From 11th to
29th leaf positions were able to maintain approximately 90% of
the maximum ACO2 , while leaves in lower leaf positions displayed
decreasing ACO2 (Fig. 6). Regression between LAI and PPFD in the
mature leaves exhibited a linear relationship. Within the canopy,
the light intensity on the mature leaves decreased with cumulative
LAI. Meanwhile, the light intensity in the lowest stratum was
so low that the leaves were nearly completely shaded (Fig. 7A).
The response of ACO2 to LAI displayed a parabolic curve. In the
upper canopy, the mature leaf ACO2 did not decrease correspondingly
with the decreasing light intensity but remained above 90% of
the maximum ACO2 with LAI between 0.3 and 1.4 m2m−2. Below the
canopy layer with 1.4 m2m−2 LAI, ACO2 decreased with the increasing
shading levels or LAI (Fig. 7B). Based on the light intensity on the
mature leaves, the canopy of ‘Tainung No. 2’ papaya was divided
into full-light upper strata (leaf position: 11th-22nd) and shaded
lower strata. The slope of the ln I/Io and LAI canopy regression curve
indicated a light extinction coefficient (k) of 1.68 (Fig. 8).