To understand long-term association between canopy-scale photosynthesis and growth, we continuously and simultaneously measured the net photosynthetic rate (Ac) and leaf area index (LAI) of a spinach canopy from transplantation to harvesting. This long-term measurement was enabled by combining an open-type flux chamber method and image analysis of top-of-view canopy photographs. The photosynthetic photon flux density (PPFD) was the major determinant of Ac during a short-term period, but the change in Ac was significantly amplified by an increase in LAI during a long-term period. LAI, measured with reliable accuracy from gap fractions (projected non-leaf areas) of top-of-view photographs, increased exponentially through the growth period. These results indicate a positive feedback loop between photosynthesis and growth (i.e., the growth of leaves amplifies photosynthesis, and the amplified photosynthesis further accelerates the growth of leaves). Consequently, a linearity was found between cumulative canopy net photosynthesis (ΣAc) and LAI, implying that throughout the growth period, a fixed proportion of dry matter was partitioned to the leaves and that the leaf area was expanded in proportion to the partitioned dry matter. This linearity between ΣAc and LAI also suggests that growth of a leafy vegetable canopy can be predicted from canopy photosynthesis.
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