O papel do cotilédone no estabelecimento de plântulas: estudo de caso Ricinus communis L. (Euphorbiaceae)

Abstract

The period of seedling establishment is a susceptible phase in the life cycle of plants, due to high mortality rates in this stage. The morphology of the cotyledons indicates what function it will have in the seedling. Cotyledons positioned above the ground by the elongation of the hypocotyl give rise to an epigeal seedling. These cotyledons can be thin with leaf-like morphology. Seedling phanerocotylar epigeal foliaceous (PEF) rapidly expand their cotyledons favouring photosynthetic activity. Premature loss of photosynthetic cotyledons can lead to increased mortality in the seedling phase. The objective of the present work is to experimentally 6 study the function of the photosynthetic cotyledon in the establishment of seedlings, using as a model Ricinus communis, a PEF seedling. It is intended to evaluate how different degrees of cotyledon removal will influence the growth and photosynthetic performance in R. communis seedlings. Ricinus communis seeds, collected near the Umuarama campus of the Federal University of Uberlândia, underwent pre-germination treatments to accelerate the germination process. The seeds were sown in 2 L pots with a mixture of soil, sand and substrate in the condition of 2: 1: 1. After seedling emergence, the cotyledons were removed forming four treatments with 20 repetitions each: 100% (control: intact seedlings), 50% (seedlings with total removal of just one cotyledon), 25% (seedlings with total removal of a cotyledon, plus 50% of the second cotyledon) and seedlings with total removal of the cotyledon area. Measurements of chlorophyll a fluorescence and Falker chlorophyll index were performed on the remaining cotyledons on the 1st, 3rd, 8th, 14th and 25th days after removing the cotyledons (DAR) and the measurements of leaf gas exchange were carried out on the 7th DAR. To analyse the growth of seedlings on the treatments applied, height, stem diameter, leaf area, dry mass, and relative growth rate were measured. In three moments of the experiment, cotyledons were collected for analysis of the structure of the cotyledon mesophyll. The values of the electron transport rate (ETR) in the treatments of 50% and 25% of the cotyledon area were higher than those of the control. Likewise, photosynthesis was superior in seedlings with 50% of the cotyledon area in relation to intact seedlings. The Falker chlorophyll indexes remained close until the 14th DAR, however the treatment with 25% of the cotyledon area exceeded to the other treatments. The loss of part of the cotyledons had a negative impact on seedling growth. Both, biomass accumulation as well as the relative growth rate decreased as the removal of cotyledons was more intense. Although it is possible to identify physiological compensation in seedlings with the removal of cotyledons, the smaller area of cotyledons present in the seedlings was not enough to balance the growth of the manipulated seedlings in relation to the control seedlings. 7 The results show the dependence of photosynthetic activity of cotyledons during initial establishment in seedlings of Ricinus communis.