11.2.4 Kiwifruit development: a case study

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Figure 11.5 Radial growth in kiwifruit is due mainly to enlargement of outer and inner pericarp. Vertical lines indicate cessation of cell division in each tissue.

(Unpublished data courtesy K. Gould and I.B. Ferguson)

Development of kiwifruit fruit is particularly complex. All tissues of the mature fruit (exocarp, outer and inner pericarp and central core) are already discernible in the ovary before anthesis and pollination. Each layer grows to a different extent and at different rates, so that the relative contribution of each to the total fruit volume varies with time (Figure 11.5). Cell division ceases first in the exocarp and last in the innermost regions of the central core. The outer pericarp is first seen as a homogeneous population of cells but by 14 d after pollination two cell types become visible, namely small isodiametric parenchyma cells full of starch grains, and much larger heavily vacuolate ovoid cells in which the frequency of starch grains per unit volume is low. The larger cells grow more rapidly than the smaller cells, and both cell types grow more rapidly in the inner regions towards the central core.

Anatomy affects our perceptions of kiwifruit quality. Hairs are developed as multicellular projections of the skin, giving a characteristic bristly appearance and rough feel. Tough skin relative to soft flesh is another important character imparted by development of primary cell wall thickenings in the hypodermal collenchyma. Ripe fruit colour is due to tannin deposits in those same cells. In addition, some small parenchyma cells in the outer pericarp deposit calcium oxalate crystals (raphides), surrounded by polysaccharide mucilage, and this tissue can cause throat irritation in many people.