Enhancement of crop photosynthesis by diffuse light: quantifying the contributing factors.
OBJECTIVE
Plants use diffuse light more efficiently than direct light. However, experimental comparisons between diffuse and direct light have been obscured by co-occurring differences in environmental conditions (e.g. light intensity). This study aims to analyse the factors that contribute to an increase in crop photosynthesis in diffuse light and to quantify their relative contribution under different levels of diffuseness at similar light intensities. The hypothesis is that the enhancement of crop photosynthesis in diffuse light results not only from the direct effects of more uniform vertical and horizontal light distribution in the crop canopy, but also from crop physiological and morphological acclimation.


METHODS
Tomato (Solanum lycopersicum) crops were grown in three greenhouse compartments that were covered by glass with different degrees of light diffuseness (0, 45 and 71 % of the direct light being converted into diffuse light) while maintaining similar light transmission. Measurements of horizontal and vertical photosynthetic photon flux density (PPFD) distribution in the crop, leaf photosynthesis light response curves and leaf area index (LAI) were used to quantify each factor's contribution to an increase in crop photosynthesis in diffuse light. In addition, leaf temperature, photoinhibition, and leaf biochemical and anatomical properties were studied.


RESULTS
The highest degree of light diffuseness (71 %) increased the calculated crop photosynthesis by 7·2 %. This effect was mainly attributed to a more uniform horizontal (33 % of the total effect) and vertical PPFD distribution (21 %) in the crop. In addition, plants acclimated to the high level of diffuseness by gaining a higher photosynthetic capacity of leaves in the middle of the crop and a higher LAI, which contributed 23 and 13 %, respectively, to the total increase in crop photosynthesis in diffuse light. Moreover, diffuse light resulted in lower leaf temperatures and less photoinhibition at the top of the canopy when global irradiance was high.


CONCLUSIONS
Diffuse light enhanced crop photosynthesis. A more uniform horizontal PPFD distribution played the most important role in this enhancement, and a more uniform vertical PPFD distribution and higher leaf photosynthetic capacity contributed more to the enhancement of crop photosynthesis than did higher values of LAI.
Horticultural Supply Chains, Wageningen University, PO Box 630, 6700AP Wageningen, The Netherlands Wageningen UR Greenhouse Horticulture, PO Box 644, 6700AP Wageningen, The Netherlands.
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T
Li
Horticultural Supply Chains, Wageningen University, PO Box 630, 6700AP Wageningen, The Netherlands.
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E
Heuvelink
Wageningen UR Greenhouse Horticulture, PO Box 644, 6700AP Wageningen, The Netherlands.
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TA
Dueck
Wageningen UR Greenhouse Horticulture, PO Box 644, 6700AP Wageningen, The Netherlands.
J
J
Janse
Biometris, Wageningen University and Research Centre, 6700AC Wageningen, The Netherlands.
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G
Gort
Horticultural Supply Chains, Wageningen University, PO Box 630, 6700AP Wageningen, The Netherlands Wageningen UR Greenhouse Horticulture, PO Box 644, 6700AP Wageningen, The Netherlands leo.marcelis@wur.nl.
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Marcelis
D000064:Acclimatization; D002734:Chlorophyll / Q000502:physiology / Q000528:radiation effects; D018556:Crops, Agricultural; D005638:Fruit / Q000502:physiology / Q000528:radiation effects; D018551:Solanum lycopersicum / Q000502:physiology / Q000528:radiation effects*; D010788:Photosynthesis / Q000187:drug effects*; D018515:Plant Leaves / Q000502:physiology / Q000528:radiation effects; D013472:Sunlight; D013696:Temperature
D002734:Chlorophyll
Diffuse light; PPFD distribution; Solanum lycopersicum; acclimation; crop photosynthesis; direct light; haze treatment; tomato