Rendimento e qualidade do melão em função do nível e da época de aplicação de águas salinas.

Abstract

In Brazil, the state of Rio Grande do Norte is the largest producer of melons for exportation, and in Chapada do Apodi, where most plantations are, farmers use irrigation water of different salinity levels. The effect of both salinity level and duration of application of saline water on crop yield is recognized. This research was conducted to study the response on soil characteristics and on growth, yield and fruit quality of melon cultivar AF646, to the application of irrigation water of different salinity levels during the entire cycle and to the increase of water salinity in three different growth stages, when waters of the initial salinity level were replaced by waters of the superior salinity levels. Two trials were conducted in the same area, in the years 2001 and 2002, in a Red Latosol at Santa Julia Farm (5° 02' 0,02" S, 37° 22' 33,6" WGr) near Mossoró, RN, Brazil. Water salinity levels were: Si = 0.6; S2 — 1.9; S3 = 3.2 and S4 = 4.5 dS m"1. Si water came from a well in the Arenito Açu Aquifer; S3 came from a well in the Calcário J and aí ra Aquifer; S2 was a mixture in equal parts of Si and S3, and S4 was obtained by addition of NaCl to S3 water. These waters, used continuously during the melon cycle, corresponded to treatments T] to T4. To compose treatments from T5 to T15, initial waters were replaced for the waters of superior salinity, beginning at 30 or 50 days after seeding (DAS): Si by S2, S3 or S4; S2 by S3 or S4 and S3 by S4. The experimental design was a completely randomized blocks with four replications, amounting sixty 36 m2 plots. Water was applied daily trough drip irrigation. Water depth necessary to supply crop évapotranspiration plus 0.10 of leaching fraction was based on T, and calculated to obtain soil field capacity trough a depth of 0.45 m Total irrigation depths were 344 mm on Trial I and 311 mm in Trial II. Three harvest were made at a three days interval, beginning at 64 DAS on Trial I, and at 62 DAS on trial Q. During melon cycle leaf area index (LAI) had a prevailing cubic response to water salinity (Si to S4) in both trials; shoot dry mass (SDM) showed a quadratic response on Trial I and a cubic one in Trial Ü; leaf area ratio of shoots (LARS) showed a negative linear response on bom trials; for specific leaf area (SLA), although different in each trial both adjusted models showed lower values at cycle end. A combined analysis of trials did not show difference in LAI, SDM, LARS and SLA, neither at 50 DAS nor at cycle end, between the continuous use of less saline water and the adoption of a higher salinity water. Water salinity levels and duration of crop exposition to saline stress affected melon yield; the longer the time and higher the salinity level lower the yield. The later the water salinity level increase occurred, less likely was the occurrence of a yield loss. The water with the lower salinity level had the higher cost but showed the higher profit. Analysis on harvest day did not show effect of salinity levels and dates of water salinity increase on fruit quality characteristics, and in a analysis 35 days after harvest pulp firmness, total soluble solids content, pH and fruit weight loss were not affected. The electrical conductivity (EC) of fruit juice showed both linear and quadratic positive effects due to salinity levels only on harvest day. In both trials, treatments Ti to T4 showed higher salt accumulation on soil upper layer, and higher profile mean salinity where more saline waters were used. Evolution of mean soil salinity during melon cycle in both trials showed: an increase in soil salinity until 50 DAS and then a decrease when only water salinity levels were considered and when water salinity level was increased at 30 DAS; a continuous increase in soil salinity when salinity of water increased at 50 DAS; overall, mean soil salinity was higher when more saline waters were used, with some exceptions. Transversal profiles of soil salinity in both trials expanded during melon cycle, becoming deeper with time. The higher soil salinity values were always in the upper soil layer, in a 0,20 m radius from the irrigation emitter, mainly where the plant was located. Salinity values in the profile were proportional to EC of the water used. Soil saturation paste pH increased from the first to the second cycle, mainly with depth, but its values were near 7. Salinity levels of the water used during melon cycle in bom trials caused positive linear effect on soil mean salinity. Trial I had higher mean soil salinity man trial n. Soil mean salinity caused a negative linear effect on marketable yield (Pcom) and total yield (Ptotal), when analyzing each trial separately or in a combined analysis. The inequality for Pcom and equality for Ptotal, obtained in the Similarity Test between models, showed that Ptotal is more related to soil salinity.