Diver­sity key to pol­li­na­tion

One chal­lenge for the av­o­cado in­dus­try is to im­prove or­chard pro­duc­tiv­ity and pol­li­na­tion is right up there.

The Orchardist - - Avocado Conference - By Denise Landow

Eight years of av­o­cado pol­li­na­tion re­search work can now be used by the in­dus­try in prac­ti­cal ways, Dr David Pat­te­more told the New Zealand Av­o­cado In­dus­try In­ter­na­tional Con­fer­ence.

He’s the team leader, api­cul­ture and pol­li­na­tion at Plant & Food Re­search, which has 19 staff in both New Zealand and Aus­tralia. His di­verse re­search projects cover the be­hav­iour of pol­li­na­tors for fruit, nut and veg­etable seed crops.

He said New Zealand’s cli­mate pushes the av­o­cado’s bound­aries be­yond its nat­u­ral range.

“We’re re­ally on the edge of the av­o­cado world. Fruit set rate is a low 0.3 per­cent. Three fruit from 1000 flow­ers isn’t un­usual for mass-flow­er­ing trees from di­verse trop­i­cal ecosys­tems. It was never the plant’s in­ten­tion to set all flow­ers into fruit.”

De­lib­er­ately cross-pol­li­nat­ing by hand on se­lected branches can get set rates of up to five per­cent or 50 fruit from 1000 flow­ers.

“Such work shows the gap be­tween nor­mal pol­li­na­tion and the po­ten­tial, even to go from 0.3 to 0.6 per­cent, rep­re­sents a dou­bling in yield,” he said.

Av­o­ca­dos have bizarre flow­er­ing be­hav­iour. Each flower opens once in a fe­male phase, then closes and sub­se­quently opens on a dif­fer­ent day in a male phase. It’s com­pli­cated fur­ther with two dif­fer­ent types of cul­ti­vars. Hass is a type A cul­ti­var in which fe­male flow­er­ing typ­i­cally oc­curs dur­ing morn­ings, then close and open as male the fol­low­ing af­ter­noon. In con­trast, type B cul­ti­vars open as fe­male in the af­ter­noon, close and next morn­ing open in the male phase.

Only male flow­ers pro­duce pollen and this is why or­chards are tra­di­tion­ally es­tab­lished with a mix of cul­ti­vars, so when Hass flow­ers are fe­male, male type B flow­ers are pro­duc­ing pollen and pro­vid­ing enough to be moved onto them.

“Cul­ti­vars are en­tirely syn­chro­nis­tic – all trees go fe­male, they close, and then go male but there can be a pe­riod of over­lap in the mid­dle,” he said.

Most over­lap is around mid­day when tem­per­a­tures are warm be­fore fe­male flow­ers have fully closed and males be­gin open­ing, but the key in­gre­di­ent now is in­sect ac­tiv­ity.

Re­searchers had to find out how much pollen dif­fer­ent in­sects deposit when they land, their ac­tiv­ity pat­terns and their cul­ti­vars of choice. Stud­ies show that as the num­ber of av­o­cado pollen grains on the stigma de­posited in­creases, so does the prob­a­bil­ity of fruit set.

“If you have one pollen grain there’s a lower prob­a­bil­ity of fruit set than if you have 50 pollen grains, there’s a di­rect re­la­tion­ship,” he said.

“In­ter­ac­tions and com­pe­ti­tion oc­cur with pollen grains and it might also be due to the vi­a­bil­ity of pollen grains – that means, to have a good prob­a­bil­ity of fruit set, dozens of pollen grains need to be de­posited on that stigma.”

Over sev­eral years sci­en­tists stud­ied flower stig­mas and counted how many pollen grains were de­posited. Time af­ter time Aus­tralian and NZ or­chards showed sim­i­lar pat­terns with the vast ma­jor­ity of flow­ers at the end of the fe­male flow­er­ing pe­riod hav­ing zero pollen grains de­posited.

“We need to find ways to in­crease pollen de­posits on av­o­cado flow­ers, be­cause that will re­move pol­li­na­tion as one of the lim­it­ing fac­tors for pro­duc­tion,” he said.

Most honey bees land­ing on Hass fe­male flow­ers had less than 20 pollen grains.

