Germinação, crescimento e desenvolvimento do coqueiro anão verde sob estresse salino.

Abstract

Three experiments were conducted to study the viability of the use of saline waters in the irrigation of the coconut (Cocos nucifera L.) cv Anão verde, in different cultivation phases. Experiments I and I I carried out in Campina Grande-PB consisted respectively of the evaluation o f the effects of the irrigation with saline waters during germination and 120 days initial development of seedlings (I - Stage I), evaluating later, the behavior of seedlings produced under saline stress conditions. alter being transplanted and irrigated with water o f 2,2 dS m"1 o f electrical conductivity in nursery during 120 days ( I - Stage II). In the Experiment I I evaluation the growth o f seedlings in the nursery ( I I - Stage I ) , evaluating later, the behavior of seedlings produced under saline stress conditions after being transplanted and irrigated with water of 2,2 dS m"1 of electrical conductivity in field during 16 months ( I I - Stage I I ) . Experiment I I I was conducted in Paranamirim-RN, during 2 years, with the objective of evaluating the effects of electrical conductivity o f water (ECw) on the morpho-physiological characteristics and the production o f coconut with 3,5 years o f age, cultivated in a sandy soil. For Experiments I and I I a completely randomized design consisting o f 5 leveis of water salinity (ECw o f 2.2, 5.0, 10.0, 15.0 and 20.0 dS m"1 at 25 °C denominated, respectively, T,, T2, T3 , T 4 and T5 ) was adopted with four replications in Experiments I (Stage I and II) and Experiment I I (Stage I ) and five replications in Experiment I I (Stage II). For Experiment I I I a randomized block design was adopted consisting o f 4 leveis o f water salinity (ECw o f 0.1, 5.0, 10.0 and 15.0 dS m"1 at 25 °C) with 4 replications. The saline waters used in the study were prepared with the addition of commercial NaCI (without iodine). The results obtained in Experiment I (Stage I) show that the increment of ECw did not significantly affect the percentage of germination that varied from 80 to 97,5%, though it significantly affected the germination speed (the seeds in T 5 on an average took 10 days more to germinate compared to Ti) and the growth of the seedlings; however, drastic reductions in the dry weight were observed only in leveis T 4 and beyond and the root system was the most affected part o f the plant by water salinity. In Experiment I (Stage I I ) it was verified that the negative effects of the salinity during the germination phase were overcome when the seedlings were irrigated with ECw = 2,2 dS m . In Experiment I I (Stage I), the seedlings showed 100% survival, and the results obtained indicate that the salinity of the irrigation water provoked substantial reductions in the growth speed o f the plants, dry weight, diameter o f the stem and leaf area, at leveis higher than 10 dS m"'; the root system and leaf area being the most affected by salinity. During Experiment I I (Stage II) it was verifíed that seedling produced in the nursery under conditions of saline stress (ECw > 5,0 dS m"1) when planted in saline soil with high water table, presented slower growth than the seedlings produced without stress; however, drastic reductions occurred in those irrigated in the nursery with ECw > 15,0 dS m"1. In Experiment I I I , the increment of water salinity reduced the total number of leaves in the plants and increased mean time for emission o f inflorescence, principally in the plants irrigated with ECw = 15 dS m"1. The coconut plant when irrigated with saline water presented mechanisms to overcome the stress effect by reducing stomatal conductance and of the number of leaves; there was decrease in the water potential o f the leaves due to increment o f salinity only in the mornings, and no alteration were verifíed in the photosynthesis efficiency due to the water salinity. The use of saline waters (>5.0 dS m"1) in the irrigation provoked significant reductions in the number of fruits in relation to control (ECw = 0,1 dS m-1), only during 15th and 19thharvest, however in the treatment ECw = 15 dS m"1 there was significant reduction o f the number of fruits in relation to control. The quality o f the production was affected by ECw increase with negative effects on the weight and format o f the fruits and volume o f the coconut water; and positive effects on °Brix and total solids content of water. The salinity of the irrigation water increased contents o f Cl, Na, K and decreased the contents of Mg in the leaf 14. The increment of water salinity significantly elevated the contents of Cl, K, Na, Ca and Mg of the coconut epicarp. At the end o f the first year, there was increase in the soil salinity (ECse and SARse) in the whole profile (up to 80 cm) in relation to the initial characteristics; the increases being proportional to the ECw and SAR o f the irrigation water. However, there were no considerable increase in ECse and SARse at the end of the second year in relation to the end o f the first year.