Adaptation of Lettuce Cultivars to High Temperatures and Different Types of Fertilization: Anticipating Management Strategies to Mitigate the Adverse Effects of Global Climate Change

Abstract

Global climate change (GCC) is already a reality and has the potential to cause significant damage to Brazilian and world agriculture. One of the most important aspects for tropical countries is the possibility of a significant increase in average air temperature which, according to the Intergovernmental Panel on Climate Change (IPCC), could vary between 1.4 ^oC and 4.4 ^oC by the end of this century. Increases in the occurrence of heat waves, extreme rainfall and droughts are also projected. The average annual air temperature in Brazil is projected to exceed 30°C. Lettuce (Lactuca sativa L.) is the most widely consumed leafy vegetable in Brazil and worldwide and originates from regions with a moderate climate and is therefore vulnerable to very high temperatures. The aim of this study was to evaluate the performance of commercial lettuce cultivars in a simulated environment with projected temperatures like those at the end of the 21st century. It also aimed to assess the positive effects of using a biofertilizer, called Hortbio®, as a strategy to mitigate the negative impacts of high temperatures. To this end, two experiments were conducted in a factorial scheme of 11 (eleven lettuce cultivars) x 2 (two types of fertigation: Hortbio® = organic and mineral fertilizers). The first experiment was conducted at a temperature of 25 ^oC/20 ^oC (day/night), representing the historical average air temperature in Brazil. The second experiment was conducted at an average air temperature of 30 ^oC/25 ^oC, representing the projected temperature for the end of the 21st century. The following morpho-agronomic attributes were assessed: total mass, commercial mass, number of leaves and plant height. The occurrence of morphophysiological disorders was also assessed, including days to setting, occurrence of leaf chlorosis and tipburn. The results showed that, although well adapted to the country's historical tropical temperatures, most of the cultivars showed low levels of adaptation to the environmental conditions projected in the GCC scenario. Only three cultivars showed good results when grown at 30 ^oC/ 25 ^oC. The main morphophysiological disorders observed were early setting, tipburn and leaf chlorosis, although leaf necrosis and even plant death were also observed when the plants were grown in the worst-case scenario. The total mass showed a tendency to increase at 30 ^oC/ 25 ^oC regime. However, this result was not reflected in the commercial mass due to the simultaneous occurrence of morphophysiological disorders. The main characteristic that differentiates the three cultivars that showed the best results under these conditions was the earliness of the production cycle.  The use of biofertilizer apparently has the potential to mitigate part of the negative impacts of high temperatures on the development of cultivated lettuce plants. This work indicates lettuce germplasm with potential use in genetic improvement programs aimed at adapting to GCCs, as well as presenting alternative management strategies aimed at adapting lettuce cultivation in GCCs scenarios.