Light-Driven Physiological Adjustments Enhance Growth Performance and Photosynthetic Efficiency in Carapa guianensis Seedlings

Identifying suitable environmental conditions for the early development and deciding how much to shade andiroba (Carapa guianensis Aubl.) seedlings in nurseries still lacks a technical basis. This study evaluated the growth and physiological responses of seedlings grown under 0, 30, 50, and 70% shading for 300 days, with six plants per treatment, combining chlorophyll content, chlorophyll a fluorescence, gas exchange, and growth analysis. Under full sun, the Fv/Fm ratio fell from 0.79 to 0.70 from 90 days onward, signaling photoinhibition, whereas under 70% shading values remained stable and total chlorophyll content reached five times the full-sun level by the end. Photosynthesis, stomatal conductance, and transpiration were highest under 30 and 50% shading, where carbon-use efficiency was also greatest; at 90 days, intrinsic water-use efficiency was up to 33% higher than in full sun, although all treatments declined after 180 days. The higher carbon-use efficiency under moderate shading was accompanied by greater relative growth rates, and the leaf area ratio of shaded plants was about 60% higher than in full sun. Moderate shading (30–50%) is recommended for nursery production, whereas full sun is not indicated for the early phase, as it induces pho-toinhibition and limits growth.