Marc W. van Iersel

Night-time transpiration can decrease hydraulic redistribution.

C(3) plants dominate many landscapes and are critically important for ecosystem water cycling. At night, plant water losses can include transpiration (E(night)) from the canopy and hydraulic redistribution (HR) from roots. We tested whether E(night) limits the magnitude of HR in a greenhouse study using Artemisia tridentata, Helianthus anomalus and Quercus laevis. Plants were grown with their roots split between two compartments. HR was initiated by briefly withholding all water, followed by watering only one rooting compartment. Under study conditions, all species showed substantial E(night) and HR (highest minus lowest soil water potential [Psi(s)] during a specified diel period). Suppressing E(night) by canopy bagging increased HR during the nightly bagging period (HR(N)) for A. tridentata and H. anomalus by 73 and 33% respectively, but did not affect HR(N) by Q. laevis. Total daily HR (HR(T)) was positively correlated with the Psi(s) gradient between the rooting compartments, which was correlated with light and/or atmospheric vapour pressure deficit (VPDa) the prior day. For A. tridentata, HR(T) was negatively correlated with night-time VPDa. Ecological implications of the impact of E(night) on HR may include decreased plant productivity during dry seasons, altered ecosystem water flux patterns and reduced nutrient cycling in drying soils.

Temperature Effects on the Development, Survival, and Reproduction of the Madeira Mealybug, Phenacoccus madeirensis Green (Hemiptera: Pseudococcidae), on Chrysanthemum

Abstract The Madeira mealybug, Phenacoccus madeirensis Green (Hemiptera: Pseudococcidae), has become an increasingly damaging pest in greenhouse ornamental production. Current management tactics of P. madeirensis require a regular chemical application schedule targeting the immature stages. Knowledge of the life cycle of P. madeirensis is important to the success of its management program. We investigated the effects of constant temperature (15, 20, 25, 30, 35, and 40°C) on the development, survival, and reproduction of P. madeirensis on chrysanthemum (Dendrathema x grandiflora Kitam.). We failed to establish colonies at 30–40°C. Between 15 and 25°C, the duration of development of all developmental stages were shortened at higher temperatures. The total duration of development of female mealybugs was ≈30 d at 25°C, 46 d at 20°C, and 66 d at 15°C. Developmental time of males was 3–9 d longer than females. Survival rates of individual instars ranged between 88 and 100% and were not influenced by temperature. Overall, >75% of eggs completed development to adulthood. Female mealybugs made up 50% of the adult populations in all temperature treatments. Adult longevity at 25°C was ≈3 and 20 d for males and ovipositing females, respectively. Females at 20°C produced the highest number of eggs (491 ± 38 eggs/female).

Interactions between temperature and fertilizer concentration affect growth of subirrigated petunias

To evaluate effects of fertilizer concentration and temperature on growth of petunia (Petunia × hybrida Hort. Vilm-Andr.), we grew petunias under three different day/night temperature regimes (35/27°C, 25/17°C, and 15/7°C) and with five different concentrations of fertilizers [electrical conductivity (EC) of 0.15, 1.0, 2.0, 3.0, and 4.0 dS m−1]. There was an interactive effect of temperature and fertilizer EC on the growth of the petunias. Optimal fertilizer EC decreased as temperature increased. Growth was better correlated with the EC of the growing medium than with the EC of the fertilizer solution. Irrespective of growing temperature, plant growth was best in treatments with a final growing medium EC of 3–4 dS m−1. The time to flowering was decreased, but flowers senesced more rapidly at higher temperature. Flower diameter decreased with increasing temperature. At the highest temperature (35/27°C), flower diameter also decreased with increasing EC of the fertilizer solution. The EC of the growing medium increased with increasing EC of the fertilizer solution and with increasing temperature. Effects on shoot nutrient concentrations were inconsistent among the three temperature treatments. At 25/17°C, a fertilizer EC of 2 dS m−1 resulted in the highest shoot N concentrations, while shoot N was not affected by fertilizer EC at 35/27°C or 15/7°C. Shoot P concentration increased with increasing fertilizer EC at a temperature of 15/7°C, but not at higher temperatures. These results indicate that fertilization guidelines for greenhouse growers should be based on maintaining the EC of the growing medium within an optimal range, instead of the more traditional recommendations based on the concentration of the fertilizer solution.

