Ecophysiology of the tall coconut growing under different coastal areas of northeastern Brazil

Coconut palm is an important crop explored as a rich source of oil, fiber, milk, and water in most tropical areas. Brazil is the fifth largest producer in the world, and extensive coastal areas of Northeastern Brazil are planted with tall coconut. However, the sustainability of these coconut plantations can be affected by different abiotic stresses, including drought, mineral nutrition, and salinity. So, the objective of the present study was to evaluate the ecophysiological responses of tall coconut (Cocos nucifera L.) adult plants in cultivated and semi-extrativist areas on coast of Ceara state, in Northeastern Brazil. The study was carried out in four areas: Irrigated, Rainfed, Foreiro (coconut trees on coastland owned by the Navy and temporarily rented to private individuals) and Preamar (coconut trees in high-tide areas owned by the Navy). The evaluations (leaf gas exchanges, Na+ on leaf surfaces, leaf concentrations of soluble carbohydrates, N, K, P, and Na, soil moisture, and soil electrical conductivity) were carried out in the dry and rainy seasons, between 2015 and 2017. Our results indicated that tall coconut palms were subjected to abiotic constraints in three of the four evaluated areas. Salinity and water-logging were evident in Preamar plants; water deficit in Rainfed plants; and nutritional stress limited the crop yield in Rainfed, Preamar, and Foreiro areas. Coconut plants that grew in Foreiro area, despite the proximity to the ocean, were not subjected to salt stress, and the higher moisture in lower soil layers significantly reduced the effect of the dry season on photosynthetic rates. Seawater spray caused accumulation of Na+ on coconut leaf surfaces, mainly in the areas close to the sea. The repeated salinity stress and waterlogging in high-tide area caused a permanent hindrance for carbon assimilation rate, limiting the capacity of fruit production by plants. Current climatic changes leading to the elevation of sea level and intrusion of sea water enhance the general concern on how to maintain these coconut plantations as source of fiber and nutrition to future generations.