12.2 - Agricultural production and light

Solar radiation is linked to agricultural productivity via biomass production and allocation to harvested parts such as grains and fruit. Radiation in this context is in relation to canopy photosynthesis. Biomass is derived from photosynthesis, but is less than the total carbon that is assimilated due to a large respiratory loss by the plant (see Case study 12.1). Carbon lost via respiration is, however, a fairly constant proportion of photosynthesis, and thus variation in canopy photosynthesis is sufficient to account for variation in biomass production.

How then does interception of photosynthetically active radiation (PAR) or photon irradiance affect biomass production and allocation to crop yield? Three steps are considered in this section: (1) variation in the incident PAR during crop growth, (2) interception of PAR by a crop canopy and (3) efficiency of PAR conversion into biomass and yield.

Crop yield commonly depends on the total amount of light intercepted, particularly when crop growth is not limited by other factors such as nutrient or water deficiency or temperature extremes. One example highlighting the importance of solar radiation for crop yield comes from a comparison of rice crops in Australia with those grown in tropical areas.

Rice in Australia is grown almost exclusively in southern New South Wales during dry summer months (November–March). Crops are fully irrigated and well fertilised and yield around 9 t ha–1. This high yield is associated with high incident solar energy (commonly 10–15 MJ m–2 d–1 PAR) during the long growing season. In tropical Asian countries, rice is commonly grown under cloudy conditions during the wet season (June–November). Yield is lower (4–5 t ha–1) even with high nutrient inputs, because of a shorter growing season and lower solar energy (often around 8–10 MJ m–2 d–1 PAR). Experiments with shading treatments have shown that growth and yield of rice and many other agricultural crops, are reduced by decreased solar radiation.

12.2.1 - Leaf area index and canopy light climate

Interception of light by a crop canopy is strongly related to total or canopy leaf area. A crop will thus intercept more light and hence grow faster if it develops the canopy 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 photoassimilates. By analogy with early canopy expansion, retention of green leaves late in a growing season also extends light interception and enhances storage of photoassimilates. This is also true for perennial crops. For some deciduous horticultural crops, leaf area expansion is also rapid because of preformed primordia which emerge rapidly and comprise the largest leaves (Greer 1996).

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 the canopy light climate. A canopy where LAI equals 1 has a leaf area equal to the soil surface area on which it grows. This does not mean all light is intercepted, because some leaves overlap, leaving gaps. Moreover, not all leaves are positioned at right angles to the incident radiation. A crop under favourable growing conditions increases LAI rapidly during early development to a maximum of 3 to 7.


Figure 12.19 Changes in leaf area index (LAI) (a), light interception (b), and the overall relationship between light interception and LAI (c) for three species, sorghum, maize and rice in South Sast Queensland. (P. Inthapan and S. Fukai, Aust J Exp Agric 28: 243-248, 1988)

An example of LAI development of three tropical cereal crops grown under well-watered conditions in South East Queensland is given in Figure 12.19. 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 the maximum LAI was none the less higher for rice than for 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 the LAI declines during grain filling as crops mature. Differences in LAI development among the three crops (Figure 12.20a) are evident in light interception by the respective canopies (Figure 12.19b); interception prior to 60 d was highest in sorghum and lowest in rice. However, in all three crops, canopy light interception increased rapidly during early stages of growth. Incident radiation was almost completely intercepted once a high LAI had been achieved.

Despite wide variation in crop phenology, sunlight interception and LAI maintain a tight curvilinear relationship (Figure 12.19c). Thus, interception increases sharply with increases in LAI to about 90% once LAI exceeds 4, and approaches an asymptote at higher LAI, see also Figure 12.20 from Khurana and McLaren (1982). In this research on potato, numerous treatments were imposed, involving different storage of seeds at low temperatures including apically and multisprouted seed treated at 4 and 12 °C and then at 8 °C before planting. In Treatment1, the trial included unsprouted seed stored at 4 °C, in Treatment 2, the trial included seed stored in the dark and in Treatment 3, two additional sprouting treatments were mixed alternately along the row. Such a relationship between LAI and light interception 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 light 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 (such as for fruit trees and grapevines, Section 12.4).