Imidacloprid Applications by Subirrigation for Control of Silverleaf Whitefly (Homoptera: Aleyrodidae) on Poinsettia

Abstract The objective of this study was to determine whether silverleaf whiteflies, Bemisia argentifolii Bellows & Perring, on poinsettia, Euphorbia pulcherrima Willdenow ex Klotsch, can be controlled with imidacloprid applied by subirrigation. Different amounts of imidacloprid uptake by the growing medium were obtained by not watering the subirrigated plants for 0, 1, 2, or 4 d before the imidacloprid application. These treatments resulted in absorption of 12–175 ml of imidacloprid solution by the growing medium. These treatments were compared with untreated control plants and plants that were treated with a standard drench application (100 ml) to the top of the growing medium. All imidacloprid treatments resulted in a significant decrease in both the survival of adult whiteflies and number of immature whiteflies on the plants. Subirrigation treatments resulted in better control of adult and immature whiteflies than drench application. Withholding water for 2 or 4 d before the imidacloprid application by subirrigation improved control of immature whiteflies. This indicates that the application of imidacloprid to poinsettia by subirrigation is a practical and efficient method to control silverleaf whiteflies.

NUTRIENT SOLUTION CONCENTRATION AFFECTS GROWTH OF SUBIRRIGATED BEDDING PLANTS

To evaluate the effects of nutrient solution concentration on growth of alyssum [Lobularia maritima (L.) Desv. ‘New Carpet of Snow’], celosia (Celosia argentea L. ‘Gloria Scarlet’), dianthus (Dianthus chinensis L. ‘Telstar Crimson’), gomphrena (Gomphrena globosa L. ‘Gnome White’), stock [Matthiola incana (L.) R. Br ‘Special Mix’], and zinnia (Zinnia elegans Jacq. ‘Dreamland Mix’), we grew plants with five different concentrations of Hoagland nutrient solution [0.125, 0.25, 0.5, 1.0, and 2.0×full strength; electrical conductivity (EC) of 0.4, 0.7, 1.1, 2.0, and 3.7 dS m−1, respectively]. Plant growth of alyssum was maximized when plants were fertilized with 1.0×concentration of Hoagland solution, resulting in a final growing medium EC of 2.1 dS m−1 (as determined by the pour-through method). Maximum shoot dry mass of celosia was obtained when plants were fertilized with 0.5 to 1.0×concentration of Hoagland solution, resulting in a growing medium EC of 1.1–2.5 dS m−1. Growth of dianthus was best with a 1.0×concentration, resulting in a growing medium EC of approximately 3.7 dS m−1. However, dianthus had the most flowers when fertilized with a 2.0×solution. The concentration of nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg), boron (B), and iron (Fe) in the shoots of dianthus increased, while the concentrations of molybdenum (Mo) and zinc (Zn) decreased with increasing fertilizer concentration. Gomphrena and stock preferred high fertilizer concentrations (1.0 to 2.0×), which resulted in maximum dry mass of both species, and the most flowers on gomphrena. The growing medium EC associated with the best growth of gomphrena and stock was relatively high (2.3–6.5 dS m−1). This suggests that both gomphrena and stock can tolerate relatively high levels of root zone salinity. Dry mass of zinnia was maximal with a 0.5×fertilizer concentration and a growing medium EC of 1.0 dS m−1. Flower diameter of zinnia decreased with increasing nutrient concentrations. In general, the growing medium pH was similar with low concentrations (0.125, 0.25, and 0.5×), but decreased with a further increase in fertilizer concentration.

Temperature response of whole-plant CO2 exchange rates of four upland cotton cultivars differing in leaf shape and leaf pubescence

It seems likely that CO2 exchange rates of cotton (Gossypium hirsutum L.) cultivars differing in leaf shape and leaf pubescence would respond differently to heat stress due to differences in boundary layer thickness. The objective of this study was to determine if CO2 exchange rates in commercially available cotton cultivars differing in leaf shape and leaf pubescence respond differently to heat stress under well-watered conditions. CO2 exchange rates of whole plants of four cultivars (‘FiberMax 832’, ‘Stoneville 474’, ‘DeltaPine 5690’ and ‘Paymaster 1220’) were measured at eight different temperatures (6 to 34°C) in 4°C steps. Net photosynthesis (Pnet) of the plants decreased at temperatures over 20C, while dark respiration (Rdark) increased exponentially with increasing temperature. Cotton cultivar did not influence the response of Pnet or Rdark to high temperature stress. Carbon use efficiency (CUE) and net assimilation rate (NAR) were highest at lower temperatures and became negative at temperatures higher than 32°C. The initial temperature response of CUE and NAR differed significantly between ‘FiberMax 832’ and the other cultivars. Carbon use efficiency and NAR of ‘FiberMax 832’ decreased at a greater rate with increasing temperature. Due to the fact that the okra leaf of ‘FiberMax 832’ is deeply lobed and smooth, one would expect this cultivar to possess a thinner boundary layer and enhanced heat tolerance under well-watered conditions, which was not the case.

