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Maren E. Veatch* and Dennis T. Ray

Rubber production in guayule (Parthenium argentatum, Gray) is highest in the winter months. It has been suggested that lower nighttime temperatures stimulate rubber production. Higher rubber production could be the result of up regulation of the enzymes involved in rubber synthesis, or acclimation of the photosynthetic apparatus to cold temperatures, making photosynthates available for rubber production when the plants are not actively growing, or a combination of the two factors. The effect of low night temperatures on photosynthesis and rubber production was examined in greenhouse grown plants. The experiment was set up as a split plot with different sub plots enclosed in heated or non-heated containers during the night. Those placed in non-heated containers were exposed to the ambient night temperatures of the greenhouse. During the growing season, growth was measured as change in height, and midday and diurnal photosynthetic measurements were collected. At harvest fresh and dry weight of both roots and shoots were taken. Resin and rubber were then extracted with acetone and cyclohexane respectively. Midday photosynthesis was as high or higher in cold night plants than in the warm night plants. In addition, the cold night treatment had lower growth than the warm night treatment. Percent resin and rubber were significantly higher in both shoots and roots of cold night plants, as was the overall yield of rubber. An increase in photosynthetic rate in the cold night treatment, without the concomitant increase in growth seen in the warm night treatment, could account for the increase in rubber production during the winter months.

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Maren E. Veatch-Blohm and Dennis T. Ray

As a native of the Chihuahuan desert, guayule (Partheniumargentatum Gray) has a history of dealing with low water availability. Agronomic studies have shown that increasing irrigation increases overall rubber yields, but decreases rubber concentration per plant. As water availability is an important factor in agricultural production, this study was conducted to examine how drought affects plant growth and secondary compound distribution throughout the plant. One-year-old guayule plants were subjected to water stress from June through August, in 2003 and 2004. The well-watered treatment was irrigated daily, and the drought-stressed plants were irrigated when the soil water potential reached 6 (0.6) or 3 (0.3) bars (megapascals) in 2003 and 2004, respectively. Plant growth was monitored by measuring height, width, and stem diameter. Fresh weight of shoots and roots was recorded at harvest, and a subset of plants were defoliated and used to determine leaf weight and area. Resin and rubber were extracted from dried and ground plant samples. Growth, leaf weight, and leaf to stem ratio were decreased in the drought-stressed plants compared to the well-watered plants. Rubber concentration, but not resin concentration, was higher in the drought-stressed plants. There were no significant differences in resin and rubber concentration in the leaves and roots of the different treatments; however, they were both higher in the stems of the drought-stressed plants. In guayule, rubber is deposited mainly in the bark parenchyma of the stems. The drought-stressed plants had a greater contribution of stem biomass to overall biomass and a reduced stem diameter with higher bark to wood ratio, which could account for the higher rubber concentration in the drought-stressed plants.

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Maren E. Veatch-Blohm and Lindsay Morningstar

Salinity is a problem for crop production worldwide, which can affect not only the yield of ornamental plants, but also their quality. Calla lilies (Zantedeschia K. Koch) are a popular cut flower and landscape plant. In this study we examined how salinity applied either pre- or post-emergence altered shoot growth and flower production compared with standard growth conditions. Salinity applied post-emergence did not significantly affect shoot growth in either the 25- or 50-mm NaCl treatments; however, irrigation with a 50-mm NaCl solution did significantly reduce dry weight and flower production compared with the control. The most drastic effect on flower production was observed when the 50-mm treatment was applied pre-emergence. Although flowering time did not vary among treatments, in the spring, flowering success was less than 50% in the 50-mm treatment, which was significantly less than the other treatments. The plants in all treatments had good visual quality and high relative water content although the shoots of the salt-stressed plants had a Na+ content two to five times greater than the control. We hypothesize that the maintenance of visual quality may in part be the result of increased Ca2+ concentration (up to four times the amount in the control) in the shoots of the salt-stressed plants, which may have helped maintain turgor and cell membrane integrity under saline conditions. Our results indicate that use of irrigation water salinized by NaCl with an electrical conductivity (EC) more than twice as high as that recommended for optimal calla growth (1.5 dS·m−1 compared with 3.4 dS·m−1) can be used without a loss in plant quality, thereby increasing irrigation options for calla lilies, particularly in areas where high-quality water is scarce.

