Long-term and continuous measurement of canopy photosynthesis and growth of spinach

To understand long-term association between canopy-scale photosynthesis and growth, we continuously and simultaneously measured the net photosynthetic rate (A_c) 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 A_c during a short-term period, but the change in A_c 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 (ΣA_c) 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 ΣA_c and LAI also suggests that growth of a leafy vegetable canopy can be predicted from canopy photosynthesis.