3. Results
3.1. The changes in gas exchange characteristics and PPFD with
leaf age
The angular position of papaya leaves follows genetic spiral
arrangements around the stem (Fig. 1A). According to the top view
of papaya tree, the papaya canopy could be divided into upper (the
leaves younger than 21st to 22nd leaf position) and lower canopy
(Fig. 1B and C). The ‘Tainung No. 2’ papaya generated three leaves
per week (Fig. 2A). In the process of leaf development, tagged leaves
gradually descended to lower positions in the canopy. Both leaf area
and chlorophyll concentration quickly increased with leaf expansion
after leaf appearance. The tagged leaves grew rapidly from
their shaded positions located inside the top of the canopy to positions
exposed to full light before the leaves expanded completely
by leaf age of 26 days (leaf position: 12th). Between 26 and 36
days after leaf appearance (leaf position: 12th–16th), the leaves
expanded fully, and the single leaf area reached approximately 0.33 m2 (Fig. 2B). By 98 days after appearing, the chlorophyll concentration
slightly decreased (Fig. 2C). The overall leaf lifespan was
approximately 106
±
5 days.
The light intensity to which an individual leaf was naturally
exposed was related to the leaf position. Relatively low light intensity
was measured on leaves aged 19 days (leaf position: 9th) and
younger (Fig. 1A and B). The newly matured leaf (leaf age 26 days,
approximately the 12th leaf position) encountered the greatest
exposure to light before it reached a leaf age of 48 days (Fig. 3),
which indicates that a mature leaf with a leaf age of 26–48 days
has moved to the top of the canopy at approximately the 12th–21st
leaf position (Fig. 1B). Consequently, the light incidence on the
leaves decreased dramatically between 56 and 62 days after leaf
appearance (leaf position: 24th to 26th) but still remained above
60% of the incident sunlight (Fig. 3). The light incidence continually
decreased with increasing leaf position until it reached 13% of
incident sunlight on leaves older than 83–98 days after appearance
(leaf position: 38th–42nd; Fig. 3). Thus, the mature tagged leaves
gradually over-shaded each other within the canopy as younger
leaves emerged.
The measured data of gas exchange characteristics with leaf age
in tagged leaves was fluctuated and thereby unable to obtain the
changing trend of gas exchange characteristics with leaf age in both cropping seasons (Figs. 4 and 5, open round symbol). However,
regression between relative gas exchange characteristics and leaf
age in tagged leaves exhibited a significant linear and quadratic
relationship (Figs. 4 and 5, closed triangle symbol). The response
of relative gas exchange parameters to leaf age displayed similar
results and trends for both cropping seasons.
In 2009–2010 cropping season, ACO2 , E and gs had similar developing
patterns with leaf growth (Fig. 5). New leaves required 26
days after appearance (leaf position: 12th) to approach the highest
level of relative ACO2 and E (Fig. 5A and B). The patterns for both leaf
area and chlorophyll concentration development were similar. In
mature leaves of field net-house grown papaya at a leaf age of 48–62
days (21st–26th leaf position), the relative ACO2 and E remained
at above 90% of the maximum ACO2 and E when shading initially
began. As the leaf aged and the light intensity decreased, the relative
ACO2 and E gradually declined to 45–55% of the maximum
(Figs. 4A and B and 5A and B).
Stomatal conductance changed more dramatically among different
leaf ages than the ACO2 and E. Relative gs reached its highest
value between 26 and 48 days after appearance and then gradually
decreased to 22%. In 2009–2010 cropping season, the sudden
decrease of relative gs 56 days after appearance (Fig. 5C) may be
due to the exposure to a short period of cold wave and low light
intensity on the cloudy days. Comparatively, the temperature during
the date of measurement in 2009 was relatively stable, which
was closer to the warm season and without cold wave. Thus, the
relative gas exchange parameters were more stable in 2008–2009
cropping season (Fig. 4). Therefore the drop in relative gs was not
very noticeable.
Ci was more stable than other photosynthetic parameters. Ci
for immature leaves measured 14 days after appearance were significantly
lower (40%) than that of mature leaves in 2009–2010
cropping season (Fig. 5D). After the leaves became mature, Ci
remained at similar levels, suggesting leaf aging did not affect Ci
(Figs. 4D and 5D).
3.2. The distribution of ACO2 and PPFD in the papaya canopy
Leaf position, corresponding to the leaf age, affected the gas
exchange characteristics of individual leaf in the developing canopy of papaya trees. ACO2 of mature leaves with well exposure in the
upper canopy, from 11th to 24th leaf positions, were more stable
than other shaded leaves in the lower canopy.