Seed weight af­fects growth rate

The rate of growth of onion seedlings seem to be de­pen­dent on seed weight, Plant & Food se­nior sci­en­tist, Bruce Searle, told the Onions New Zealand re­search sem­i­nar.


“They’re com­ing out of the ground at a dif­fer­ent size al­ready,” he said.

“We think that’s be­cause of the seed weight and that does af­fect the rate of growth.”

He is based in Hawke’s Bay and has worked on look­ing into emer­gence of onion crops and their vigour for the last two years.

There was a lot of vari­abil­ity seen with weights of 153 grams to 464g per square me­tre due to en­vi­ron­men­tal fac­tors, with plants un­able to catch that deficit up.

“We’re in­ter­ested in when the rad­i­cal pops out of the seed and starts to form the root be­cause that’s a fairly con­sis­tent time,” he said.

“Tem­per­a­ture is one of the most im­por­tant fac­tors.”

An ex­per­i­ment was set up look­ing at dif­fer­ent tem­per­a­tures and field ca­pac­ity to see how these fac­tors changed onion seedling growth.

“As tem­per­a­tures got hot­ter soil mois­ture was more im­por­tant,” he said. At 25 de­grees C growth slowed down so late onion plant­ings were go­ing to need more mois­ture to get seeds out of the ground.

Searle said a trac­tor had been de­vel­oped over­seas which was able to sense soil mois­ture and plant sweet­corn seeds at the op­ti­mal depth for growth.

“I don’t know if you could do it with onions,” he said.

Soil com­paction and crust­ing through lack of mois­ture could also in­ter­fere with onion emer­gence, he said.

“The plant is not go­ing to re­cover so you’ve got to have the soil in good con­di­tion.”

Re­search over the last 25 years had shown a range of time be­fore emer­gence took place, from 10 to 20 days.

“It’s much easier to man­age a crop com­ing out of the ground fast, but you don’t have a lot of con­trol over that,” he said.

“You do over soil con­di­tions.” When rain­fall statis­tics were over­laid on the emer­gence tim­ing it was seen that there could be from five to 10 mil­lime­ters to 120mm re­ceived.

“Nor­mally onion crops are get­ting enough mois­ture but in some years there was too much,” he said.

“If you’ve got a com­paction layer 120mm is a prob­lem and man­ag­ing the soil is the only way to han­dle that.”

A root stunt­ing ab­nor­mal­ity had been seen in Hawke’s Bay where ei­ther the seed did not get out of the ground, or else it did and then died.

“It hap­pens in seed pro­duc­tion in tem­per­ate re­gions and we think it’s caused by seed that hasn’t reached phys­i­o­log­i­cal ma­tu­rity,” he said.

“It’s worse when the soil gets wet and we don’t know why. The so­lu­tion is to use primed seed.”

Searle said some work on seed char­ac­ter­is­tics had been car­ried out last sum­mer, which found that small, dense seed was bet­ter than big, light seed.

Look­ing at vigour dur­ing es­tab­lish­ment a re­cent trial stud­ied the ef­fect of field ca­pac­ity. It found that onion seed grown in soil at 90% field ca­pac­ity was twice the height of one grown in soil at 70% field ca­pac­ity at the three-leaf stage.

“Keep­ing wa­ter up is very im­por­tant,” he said.

“If the plant is twice the size at that stage it’s go­ing to be a lot big­ger at har­vest.”

A light ir­ri­ga­tion be­tween emer­gence and the three-leaf stage if needed could help.

With nu­tri­ents he said to start with there wasn’t a lot of up­take by the plant, but then when the onion bulb is filling 80% of ni­tro­gen is taken up, com­pared with 20% at ear­lier stages. This meant grow­ers needed to ap­ply ni­tro­gen later on in the plant’s growth, but they might choose to make one ap­pli­ca­tion at an ear­lier stage if they were wor­ried they would not get back on the pad­dock be­cause of wet con­di­tions. A Hawke’s Bay trial showed that vir­tu­ally all the ni­tro­gen ap­plied early could be leached out of the top 30 cen­time­tres of the soil with an ap­pli­ca­tion of 194mm of wa­ter.

“The onion roots are of­ten not be­low 30cm so you can eas­ily lose ni­tro­gen if you are putting it on early,” he said.

“You might want to keep soil ni­tro­gen lev­els low if you know you are go­ing to have events where you will get leach­ing. A lot of ni­tro­gen is not a good thing be­cause it can cause dis­eases.”

A trial was now be­ing car­ried out us­ing ni­tro­gen test strips to show if it was nec­es­sary to make fur­ther ap­pli­ca­tions.

Searle said that work with bar­ley seedlings had shown that a ni­tro­gen de­fi­ciency early on was ben­e­fi­cial later as the plants were more re­sis­tant to aphids and thrips.

“There are good rea­sons to look at how much ni­tro­gen you’re putting on and what you’re do­ing to your sys­tem,” he said. A United States trial had shown dif­fer­ent re­sponses with wheat and maize ro­ta­tions, mean­ing some­thing dif­fer­ent was go­ing on. But new tools were com­ing on­line to help sci­en­tists bet­ter un­der­stand what was hap­pen­ing.

“... some work on seed char­ac­ter­is­tics had been car­ried out last sum­mer, which found that small, dense seed was bet­ter than big, light seed.”

It was also thought that small seeds grow­ing in cold con­di­tions could need more phos­phate, he said.

An­other trial had been car­ried out look­ing at whether there was a cor­re­la­tion be­tween seed weight and seed area but this couldn’t be proved.

“Heav­ier seed doesn’t grow faster but those with the low­est seed weight had the low­est rate of growth and great­est vari­a­tion in size,” he said.

And heav­ier seed pro­duced a more even canopy at har­vest.

“Un­der­stand­ing the mi­cro­biome and how to use ni­tro­gen will be im­por­tant for the fu­ture,” he said.

“We will know more about vari­abil­ity and will be able to pro­duce more uni­form crops.”

▴ Bruce Searle – big vari­a­tions in onion seedling growth.

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