12.3.2 Leaf area index and canopy light climate

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Interception of PAR by a crop canopy is strongly related to total leaf area. A crop will thus intercept more PAR and hence grow faster if it develops leaf area rapidly. This principle applies to both annual crops which are usually planted at the beginning of a growing season and to perennial crops which resume growth after a dormant season. Leaf area development of sugar cane, for example, is generally slower in the year of planting compared with a subsequent ratooned crop where canopy regrowth is enhanced by stored photoassimilate. By analogy with early canopy expansion, retention of green leaves late in a growing season also extends sunlight interception.

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Figure 12.20 Changes in (a) leaf area index (LAI), (b) light interception, and  (c) the overall relationship between light interception and LAI for all three species, sorghum, maize and rice. (All grown in southeast Queensland)

(Based on Inthapan and Fukai 1988)

The leaf area index (LAI) is the ratio of total projected leaf area (one side only) per unit ground area, and is widely used to characterise canopy light climate. A canopy where LAI equals 1 has a leaf area equal to the soil surface area on which it grows, but this does not mean all PAR is intercepted because some leaves overlap, leaving gaps. Moreover, not all leaves are positioned at right angles to incident radiation. A crop under favourable growing conditions increases LAI rapidly during early development to a maximum of 3 to 7. An example of LAI development of three tropical cereal crops grown under well-watered conditions in southeast Queensland is given in Figure 12.20. Sorghum showed a more rapid increase in LAI than did maize largely because of a higher sowing density (33 v. 5.6 plants m–2). A late maturing rice crop showed slowest leaf area development during early stages of growth, but maximum LAI was none the less higher in rice than in maize. As a general rule, maximum LAI is achieved just prior to flowering in cereal crops. By that stage, growing points are differentiating floral rather than leaf primordia, and initiation of new leaves has ceased.

Some cereal crops lose leaves and LAI declines during grain filling as crops mature. Differences in LAI development among the three crops (Figure 12.20a) are evident in PAR interception by respective canopies (Figure 12.20b); interception prior to 60 d was highest in sorghum and lowest in rice. However, in all three crops, canopy PAR interception increased rapidly during early stages of growth. Incident radiation was almost completely intercepted once high LAI had been achieved.

Despite wide variation in crop phenology, sunlight interception and LAI maintain a tight relationship (Figure 12.20c). Interception increases sharply with increase in LAI to about 90% once LAI exceeds 4, and approaches an asymptote at higher LAI. Such a relationship applies to many crops, and emphasises (1) the importance of a rapid increase in LAI during early stages of growth, and (2) a requirement for only moderate LAI to achieve effective interception. Indeed, excessive leaf area development can be counter-productive because reproductive development, and hence economic yield, may be reduced due to self-shading and resource allocation to leaf production (Section 12.5).

The time-course of radiation interception during crop growth can be manipulated to some extent by farmers. For example, seeding rate is an important management option which affects interception and subsequent crop growth and yield. A higher seeding rate would produce a higher plant population density and a higher LAI at crop establishment. This hastens canopy interception and hence biomass pro-duction would be promoted. Any advantage of a high plant density may disappear with time during crop growth because radiation interception of a medium plant density may eventually catch up with that of the high density (Figure 12.21). In this case, density 3 (35 plants m–2) was sufficient for radiation interception and plant dry matter production. If plant density is very low, shown as density 1 (1.4 plants m–2) or density 2 (7 plants m–2) in Figure 12.21, LAI never exceeded 2 and final biomass at harvest was much smaller than values returned from higher densities. Solar radiation was wasted at low planting density, and potential yield (dry mass produced per unit area) was never realised.

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Figure 12.21 Changes in (a) dry matter, (b) leaf area index (LAI), and (c) photon irradiance at ground level for wheat crops grown at five different planting densities, 1.4, 7, 35, 184 and 1078 plants m-2 for treatments 1 to 5 respectively. Some self-thinning occureed in plots sown at higher densities, and by 26 weeks after sowing treatment densities were 1.4, 7, 35, 154 and 447 plants m-2 respectively

(Based on Puckridge and Donald 1967)

As solar radiation penetrates a crop canopy, PAR is intercepted by leaves and photon irradiance commonly declines exponentially with cumulative leaf area (i.e. depth in Figure 12.23), according to the simple relationship:

I = I0 exp (-kL) (12.5)

where I is horizontal photon irradiance within a canopy, I0 is horizontal photon irradiance above that canopy, L is LAI from the top of the canopy to the point where I is determined, and

k is an extinction coefficient (a more explicit formulation for PAR attenuation through a canopy appears in Section 12.4).

Large k values imply that photon irradiance decreases rapidly with depth, whereas a canopy with a small k would allow solar radiation to penetrate deeply, for a similar leaf area profile. Variation in k value is commonly associated with leaf angle. Canopies with more horizontal leaves, such as sun-flower or cotton, have large k values, often 0.7–1.0, whereas those with more erect leaves, such as barley and sugar cane, have small values, often 0.3–0.6.

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