Photosynthetic flexibility in maize exposed to salinity and shade

C₄ photosynthesis involves a close collaboration of the C ₃ and C₄ metabolic cycles across the mesophyll and bundlesheath cells. This study investigated the coordination of C₄ photosynthesis in maize plants subjected to two salinity (50 and 100 mM NaCl) treatments and one shade (20% of full sunlight) treatment. Photosynthetic efficiency was probed by combining leaf gas-exchange measurements with carbon isotope discrimination and assaying the key carboxylases [ribulose-1,5- bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC)] and decarboxylases [nicotinamide adenine dinucleotide phosphate malic enzyme (NADP-ME) and phosphoenolpyruvate carboxykinase (PEP-CK)] operating in maize leaves. Generally, salinity inhibited plant growth and photosynthesis to a lesser extent than shade. Salinity reduced photosynthesis primarily by reducing stomatal conductance and secondarily by equally reducing Rubisco and PEPC activities; the decarboxylases were inhibited more than the carboxylases. Salinity increased photosynthetic carbon isotope discrimination (δ_p) and reduced leaf dry-matter carbon isotope composition (13δ) due to changes in p_i/pa (intercellular to ambient CO ₂ partial pressure), while CO₂ leakiness out of the bundle sheath (θ) was similar to that in control plants. Acclimation to shade was underpinned by a greater downregulation of PEPC relative to Rubisco activity, and a lesser inhibition of NADP-ME (primary decarboxylase) relative to PEP-CK (secondary decarboxylase). Shade reduced δ_p and without significantly affecting leaf 13δ or p_i/pa relative to control plants. Accordingly, shade perturbed the balance between the C₃ and C₄ cycles during photosynthesis in maize, and demonstrated the flexible partitioning of C₄ acid decarboxylation activity between NADP-ME and PEP-CK in response to the environment. This study highlights the need to improve our understanding of the links between leaf 13δ and photosynthetic δ_p, and the role of the secondary decarboxylase PEP-CK in NADP-ME plants such as maize.