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.