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Paul R. Adler and Gerald E. Wilcox

Two mechanisms that reduce water and salt stress, respectively, are an increase in root hydraulic conductivity (LP) and reduction in Na and Cl absorption and transport to the leaf. NH4 +-N decreased muskmelon LP 55-70% while under 100 mM NaCl stress and 40-50% in the absence of NaCl stress. A decrease in LP increases the rate of water stress development as the transpiration rate increases. Although dry weight decreased about 70%, with NO- 3-N, muskmelon remained healthy green, while with NH+ 4-N they became chlorotic and necrotic with a 100% and 25% increase in leaf blade Na and Cl compared to NO- 3-N, respectively. Further investigation indicated that NH+ 4-N increased muskmelon sensitivity to NaCl through both an increased rate of net Na influx and transport of Na to the leaf. Since Na influx partitioning is controlled by mechanisms K/Na selectivity and exchange across membranes, the NH+ 4-N inhibition of K absorption may impair K/Na exchange mechanisms. Reduced K/Na selectivity or Na efflux are implicated as the source of the increased net Na influx with NH+ 4-N. The importance of K in preventing Na partitioning to the leaf was confined through removal of K from the nutrient solution thereby simulating the NH+ 4-N-induced gradual K depletion in muskmelon. Our work indicates that at a given level of water or NaCl stress, NO- 3-N reduces the level of stress experienced by muskmelon through increasing LP and reducing the net rate of Na influx and transport to the sensitive leaf blade. This avoidance mechanism should enable muskmelon plants fertilized with NO- 3-N to tolerate greater levels of stress.

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Paul R. Adler and Gerald E. Wilcox

Two mechanisms that reduce water and salt stress, respectively, are an increase in root hydraulic conductivity (LP) and reduction in Na and Cl absorption and transport to the leaf. NH4 +-N decreased muskmelon LP 55-70% while under 100 mM NaCl stress and 40-50% in the absence of NaCl stress. A decrease in LP increases the rate of water stress development as the transpiration rate increases. Although dry weight decreased about 70%, with NO- 3-N, muskmelon remained healthy green, while with NH+ 4-N they became chlorotic and necrotic with a 100% and 25% increase in leaf blade Na and Cl compared to NO- 3-N, respectively. Further investigation indicated that NH+ 4-N increased muskmelon sensitivity to NaCl through both an increased rate of net Na influx and transport of Na to the leaf. Since Na influx partitioning is controlled by mechanisms K/Na selectivity and exchange across membranes, the NH+ 4-N inhibition of K absorption may impair K/Na exchange mechanisms. Reduced K/Na selectivity or Na efflux are implicated as the source of the increased net Na influx with NH+ 4-N. The importance of K in preventing Na partitioning to the leaf was confined through removal of K from the nutrient solution thereby simulating the NH+ 4-N-induced gradual K depletion in muskmelon. Our work indicates that at a given level of water or NaCl stress, NO- 3-N reduces the level of stress experienced by muskmelon through increasing LP and reducing the net rate of Na influx and transport to the sensitive leaf blade. This avoidance mechanism should enable muskmelon plants fertilized with NO- 3-N to tolerate greater levels of stress.

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James E. Simon, W. Dennis Scott, and Gerald Wilcox

A study was initiated at the Southwest Purdue Ag Center to demonstrate the effect of transplant age and transplanting date on the scheduling of melon harvests. Muskmelon (Cucumis melo cv. Superstar) was seeded into #38 growing trays with Jiffy-mix media. Seeding dates were such that 14 and 21 day old seedlings were transplanted April 25, May 9, 17 and 24. The plants were grown on black plastic with trickle irrigation Marketable fruit were harvested starting on June 28 and continuing through August 12. Neither transplant age or date had a significant effect on the number of fruit harvested or on total yield. However, each transplant date showed a distinctive harvest peak beginning June 30 for the April 26 transplant then June 7, 14 and 21 for each successive transplant date.

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Gerald E. Wilcox, Paul R. Adler, and Mohamad Errebhi

A study was made to investigate the effects of liming and N source fertilization on soil acidity, nutrient uptake an yield of muskmelon on a Princeton loamy-sand (fine sandy, mixed, mesic, type Hapludalf) at Southwest Purdue Agricultural Center, Vincennes, IN. The experiment consisted of lime and no lime treatments with five N treatments of 0 N, 50 kg·ha-1 N as urea and 100 kg·ha-1 N as urea, NH4NO3, and (NH4) SO4. The unlimed soil tested pH 4.6, 4.2 and 4.1 and the limed soil was pH 5.5, 5.6 and 5.2 with 100 kg N·ha-1 as urea, NH4NO3 and (NH4) SO4, respectively. With NH4NO3 the NO3-N declined from 268 ppm on 6/1 to 64 ppm on 7/7 in the saturation extract (SE). Highest NH4-N was from (NH4)2SO4 followed by NH3NO4 and urea. The NH4-N concentration from (NH4)2SO4 in the SE decreased from 152 ppm to 19 ppm during the season on unlimited soil and from 56 ppm to 8 in the SE decreased from 152 ppm on limed soil. Symptoms of Mn toxicity in the leaves became apparent on unlimed plots 7 weeks after transplanting. As the rate of N increased in the range of 0, 50 and 100 kg·ha-1 from urea the Mn contents were 372,459 and 607 ppm respectively. The muskmelon fruit yield increase due to 100 kg N·ha-1 was 13279 kg·ha-1, 12161 kg·ha-1 and 8502 kg·ha-1 for ureas, NH4NO3 and (NH4)2SO4 respectively.

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Richard G. Snyder, James E. Simon, Richard A. Reinert, Michael Simini, and Gerald E. Wilcox

Watermelon, Citrullus lanatus (Thunb.) Matsum & Nakai cv. Sugar Baby, were grown in the field as a fall crop in open-top chambers (OTC) in southwestern Indiana with either charcoal-filtered (CF) or nonfiltered (NF) air. Ozone and sulfur dioxide were continuously monitored in OTC and ambient air. There was a significant decrease in marketable yield by weight (19.9%, P = 0.05), percentage of marketable fruit by number (20.8%, P = 0.10), and total yield by weight (21.5%, P = 0.05) from plants grown in the NF air treatment compared with those grown in CF air. Ozone-induced foliar injury was significantly greater on plants grown under NF conditions. Ambient concentrations of 03 in southwestern Indiana caused foliar injury (P = 0.10) and significant yield loss to a fall crop of watermelons.

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Glenn H. Sullivan, William J. Ooms, Gerald E. Wilcox, and Douglas C. Sanders

A management expert system that enables producers to fully assess the integrated resource requirements, management risks, and profit potential for growing muskmelon was developed. The expert system environment Guru was used as the development software.