Exploit physiology to transform yields

Will 200 tonnes per hectare become the new normal? By Anna Mouton.

Debates about the relationship of yield to planting systems and tree structure are as old as horticulture, according to plant physiologist Dr Ken Breen of the New Zealand Institute for Plant and Food Research. His team has been developing physiology-based metrics that can guide pruning and crop-loading decisions.

“Photosynthesis is the basic function that regulates crop productivity,” explained Breen. “Yield at an orchard level is related to the light interception of the whole orchard. Yield at a canopy level is related to the light penetration into that canopy.”

Light interception and penetration are affected by the design and management of both the orchard and canopy. The relationship between light interception and gross yield is roughly linear but Breen stressed that growers could shift the curve in their favour.

“We know we can get 100–110 tonnes per hectare quite comfortably with Royal Gala. With precision canopy management we’re talking about 120–130 tonnes per hectare. For a late-season variety — large fruit and high dry matter — like Scilate we can talk about 150 tonnes per hectare commercially and 170 tonnes per hectare experimentally,” said Breen.

Light penetration impacts both yield and quality. Bud quality and flowering — leaf number and size — depend on the light environment in the previous season. Fruit quality and colour depend on the light environment this season.

The problem with current orchard design is that row width is frequently dictated by the need for vehicle access. Reducing row width without changing canopy shape can limit light penetration. Breen stated that current orchards only intercept about 55%–60% of incoming light, and not all that light is productive because light penetration is suboptimal.

Breen believes that there is a large opportunity to improve yields and quality through management practices that maximise light interception and penetration.

Less is more

Breen presented results on trials conducted in a commercial Royal Gala orchard planted at 3.4 x 1.25 metres. The trees were trained as tall spindles and had an average of 9.1 limbs per metre of canopy translating into 29 limbs per tree. Limbs were reduced to 6.3 per metre of canopy — 20 limbs per tree — in the treatment group.

Light interception was monitored throughout the season and found to peak at around 60%. There was no significant difference between the treatment and the control. “We can cut down the number of branches by a third with no effect on potential yield — on the ability to capture light — in these orchards,” reported Breen.

The effects of artificial spur extinction were also tested. Trees with and without limb removal had about the same number of buds — 14 and 13 per cm² of basal branch cross-sectional area. Buds were reduced to 6, 4, and 2 per cm² respectively in trees with limb removal. A reduction in light interception was only seen at 2 buds per cm².

Similar results for branch and bud removal were obtained in a commercial tall-spindle Cripps Pink orchard planted at 3.5 x 1.5 metres.

“We actually had an increase in light interception throughout most of the season when reducing bud densities to 6 per cm²,” said Breen. “We can safely reduce buds to 4 per cm² without reducing the ability of these canopies to capture light. Reducing bud density also increases light penetration.”

What was the effect of all these interventions on yield? Branch removal without spur extinction did not reduce yields, fruit numbers, or fruit mass over two seasons. There was no third season. “We lost our control here because the grower decided that we’re on to something and removed all the extra branches.”

Spur extinction led to fewer fruit per tree and greater fruit mass. The highest tonnes per hectare were seen at 6 buds per cm² but the heaviest fruit at 2 buds per cm². Similar results for spur extinction were obtained in a tall-spindle Lady in Red orchard.

“To maximise light interception and penetration without reducing yield, we only need 6 branches per metre and 5 buds per cm²,” advised Breen. “Any more than this is surplus to requirements. Trees naturally produce many more branches and buds than we want for commercial production.”

He added that they maintain branch angles at 10–15 ° below horizontal to induce flowering and fruiting. Pruning is kept to the minimum as it promotes vigour.

Planar sailing

Increasing tree density by reducing row width requires changing canopy design. “We’ve been testing something called a planar cordon design over the last ten years,” said Breen.

Trees are spaced 3 metres apart in the row and rows are 1.5–2.0 metres wide. Each tree has a basal cordon with 10–12 upright fruiting stems trained either as verticals or a narrow vee. Breen presented results from trials conducted on Royal Gala trained as verticals. The trees have completed their ninth season.

“The question was whether we can increase light interception to 80%–85% and get the expected increases in yield,” said Breen. The answer is yes. Light interception in the 1.5 metre-row treatments approached 80% in seasons 7 and 8. Yields were above 100 tonnes per hectare in seasons 6–8 for all treatments and approached 150 tonnes per hectare for the 2.0 metre-row treatment in season 7.

Season 8 had a cold, wet start, leading to smaller fruit. “This makes it worth asking, if we’re better able to predict fruit size given certain climatic inputs, could we model this better and reduce fruit numbers earlier to get better fruit size?” wondered Breen.

Fuji, Scilate, and Scifresh gave similar or even better yields than Royal Gala. Fuji produced 220 tonnes per hectare in the 7^th leaf at about 85% light interception, demonstrating that 200 tonnes can indeed be achieved. In addition, fruit quality and colour were exceptional with 86% of fruit meeting export-grade standards on average.

Fruit quality and colour for Royal Gala were not significantly different at row widths of 1.5 and 2.0 metres. Fruit quality was slightly reduced in Scifresh at 1.5 metres and Scilate at 2.0 metres. Red colour development was reduced for Fuji, Scilate, and Scifresh at row widths of 1.5 compared to 2.0 metres. No reflective mulch was applied in these trials.

“In apples, cherries and apricots, the planar cordon system has huge potential to increase yields of high-quality fruit,” concluded Breen. He said that growers are starting to embrace this technology in their new plantings — 200 tonnes per hectare might be the new normal before we know it!

Image: Dr Ken Breen, New Zealand Institute for Plant and Food Research. Supplied by Dr Ken Breen.