“If you just have 20 pollen grains on a bee the prob­a­bil­ity of get­ting enough pollen grains on the stigma is van­ish­ingly small and shows a prob­lem; bees vis­it­ing flow­ers don’t have suf­fi­cient pollen grains.

“Our stud­ies show bees carry so few pollen grains that there’s no harm in in­creas­ing other pol­li­na­tor ac­tiv­ity in or­chards. More in­sects in­crease vis­its dur­ing male flower open­ing, and this in­creases the prob­a­bil­ity of more pollen grains be­ing de­posited on stig­mas, which builds up enough pollen grains for good fruit set.”

Re­search shows the crit­i­cal ef­fect tem­per­a­ture has on flow­er­ing. One three-year study had cam­eras and tem­per­a­ture mon­i­tors take record­ings of av­o­cado flow­ers ev­ery five min­utes dur­ing flow­er­ing. As tem­per­a­tures de­clined at night fe­male flow­ers opened later the next day. Min­i­mum overnight tem­per­a­tures of 11°C and 16°C had flow­ers open in the ideal time for honey bee ac­tiv­ity from 9am to mid­day.

But as tem­per­a­tures dropped to 9°C – 11°C flow­ers opened be­tween mid­day and 4pm, past the peak of honey bee ac­tiv­ity. Even colder tem­per­a­tures from 4°C – 8°C had flow­ers not open­ing un­til 2pm, and some­times as late as 6pm.

And stud­ies found what­ever phase the flower is in when the sun sets, it es­sen­tially freezes in that phase overnight.

“Not only do flow­ers open in the late af­ter­noon when honey bees are head­ing for bed, but they stay open all night and don’t close un­til about around 9am the next day,” he said.

“This is im­por­tant to know if we want to max­imise the amount of pollen grains be­ing moved and de­posited. In cooler tem­per­a­tures, honey bees are not the ideal in­sects be­cause they’re not ac­tive in those con­di­tions.” Honey bees have a dis­tinct ac­tiv­ity pat­terns which peak around mid­day to 2pm, but other in­sects are ac­tive in af­ter­noon, evening and overnight.

NZ ob­ser­va­tions show sig­nif­i­cant num­bers of male flow­ers open and pro­duc­ing pollen most nights, re­gard­less of the tem­per­a­ture which rep­re­sents a real op­por­tu­nity to har­ness pollen trans­fer.

Diver­sity of in­sect pol­li­na­tors means diver­sity in in­sect ac­tiv­ity pat­terns. Aus­tralian re­search proves some or­chards don’t have any honey bee ac­tiv­ity and good pol­li­na­tion is se­cured al­most en­tirely by flies and bee­tles. NZ or­chards have low num­bers of al­ter­na­tive in­sects but David’s team is look­ing at ways wild pol­li­na­tors can be used and heav­ily boosted in num­bers.

Moths could pro­vide help be­cause they’re present and busy at night in av­o­cado or­chards, how­ever as yet it’s not known which species might trans­fer pollen.

Bum­ble bees are an ex­cit­ing al­ter­na­tive to honey bees be­cause their large and hairy bod­ies cross-pol­li­nate more ef­fec­tively.

Vari­abil­ity is the rule.

“We know that pol­li­na­tors vary in ac­tiv­ity, be­hav­iour and ef­fi­ciency,” he said.

“Key in think­ing about the pol­li­na­tion cy­cle is there’s not much we can do about bi­o­log­i­cal facts about the av­o­cado plant’s flow­er­ing, how­ever, we can do things about the over­lap of cul­ti­vars when set­ting up or­chards.

“We can use dif­fer­ent pol­li­na­tors and that’s what our work has come to – fo­cus­ing on how we can pick the right pol­li­na­tors, then boost­ing their num­bers so we can en­sure we can get suf­fi­cient pollen flow­ing onto the fe­male av­o­cado flower.”

“We need to find ways to in­crease pollen de­posits on av­o­cado flow­ers, be­cause that will re­move pol­li­na­tion as one of the lim­it­ing fac­tors for pro­duc­tion.”

Dr David Pat­te­more

Newspapers in English

Newspapers from New Zealand

© PressReader. All rights reserved.