6.2.1  Light

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Figure 6.6 Area of individual leaves on cucumber (Cucumis sativus) responds to daily irradiance and reaches a maximum above about 2.5 MJ m-2 d-1. Area increase (node 2 in this example) is due to greater cell number under stronger irradiance. Mean size of mesophyll cells is little affected and has no influence on area of individual leaves (Based on Newton 1963)

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Figure 6.7  A sun-adapted plant such as Helianthus annuus adjusts LAR to some extent in response to lower daily irradiance but not enough to maintain RGR. By contrast, a shade-adapted plant such as Impatiens parviflora with somewhat higher LAR and RGR in full sun makes further adjustment in LAR so that RGR does not diminish to the same extent in moderate of deep shade as does that of H. annuus (Based on Blackman and Wilson 1951b; Evans and Hughs 1961)

 

Light has an impact on both extent and activity of plant canopies. Taking cucumber as an archetype for herbaceous crop plants (Figure 6.6) leaf growth increases with daily irradiance due to increased cell number rather than increased cell size. Leaf thickness is also positively affected by daily irradiance, principally resulting in a greater depth of palisade (Table 6.1). Indeed, mean cell volume is more than doubled under strong irradiance (3.11 × 10–5 mm3 at 3.2 MJ m–2 s–1 cf. 1.46 × 10–5 mm3 at 0.5 MJ m–2 d–1 in Table 6.1), and because cross-sectional area is virtually unchanged cell depth is responsible. This greater depth of palisade in strong light confers a greater photo-synthetic capacity on such leaves (expressed on an area basis) and translates into larger values for NAR and a potentially higher RGR. At lower irradiance (Table 6.1) leaves are thinner and SLA will thus increase with shading, and because LAR = SLA × LWR (recall Equation 6.9) a smaller absolute size at lower irradiance can be offset by larger SLA resulting in LAR increase.

G.E. Blackman (Agriculture Dept, Oxford University) appreciated the significance of such LAR × NAR interaction for whole-plant growth, and in a series of comprehensive papers with a number of collaborators documented shade-driven growth responses for many species. RGR response to growing conditions such as shade, and the degree to which upward adjustment in LAR could offset reduced NAR, was a recurring theme. Plants were commonly held in either full sun or under combinations of spectrally neutral screens that reduced daily irradiance to either 24% or 12% of full sun. These three treatments commenced with onset of rapid growth by established seedlings, and harvests taken as plants were judged to have doubled in size over successive intervals. Steady exponential growth ensured that treatment effects on RGR could be resolved into component responses by NAR and LAR.

In a series of 20 pot experiments, Blackman and Wilson (1951a) first established a close relationship between NAR and daily irradiance where shade-dependent reduction in NAR was similar for 10 species. More precisely, NAR was linearly related to log irradiance and extrapolation to zero NAR corresponded to a light-compensation point of 6–9% full sun for eight species, and 14–18% full sun for two others. Significantly, neither slope nor intercept of NAR versus log10 daily irradiance differentiated sun-adapted plants such as barley, tomato, peas and sunflower from two shade-adapted species (Geum urbanum and Solanum dulcamara). LAR proved especially responsive to light and accounted for contrasts between sun plants and shade plants in their growth response to daily irradiance.

Concentrating on sunflower seedlings, Blackman and Wilson (1951b) confirmed that NAR increased with daily irradiance (Figure 6.7a) and that LAR was greatly increased by shading especially in young seedlings (uppermost line in Figure 6.7b). Response in RGR tracked LAR and especially in young seedlings which also showed highest RGR and were most sensitive to shading. LAR appeared sensitive to both daily maxima as well as daily total irradiance. Variation between species in adjustment to shade, and ultimately their long-term shade tolerance, would then derive from plasticity in LAR.

A subsequent comparison between sunflower and the wood-land shade plant Impatiens parviflora by Evans and Hughes (1961) confirmed this principle of LAR responsiveness to irradiance (Figure 6.7). Sunflower achieved noticeably higher NAR in full sun than did I. parviflora, but LAR was consider-ably lower and ironically translated into a somewhat slower RGR for sunflower. This species contrast was, however, much stronger in deep shade (12% full sun) where RGR for I. parviflora had fallen to 0.090 d–1 whereas sunflower was only 0.033 d–1. Clearly, I. parviflora is more shade tolerant, and retention of a faster RGR in deep shade is due both to greater plasticity in LAR as well as a more sustained NAR. Adjustments in both photosynthesis and respiration of leaves contribute to maintenance of higher NAR in shade-adapted plants growing at low irradiance (Chapter 12).

A note on irradiance

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Figure 6.8 NAR for open-grown seedlings of sunflower (Helianthus annuus) responds linearly to daily irradiance across a wide range from low values recorded at Oxford to the highest recorded value for NAR of about 30 g m-2 d-1 at Deniliquin under daily irradiance of 13.5 MJ m-2 d-1 (Based on Warren Wilson 1969)

Daily irradiance (photosynthetically active energy) at low to mid latitude (20–30°) can reach 15 MJ m–2 on clear days in midsummer. The tropics can be lower due to cloud cover, while at higher latitudes (30–50°) lower daily maxima are offset by long days. Plant growth and reproductive development vary accordingly, and some early results, including those from northern hemisphere experiments, must be viewed in this context. Warren Wilson (1966, 1967) analysed the performance of open-grown seedling sunflowers at Deniliquin and recorded the highest known value for NAR, namely 29.9 ± 0.4 g m–2 d–1. Pooling data from Deniliquin and Oxford (Figure 6.8), NAR in widely spaced and nutrient-rich sunflower plants was linearly related to daily irradiance with a mean maximum NAR of about 25 g m–2 d–1 at about 15 MJ m–2 d–1. In assimilatory terms, sunflower shows remarkable capacity and plasticity.

 

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