.

Why florists’ bou­quets don’t smell of much

You’ve prob­a­bly no­ticed that flow­ers at florists shops do not al­ways have great scents. That’s be­cause cut flow­ers are typ­i­cally bred to im­prove ap­pear­ance and fo­cus on colour, stem length and flower qual­ity.

Scent doesn’t usu­ally get a look in, which has led to a re­duc­tion in fra­grant blooms from the florist.

Cut flow­ers also of­ten travel long dis­tances, so tough­ness is deemed more im­por­tant than scent. And as cul­ti­vated plants have no use for scent – at least not in the sense of re­pro­duc­tion – the lack of fra­grance is sim­ply tough luck for the con­sumer.

In com­par­i­son, if a plant in the wild was to lose its scent – think wild roses – the re­sult would be dev­as­tat­ing. Wild flow­ers could not sur­vive with­out their aro­matic lures to en­tice pol­li­na­tors.

So next time you stop to smell the roses, re­mem­ber, that fra­grance is not ac­tu­ally cre­ated to ben­e­fit you. But you can en­joy it any­way. As Bri­tish writer Bev­er­ley Ni­chols said, “To be over­come by the fra­grance of flow­ers is a de­lec­ta­ble form of de­feat.”

Scent sig­nals change or shut down af­ter pol­li­na­tion so that the plant doesn’t have to waste en­ergy pro­duc­ing them.

The owlet moth is a noc­tuid.

Red ad­mi­ral but­ter­fly.

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