SUMMARYWatermelon is a crop with a high water demand and is frequently grown under conditions of higher than normal root-zone salinity. In the present study, seedlings of watermelon (cv. Fantasy, Citrullus lanatus (Thunb.) Matsum & Nakai) were grown either ungrafted or grafted on three rootstocks: Strong Tosa, S1 (both Cucurbita maxima×Cucurbita moschata), or Emphasis (Lagenaria siceraria). All the plants were exposed to an NaCl-induced salinity stress (electrical conductivity, EC=2·2, 4·0, or 6·0 dS/m). The vegetative growth of all the plants substantially reduced after 2 weeks of exposure to 6·0 dS/m; however, growth of the plants grafted on Strong Tosa reduced less than that of the others. The leaf water content and specific leaf area (SLA, m2/g) decreased with an increasing salinity in grafted plants, but not in ungrafted plants. Salinity induced an increase of superoxide dismutase (SOD) activity in grafted plants up to two-fold depending on the rootstock, whereas it had no effect on this enzyme activity in ungrafted plants. Leaf Na+ concentration increased with increasing salinity in ungrafted and S1 grafted plants, whereas there was no significant leaf Na+ accumulation in Emphasis and Strong Tosa grafted plants. Leaf K+ concentration was affected by the rootstock but not by salinity, thus, the ability to keep a high K+/Na+ ratio was achieved mainly by limiting leaf Na+ concentration. The rootstock determined the leaf Cl− accumulation, with lower overall concentrations found if plants were grafted on the S1 rootstock than on Emphasis or ungrafted plants. Salinity significantly decreased the leaf NO3− concentration on Emphasis grafted plants only, while the NO3−/Cl− ratio was reduced in all the rootstocks. The capacity of Strong Tosa to withstand salt stress better than other tested rootstocks was probably due to the ability to induce anatomical adaptation (SLA) and SOD activity in response to salt stress, and also to the efficiency of Na+ exclusion from the shoot.

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Suggested watermelon planting densities and N rates vary on a large scale, indicating that there is insufficient knowledge about their effects. Therefore, the objective of this study was to evaluate the effects of N rate and planting density on growth, yield and quality of watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai] grown on black polyethylene mulch. The field experiments with `Crimson Sweet' watermelon were conducted in two climatologically different growing regions. The treatments were factorial combinations of three in-row plant spacings (0.5, 1.0, and 1.5 m) and three N rates (115, 195, and 275 kg·ha-1). Part of the N (35 kg·ha-1) was applied preplant and the remainder was fertigated. Vine length increased linearly up to 7 weeks after planting (WAP) as N rate increased from 115 to 275 kg·ha-1, and up to 9 WAP as plant spacing increased from 0.5 to 1.5 m. Total and marketable yields per ha or per plant did not increase with N rates above 115 kg·ha-1. Average fruit weight and fruit size distribution were generally unaffected by N rate. Leaf N concentration increased as N rate increased, although leaf N concentrations at the lowest N rate (115 kg·ha-1) even at 9 WAP were relatively high (43.3 to 47.3 g·kg-1). Total and marketable yields per ha were linearly decreased with an increase in plant spacing from 0.5 to 1.5 m, and the same was noticed with the total and marketable number of fruit per ha. With increased plant spacing average fruit weight increased and fruit size distribution shifted to larger categories.

Successful field establishment of vegetable transplants often depends on the ability of young seedlings to tolerate various biotic and abiotic stresses after transplanting. Treatments that limit transpirational water loss could improve plant survival and stand establishment. In this study we evaluated growth and physiological responses of pepper (Capsicum annuum L.) seedlings to foliar application of chemical plant regulators [abscisic acid (ABA) and aminoethoxyvinylglycine (AVG)] or physical film-forming barriers [AntiStress (AS), Transfilm (TF), and Vapor Gard (VG)] during transient 4-day water deficit cycles. During two 4-day water deficit cycles, stomatal conductance (g s) and net CO2 assimilation rate (ACO2) were unaffected by the application of physical materials, but differed for ABA and AVG. Compared with untreated control plants, ABA reduced g s (47% to 69%) and ACO2 (37% to 57%) by the end of the second water deficit cycle, whereas AVG increased gs (27% to 60%) during the first desiccation cycle. Leaf (ψlf) and stem (ψst) xylem water potential of plants treated with film-forming materials generally decreased at the same rate as those of untreated plants, whereas application of AVG caused earlier and more pronounced decline of ψlf. Application of ABA enabled the maintenance of ψlf and ψst during two desiccation cycles, and thus prevented an increase of electrolyte leakage and leaf abscission. Growth rates of all plant components were reduced after ABA applications. However, allometric relationships showed similar patterns of dry matter allocation in leaves and shoots among ABA, TF, VG, and untreated control plants. Application of AS reduced allocation of dry matter to leaves, whereas AVG enhanced it at the expense of roots. These data indicate that water deficit tolerance of pepper seedlings only occurred with foliar application of ABA. This effect was associated with improved plant water relations, increased cell membrane stability, reduced leaf abscission, and a transient reduction in plant growth rates.

Olive trees (Olea europaea L.) are considered moderately tolerant to salinity, with clear differences found among cultivars. One-year-old self-rooted olive plants of the Croatian cv. Oblica and Italian cv. Leccino were grown for 90 d in nutrient solutions containing 0, 66, or 166 mM NaCl, respectively. The shoot length and the number of nodes and leaves for both cultivars were not affected by salinity up to 66 mM NaCl. However, at 166 mM NaCl, growth of Leccino was reduced earlier and to a higher extent than growth of Oblica. After 10 d of exposure to 66 and 166 mM NaCl, increased activity of superoxide dismutase (SOD) was observed in Leccino, whereas there was almost no response in Oblica. Reduced SOD activity in Leccino at 166 mM NaCl was observed after prolonged stress (90 d), whereas in Oblica SOD was increased at 66 mM compared to control or 166 mM NaCl.Electrolyte and K + leakage were increased and relative water content decreased as NaCl concentration increased with similar intensity of response measured in both cultivars. Oblica exhibited an ability to keep a higher K + : Na + ratio at all salinity levels compared to Leccino, but since no difference was found in leaf K + concentration, this was mainly achieved by less Na + ions reaching the younger leaves. The antioxidative system represents a component of the complex olive salttolerance mechanism, and it seems that the role of SOD in protection from oxidative stress depends on sodium accumulation in leaves.

Areas with mild climate conditions are suitable for growing winter spinach (Spinacia oleracea L.). Successful production depends on choosing slow-bolting cultivars resistant to major diseases in each area. Field experiments with a total of 18 cultivars were conducted during 8 years in the Winter Garden region of Texas, an area known for its high white rust (Albugo occidentalis G.W. Wils) inoculum. Spinach cultivars differed widely in their resistance to both white rust and bolting, and the incidence of both traits was more severe as the season progressed. White rust infection increased linearly with average monthly minimal air temperature. Cultivars Fidalgo, Springfield, and Springer were slow bolting and are suitable for areas with no white rust incidence, while cvs. ASR-318, DMC 66-09, Fall Green, Samish, and San Juan were more white rust resistant.

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