An in-depth examination of the factors controlling
cracking sensitivity of carrot roots was conducted from 2000-2002.
More than 30 commercial fields of “Sugar Snax” carrot
(a popular variety for cut and peel use) were sampled and the roots
evaluated for cracking sensitivity in several standardized tests.
Information was compiled on soil, environmental and management factors.
In a number of these fields, replicated tests were also conducted
evaluating the effect of nitrogen fertilizer rate on cracking. The
following summarizes the results of this project:
(1) Excessive N Fertilizer increases cracking
Averaged across 10 field trials, applying 80- 100 lb N/acre above
that necessary to produce optimum yield increased root cracking approximately
30 percent. Current tissue N sufficiency recommendations used in the
California industry are higher than required and may induce growers
to use excessive N rates.
(2) Irrigation management and root water status
appear to play a minor role in cracking sensitivity.
This was a surprise, given the central role of water relations in
the cracking of other horticultural commodities (tomatoes and tree
fruit, for example).
(3) Heavier textured soils tend to produce roots
more prone to cracking.
More than 80 percent of fields with soil cation exchange capacity
(CEC, a characteristic closely linked to soil texture) greater than
10 meq/ 200g had a high cracking percentage in the standardized tests,
compared to less than 40 percent of fields with lighter soil texture.
(4) Warm weather appears to increase cracking
When mean air temperature over the final month
before harvest was greater than
60 degrees F, the average incidence of cracking was only half that
of fields growing at higher temperatures.
(5) The strength of the periderm tissue (the “skin”)
controls cracking sensitivity.
The periderm, which is only a few cell layers thick, has relatively
little fexibility and does not deform as readily as the parenchyma
tissue underneath it to impacts during harvesting an handling. When
the periderm ruptures, the crack propagates readily through the parenchyma
tissue. However, if the periderm is peeled off, the parenchyma tissue
deform easily to absorb impacts; peeled roots are highly crack resistant.
(6) Varieties vary widely in their cracking sensitivity.
Among horticulturally accepted varieties there is a wide range in
the incidence of cracking observed in the standardized tests. However,
once the carrots were peeled the varietal difference completely disappeared.
This emphasizes the overwhelming importance of periderm structure
Editor’s note: The information presented here was part of a
Great Lakes Fruit, Vegetable & Farm Market Expo held in December
2003 at the DeVos Place Convention Center in Grand Rapids, Mich.