Photosynthesis, productivity and environment

The relation between photosynthetic rate per unit leaf area (Pn), total photosynthesis by canopies and dry matter production (DMP) of crops is reviewed. Although Pn is the driving force for all plant growth, total DMP is determined by processes integrated over the canopy, primarily light interception and thus by leaf area index (LAI) and canopy architecture and leaf area duration (LAD). The processes are not linearly related so that effects on DMP of changes in the efficiency of conversion of energy of radiation to dry matter are smaller than those associated with LAI. Photosynthesis is much less sensitive to changing environmental conditions than development of leaf area. This explains the apparent anomaly that Pn does not determine the variation in productivity of a crop under normal agronomic practice, despite the role of photosynthesis in providing all the assimilates for DMP. Breeding and selection of crops for higher yield has not resulted in improvements in dry matter production. Indeed, potential photosynthetic rate has decreased under selection breeding, compensated by increased leaf area: the causes are considered. Crops growing at or near their potential rate use most of the available solar energy and there is a strong correlation between radiation absorbed and DMP and canopy photosynthesis is not light saturated. To increase production it will be necessary to extend either the growing season or to improve light conversion efficiency; the latter is best achieved by increasing Pn. The limiting factors in photosynthetic metabolism which determine Pn under different environmental conditions, are reviewed. Increased Pn is not likely to come from altering light harvesting, electron transport or ATP and NADPH synthesis which are potentially very flexible and have large capacity: there is large genetic variation. Synthesis of ribulose bisphosphate (RuBP) depends on activity of Calvin cycle enzymes which may be limiting due to environmental factors. Increasing the atmospheric CO2 supply increases production of C3 plants under current conditions: this is due to a decrease in the oxygenase function of ribulose bisphosphate carboxylase-oxygenase (Rubisco). Improving the specificity factor of Rubisco is a longterm goal to decrease photorespiration. Even if Pn is increased, evidence from theoretical and crop canopy studies suggest that the increase in biomass production will only be a small proportion of the increase in Pn, and DMP also depends on the crop's sink capacity. Crop production is a function of many processes, at the level of the chloroplast, the leaf and the canopy. To increase Pn and production of crops will require knowledge of processes at all levels in the plant-environment system.