Growth and photosynthetic acclimation of coffee plants under contrasting irradiance and nitrogen supplies

Coffee (Coffea arabica L.) evolved in shady environments but often produces better at full sunlight, although at the cost of high fertilization loads, especially nitrogen (N). Coffee displays low net photosynthetic rates (A) and low photosynthetic N-use efficiency (PNUE), and the underlying mechanisms by which the N supply affects coffee's growth and photosynthetic performance in distinct light environments remain elusive. Here we investigated two coffee cultivars with presumably distinct light requirements. Potted plants, grown at full sunlight (HL) or at 80 % of light restriction (LL), were fertilized with either low (LN) or high (HN) N supplies. Marked photosynthetic responses to varying light and N supplies were observed, but differences between cultivars were limited. Decreases in A due to (i) LL could be largely explained by the exacerbation of stomatal limitations irrespective of N, and (ii) N deficiency by the increased mesophyll limitations at shade and increased biochemical constraints at HL. The specific activities of major antioxidant enzymes were upregulated by LN in HL. The low coffee's PNUE was unresponsive to N and primarily associated with low A despite the relatively low N fractions invested into photosynthesis; at LL, however, some decreases in PNUE might be linked to an overinvestment in RuBisCO. Cultivar variations in shade tolerance were chiefly driven by canopy architectural aspects rather than by leaf morphophysiological traits. Our findings have direct practical importance for breeding programs which should currently centre their attention for selecting coffee cultivars specifically for shade environments.