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Maren E. Veatch-Blohm, Douglas Sawch, Nicole Elia, and Dominic Pinciotti

Salinity is a problem for crop production worldwide and may be particularly problematic for ornamental plants because it has the potential to degrade visual quality. Daffodils [Narcissus sp. (L.) Amaryllidaceae] are a popular bulb plant, in demand for both landscapes and cut flowers. In this study, we examined how salinities ranging from 0 to 300 mm NaCl affected growth, flower production, and leaf physiology of three of the most popular cultivars of daffodil (‘Dutch Master’, ‘Ice Follies’, and ‘Tete-a-Tete’). Salinity reduced growth rate and biomass production at salinities greater than or equal to 100 mm NaCl with the greatest reductions in the highest salinity treatments (200 and 300 mm NaCl). Despite reductions in biomass, there was no significant chlorosis of the leaves. Flower quantity was unaffected by salinity in ‘Dutch Master’ and ‘Ice Follies’, but anthesis was delayed and flower duration was reduced by 40% to 70% at salinities of 150 mm NaCl and above. Anthesis and flower duration in ‘Tete-a-Tete’ were unaffected by salinity, but the number of flowers produced was negatively affected (reductions of 50% or more) by salinities of 150 mm NaCl and above. Sodium concentration in the leaves and bulbs increased 53% to 400% compared with the 0 mm NaCl control with lower accumulation in the bulbs than in the shoots. Sodium accumulation occurred at or above 50 mm NaCl in ‘Tete-a-Tete’, but at salinities greater than 150 mm NaCl in ‘Dutch Master’ and only in the 300-mm NaCl treatment in ‘Ice Follies’. Despite the Na+ accumulation in the leaves, the plants in most of the salinity treatments were able to maintain a K+:Na+ ratio above 1 (except in ‘Tete-a-Tete’ at salinities 150 mm NaCl or greater), which may have helped the daffodils tolerate the negative affects of Na+ and maintain good visual quality. ‘Dutch Master’, ‘Tete-a-Tete’, and ‘Ice Follies’ can be considered highly salt-tolerant because they maintain visual quality (leaf greenness, flower production, and flower duration) at soil NaCl-induced electrical conductivities greater than 6 dS·m−1.

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Maren E. Veatch-Blohm, Dorothy Chen, and Matthew Hassett

Salinity affects crop production worldwide and can be particularly problematic for ornamental plants. Daffodils (Narcissus sp. L.) are one of the most popular bulb plants, in demand for both landscapes and cut flowers. In this study we examined how moderate salinity, imposed as NaCl, affected growth, flower production, and leaf physiology when salinity treatment began either pre- or postemergence. Salinity did not affect growth rate or flower production for the three early-season cultivars (‘Dutch Master’, ‘Ice Follies’, and ‘Tete-a-Tete’) regardless of when salinity treatment began, but there were reductions in dry shoot biomass production under the highest salinity (50 mm NaCl). The mid-/late-season cultivar (‘Actaea’) had growth reduction and decreased flower production at the highest salinity when salinity treatment began preemergence. Within 28 days of greenhouse growth, the plants in the 50 mm NaCl treatment had carbon assimilation rates of only 25% to 32% of the 0 mm NaCl control. Sodium concentration increased 35% to 900% in the leaves in response to salinity for all cultivars when salinity treatment began preemergence but significantly increased only for ‘Actaea’ and ‘Tete-a-Tete’ (42% to 235%) when salinity treatment began postemergence. Potassium concentration in the shoot was mostly stable in response to salinity. The ability to maintain potassium (K) content within the plant may help daffodils tolerate the negative effects of sodium (Na). We believe that ‘Actaea’ was more negatively affected by salinity than the other three cultivars because, as a mid-/late-season cultivar, it had a much longer exposure period to salinity than the three early-season cultivars. However, moderate salinity does not seem to have a significant impact on growth and visual quality for ‘Dutch Master’, ‘Ice Follies’, or ‘Tete-a-Tete’ indicating that at least some saline water can be used for plant production without losses in plant quality.

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Maren E. Veatch-Blohm, Dennis T. Ray, and Valerie H. Teetor

Guayule (Parthenium argentatum Gray) has been difficult to improve through classical plant breeding because of its facultative apomictic reproductive system. Attempts have been made to increase rubber concentration and yield by examining traits correlated with rubber production and their heritabilities. We propose a new way of estimating heritability in guayule that more accurately accounts for the contributions of apomictic and sexual reproduction. At two years of growth, there was a significant relationship between the parents and the progeny for all traits measured, except for rubber and guayulin B concentrations. Due to the facultative apomictic nature of guayule reproduction, heritabilities are more accurately presented as a range of values between the narrow and broad sense heritabilities. Since guayule is more apomictic than sexual, most heritabilities will be closer to the broad sense values. To increase resin and rubber yield in the progeny, selection should focus on height and width in the parents because height and width are highly correlated with rubber yield, with the highest heritabilities (0.65 to 1.00 and 0.97 to 1.0, respectively) of the traits measured.