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Cell di­vi­sion win­dows

Quality from density Episode 4 4 min read

The short version

A fruit, tuber or pod makes most of its cells in a short window early on. After that it grows mostly by filling the cells it has. So how many cells there are, and how well they’re built, is mostly settled early: in days for row crops like beans, and by Steve’s rule of thumb a few weeks for tree crops like avocados.

Straight after flowering, the fruit makes its cells: over a few days in a bean, a few weeks in an avocado.

Quality is set early

Steve’s line is that quality is all about cell division. A fruit, a tuber or a pod grows in two stages. First it makes most of its cells, fast, in a short window just after it sets. Then it grows mostly by filling the cells it has made. A few crops, like avocados and potatoes, keep adding cells more slowly as they grow. But in most, the number of cells is largely settled in that first window.

  • An apple spur a few weeks after bloom: six small green fruitlets, the one in the middle the biggest and round, among young leaves.

    Just after it sets

    an apple spur, a few weeks after bloom; one patch of flesh, magnified

    it makes most of its cells now, fast, in a short window

  • The same spur at harvest: one big, ripe red apple hanging where the biggest fruitlet was, among full-grown leaves.

    At harvest

    the same fruit; the same patch, magnified the same

    it grew mostly by filling the cells it made in that window

A fruit grows in two stages. First it makes most of its cells, fast, in a short window just after it sets. Then it grows mostly by filling the cells it has made. In most crops, the number of cells is largely settled in that first window. An illustration, simplified.

Quality from density

Two fruit can end up the same size with very different insides. One has plenty of small, well-built cells, packed tight. The other has fewer cells, stretched to fill the same space. The first is firmer, holds up better in the packing shed and stores longer. Steve’s slide puts it in three words: quality from density.

  • An apple cut in half from stalk to tip: cream-white flesh, the core with its pips, and a thin red-and-green skin.

    More cells

    an apple, cut open

    firm, holds up in the shed, and stores longer

  • An apple cut in half from stalk to tip: cream-white flesh, the core with its pips, and a thin red-and-green skin.

    Fewer cells

    another apple, the same size

    softer, bruises, and stores less well

Two fruit the same size, cut open, their flesh magnified: very different insides. The cell count is mostly set early, and the fruit fills out from there. An illustration, simplified.

How long the window lasts

It depends on the crop. Steve’s rule of thumb: in row crops like beans, the window might be seven days, or even three or four days. In tree crops like avocados and limes, it’s more like three to four weeks. In potatoes, he says, it starts when the stolon tips hook.

The fruit making the cells it will fill.

  • A young bean pod just set, its withered flower at its tip, above a young avocado fruitlet on its stalk.

    The cell division window

    Steve’s rule of thumb, from fruit set

    days in row crops like beans, weeks in tree crops

Days in row crops like beans, weeks in tree crops: Steve’s rule of thumb for how long the window stays open. An illustration, simplified.

Have the foundation in first

Cells are built inside the window, so what they’re built from has to be there when it opens. That’s why the foundation goes in ahead of it: calcium, boron and silicon. Calcium in particular travels with the water. Fruit draws far less of it than the leaves do, and less again once the fruit starts to size. So a fruit takes its calcium in fastest early, while it’s small, and the calcium has to keep arriving right through the window. And keep the water steady through the window: a stressed plant builds fewer cells.

Water, carrying calcium.

  • An apple spur a few weeks after bloom: six small green fruitlets, the one in the middle the biggest and round, among young leaves.

    Just after it sets

    an apple spur, a few weeks after bloom

    the fruitlets take calcium in fastest now, while they’re small and building their cells

  • The same spur at harvest: one big, ripe red apple hanging where the biggest fruitlet was, among full-grown leaves.

    Later

    the same spur, the fruit grown

    the leaves take most of the water, and the fruit gets less and less

Calcium travels with the water, and a fruit draws far less of it than the leaves do, and less again once it starts to size. So a fruit takes its calcium in fastest early, while it’s small: have the foundation in before the window opens, and keep it arriving right through. An illustration, simplified.

A bean crop’s sap tests

Steve tells this one from a bean crop’s report. Through the season’s sap tests, calcium dropped hard right through cell division. Steve points it out on the chart: “See, calcium, it’s gone boom, boom, gone down. So that’s that cell division.” His reading: the plant was using the calcium. So next season’s program puts a calcium foliar spray in at that stage. The tests showed him what the crop ran short of, and when. That’s what a run of sap tests is for.

One sap test: its calcium. A spray of calcium.

  • Five bean plants across one season, left to right: a seedling just up, a young plant, one in flower, one with young pods, and one with full pods.

    A bean crop’s sap tests

    calcium, test by test, through one season

    calcium dropped hard right through cell division: next season, a calcium foliar goes in there

Through one season’s sap tests, a bean crop’s calcium dropped hard right through cell division: the plant was using it. So next season’s program puts a calcium foliar spray in at that stage. The tests showed what the crop ran short of, and when. An illustration, not real results.

Where it fits in a program

  • Find your crop’s window: when it opens, and how long it lasts.
  • Have the foundation in before it opens: calcium, boron and silicon.
  • Keep the water steady right through it.
  • Keep potassium out of it. In Steve’s picture of a house, you pour the foundation, build the house, then fill it with potassium.
  • Sap test around it, so next season you know what the window was short of.

“Quality is all about cell division.”

— Steve, on fruit quality