Figure 12.20 The proportion of incident radiation intercepted by potato canopies as a function of leaf area index. The different symbols indicate agronomic treatments over two growing seasons. In 1979: □ cv. Record, ○ cv. Pentland Crown. In 1980: cv. Pentland Crown with three treatments (see text): ●Treatment 1, ▲Treatment 2, ■ Treatment 3. (Based on Khurana and McLaren 1982)

The time-course of light 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 high seeding rate would produce a high plant population density and a high LAI at crop establishment. This hastens canopy interception and hence biomass production would be promoted. Any advantage of a high plant density may, however, 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).


Figure 12.21 Changes in dry matter (a), leaf area index (LAI) (b), and photon irradiance at ground level (c) for wheat crops grown at five different densities, 1.4, 7, 35, 154 and 447 plants m-2 for treatments 1 to 5 respectively. (D.W. Puckridge and C.M. Donald, Aust J Agric Res 18: 193-211, 1967)

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 not fully intercepted and hence wasted at low planting density, and potential yield (dry mass produced per unit area) was never realised.

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 exponential relationship:

\[ I = I_0 e^{-kL} \tag{12.1} \]

where \(I\) is horizontal photon irradiance within a canopy, \(I_0\) 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 forest canopy is given in the next section).


Figure 12.22 Total dry matter production at harvest of several different crops as a function of the total amount of solar radiation intercepted over the whole growing season. (Based on Monteith 1977)

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.

Irrespective of the canopy extinction, there is a strong relationship between the light intercepted by the canopy over the growing season and the total dry matter produced for a number of crops (Fig. 12. 22). Thus, it is imperative on growing crops to manage the LAI to achieve maximum light interception from the early part of the season to maximise production of dry matter.


12.2.2 - Light use efficiency


Figure 12.23 A hypothetical wheat ideotype with features presumed conducive to high grain yield as a crop community. (C. M. Donald, Euphytica 17: 385-403, 1968)

Sunlight intercepted is not utilised with similar efficiency by different crops. There are clear differences in light use efficiency between crop species, particularly between those with C3 and those with C4 photosynthetic pathways. The photosynthetic advantage of C4 species at a leaf level is evident here at a canopy level, where efficiency is higher by 30–100%. Expressed in terms of dry mass formed (g) per unit of photosynthetically active energy absorbed (MJ), the efficiency of sorghum and maize (C4 photosynthesis) in Figure 12.19 was 1.32 g MJ–1, while that of rice (C3 photosynthesis) was only 0.93 g MJ–1.

Canopy structure, and particularly the spatial distribution of leaf angles, has an important bearing on the canopy light climate and energy conversion. Large leaf angles, with leaves close to vertical, ensure good light penetration when solar angle is high, and a high proportion of leaves receive similar photon irradiances. An even distribution of light at leaf surfaces is advantageous for canopy photosynthesis and improves light use efficiency over canopies where upper horizontal leaves intercept most solar radiation and lower leaves experience greatly attenuated levels. Small and erect leaves, particularly in top canopy layers, are thus a key feature of an ideal plant type, or ‘ideotype’ for high-density cropping (Figure 12.23).

Canopy radiation climate is especially complex in mixed crops and pastures where species with contrasting forms grow together. In grass–legume pastures, grass is generally taller than the legume component and is better placed to intercept incident radiation. Legumes then exist in permanent shade. Height is, therefore, an important determinant of light interception within a mixed sward, and thus species composition. In such mixed swards, management options such as nitrogen fertiliser application, grazing time or cutting frequency all affect the relative height and hence radiation interception by component species. High-nitrogen fertiliser tends to favour grass, while clover may become dominant under nitrogen-limiting conditions.


Figure 12.24 Photon irradiance declines with depth (cumulative LAI) in any plant community. That rate of decline is accentuated by a preponderance of horizontal leaves. (W.R. Stern and C.M. Donald, Aust J Agric Res 13: 599-614, 1962)

Light profiles within a pasture are, therefore, affected by LAI profiles of component species (Figure 12.24), and a clover-rich sward with more horizontal leaves (N0, no added nitrogen) shows stronger attenuation of sunlight than a grass-rich sward with a preponderance of vertical leaves (N225, nitrogen added). In common with monocultures, pasture productivity is enhanced by a species balance that ensures even distribution of sunlight within a mixed community.