Growth Rate, Photosynthesis and Respiration in Relation to Leaf Area Index

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Abstract
This work examined three possible explanations of growth rate responses to leaf area index (LAI) in which growth rate per unit of ground area (crop growth rate, CGR) increased to a plateau rather than decreasing above an optimum LAI at which all light was intercepted. Single leaf photosynthetic measurements, and whole plant 24 h photosynthesis and respiration measurements were made for isolated plants and plants in stands using Amaranlhus hybridus, Chenopodium album, and two cultivars of Glycine max grown at 500 and 1000 μimol m−2 S−1 photosynthetic photon flux density at 25 °C. CGR, relative growth rate (RGR), and LAI were determined from 24 h carbon dioxide exchange and leaf area and biomass measurements. Respiration increased relative to photosynthesis with crowding in A. hybridus and there was an optimum LAI for CGR. In contrast, the ratio of respiration to photosynthesis was constant across plant arrangement in the other species and they had a plateau response of CGR to LAI. Neither increased leaf photosynthetic capacity at high LAI nor a large change in biomass compared to the change in LAI could account for the plateau responses. It was calculated that maintenance respiration per unit of biomass decreased with decreasing RGR in C. album and G. max, but not A. hybridus, and accounted for the plateau response of CGR to LAI. Since similar decreases in maintenance respiration per biomass at low RGR have been reported for several other species, a constant ratio of respiration to photosynthesis may occur in more species than constant maintenance respiration per unit of biomass.