Sun and shade plants of an evergreen shrub

A number of the general points above are illustrated by a study of the evergreen shrub, Heteromeles arbutifolia. This plant grows both in chaparral habitats in California, where shoots in the upper crown are consistently exposed to full sunlight and high temperatures, especially during the dry season, and also in woodland habitats, where the plant grows both in open sites and in the shaded understory (Valladares & Pearcy, 1998). Shade plants from the understory were compared with sun plants from the chaparral, where they received around seven times as much radiation ('photon flux density', PFD). Compared to those from the shade (Figure 3.9 and Table 3.1a), sun plants had leaves that were inclined at a much steeper angle to the horizontal, were smaller but thicker, and were borne on shoots that were themselves shorter (smaller internode distances). The sun leaves also had a greater photosynthetic capacity (more chlorophyll and nitrogen) per unit leaf area but not per unit biomass.

The 'architectural' consequences of these differences (Table 3.1b) were first that shade plants had a much greater 'projection efficiency' in the summer, but a much lower efficiency in the winter. Projection efficiency expresses the degree to which the effective leaf area is reduced by being borne at an angle other than right angles to the incident radiation. Thus, the more angled leaves of sun plants absorbed the direct rays of the overhead summer

Figure 3.9 Computer reconstructions of stems of typical sun (a, c) and shade (b, d) plants of the evergreen shrub Heteromeles arbutifolia, viewed along the path of the sun's rays in the early morning (a, b) and at midday (c, d). Darker tones represent parts of leaves shaded by other leaves of the same plant. Bars = 4 cm. (After Valladares & Pearcy, 1998.)

sun over a wider leaf area than the more horizontal shade plant leaves, but the more sidewards rays of the winter sun struck the sun plant leaves at closer to a right angle. Furthermore, these projection efficiencies can themselves be modified by the fraction of leaf area subject to self-shading, giving rise to 'display efficiencies'. These were higher in shade than in sun plants, in the summer because of the higher projection efficiency, but in the winter because of the relative absence of self-shading in shade plants.

Whole plant physiological properties (Table 3.1b), then, reflect both plant architecture and the morphologies and physiologies of individual leaves. The efficiency of light absorption, like display efficiency, reflects both leaf angles and self-shading. Hence, absorption efficiency was consistently higher for shade than for sun plants, though the efficiency for sun plants was significantly higher in winter compared to summer. The effective leaf ratio (the light absorption efficiency per unit of biomass) was then massively greater for shade than for sun plants (as a result of their thinner leaves), though again, somewhat higher for the latter in winter.

Table 3.1 (a) Observed differences in the shoots and leaves of sun and shade plants of the shrub Heteromeles arbutifolia. Standard deviations are given in parentheses; the significance of differences are given following analysis of variance. (b) Consequent whole plant properties of sun and shade plants. (After Valladares & Pearcy, 1998.)

Sun Shade P

Table 3.1 (a) Observed differences in the shoots and leaves of sun and shade plants of the shrub Heteromeles arbutifolia. Standard deviations are given in parentheses; the significance of differences are given following analysis of variance. (b) Consequent whole plant properties of sun and shade plants. (After Valladares & Pearcy, 1998.)

Sun Shade P

Internode distance (cm)

1.08

(0.06)

1.65

(0.02)

< 0.05

Leaf angle (degrees)

71.3

(16.3)

5.3

(4.3)

< 0.01

Leaf surface area (cm2)

10.1

(0.3)

21.4

(0.8)

< 0.01

Leaf blade thickness (mm)

462.5

(10.9)

292.4

(9.5)

< 0.01

Photosynthetic capacity, area basis (mmol CO2 m-2 s-1)

14.1

(2.0)

9.0

(1.7)

< 0.01

Photosynthetic capacity, mass basis (mmol CO2 kg-1 s-1)

60.8

(10.1)

58.1

(11.2)

NS

Chlorophyll content, area basis (mg m-2)

280.5

(15.3)

226.7

(14.0)

< 0.01

Chlorophyll content, mass basis (mgg-1)

1.23

(0.04)

1.49

(0.03)

< 0.05

Leaf nitrogen content, area basis (g m-2)

1.97

(0.25)

1.71

(0.21)

< 0.05

Leaf nitrogen content, mass basis (% dry weight)

0.91

(0.31)

0.96

(0.30)

NS

Sun plants Shade plants

Summer

Winter

Summer

Winter

EP

0.55a

0.80b

0.88b

0.54a

ED

0.33a

0.38a, b

0.41b

0.43b

Fraction self-shaded

0.22a

0.42b

0.47b

0.11a

EA, direct PFD

0.28a

0.44b

0.55c

0.53c

LARc (cm2 g-1)

7.1a

11.7b

20.5c

19.7c

EP, projection efficiency; ED, display efficiency; EA, absorption efficiency; LARe, effective leaf area ratio; NS, not significant. Letter codes indicate groups that differed significantly in analyses of variance (P < 0.05).

Overall, therefore, despite receiving only one-seventh of the PFD of sun plants, shade plants reduced the differential in the amount absorbed to one-quarter, and reduced the differential in their daily rate of carbon gain to only a half. Shade plants successfully counterbalanced their reduced photosynthetic capacity at the leaf level with enhanced light-harvesting ability at the whole plant level. The sun plants can be seen as striking a compromise between maximizing whole plant photosynthesis on the one hand while avoiding photoinhibition and overheating of individual leaves on the other.

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