Physiological and molecular responses to drought in Petunia: the importance of stress severity

Plant responses to drought stress vary depending on the severity of stress and the stage of drought progression. To improve the understanding of such responses, the leaf physiology, abscisic acid (ABA) concentration, and expression of genes associated with ABA metabolism and signalling were investigated in Petunia × hybrida. Plants were exposed to different specific substrate water contents (θ = 0.10, 0.20, 0.30, or 0.40 m3·m–3) to induce varying levels of drought stress. Plant responses were investigated both during the drying period (θ decreased to the θ thresholds) and while those threshold θ were maintained. Stomatal conductance (gs) and net photosynthesis (A) decreased with decreasing midday leaf water potential (Ψleaf). Leaf ABA concentration increased with decreasing midday Ψleaf and was negatively correlated with gs (r = –0.92). Despite the increase in leaf ABA concentration under drought, no significant effects on the expression of ABA biosynthesis genes were observed. However, the ABA catabolism-related gene CYP707A2 was downregulated, primarily in plants under severe drought (θ = 0.10 m3∙m–3), suggesting a decrease in ABA catabolism under severe drought. Expression of phospholipase Dα (PLDα), involved in regulating stomatal responses to ABA, was enhanced under drought during the drying phase, but there was no relationship between PLDα expression and midday Ψleaf after the θ thresholds had been reached. The results show that drought response of plants depends on the severity of drought stress and the phase of drought progression.

Slowly developing drought stress increases photosynthetic acclimation of Catharanthus roseus.

Our understanding of plant responses to drought has improved over the decades. However, the importance of the rate of drought imposition on the response is still poorly understood. To test the importance of the rate at which drought stress develops, whole-plant photosynthesis (P(net) ), respiration (R(dark) ), daily carbon gain (DCG), daily evapotranspiration (DET) and water use efficiency (WUE) of vinca (Catharanthus roseus), subjected to different drought imposition rates, were investigated. We controlled the rate at which the substrate dried out with an automated irrigation system that allowed pot weight to decrease gradually throughout the drying period. Fast, intermediate and slow drying treatments reached their final pot weight [500 g, substrate water content (θ) ≈ 0.10 m³ m(-3) ] after 3.1, 6.6 and 10 days, respectively. Although all drying treatments decreased P(net) and R(dark) , slow drying reduced P(net) and R(dark) less than fast drying. At a θ < 0.10 m³ m(-3) , DCG and DET in the slow drying treatment were reduced by ≈50%, whereas DCG and DET in the fast drying treatment were reduced by 85 and 70% at a θ of 0.16 m(3) m(-3) . Plants exposed to slow drought imposition maintained a high WUE, even at θ < 0.10 m³ m(-3) . Overall, physiological responses to low θ were less severe in plants subjected to slow drying as compared with fast drying, even though the final θ was lower for plants exposed to slow drying. This suggests that the rate at which drought stress develops has important implications for the level of acclimation that occurs.

Application Technique and Irrigation Method Affect Imidacloprid Control of Silverleaf Whiteflies (Homoptera: Aleyrodidae) on Poinsettias

Abstract Subirrigation systems are increasingly used to water and fertilize greenhouse crops. They also appear to be well suited for the application of systemic pesticides. We conducted two studies to look at interactive effects of imidacloprid application technique and irrigation method on plant uptake of imidacloprid and whitefly control. Drench applications of imidacloprid resulted in much higher concentrations in the leaves than applications to the bottom of pots after 14 d. However, imidacloprid efficacy in subirrigated plants was better if the imidacloprid was applied to the bottom of the pot than when an equal amount was applied as a drench. In drip-irrigated plants, imidacloprid efficacy was greater after a drench than after an application to the bottom of the pots. A second study showed that drench applications to drip-irrigated plants result in high imidacloprid concentrations in the bottom of the canopy, whereas bottom applications to subirrigated plants result in a more even distribution of imidacloprid throughout the plant. Surprisingly, the high leaf imidacloprid concentrations in the bottom layer of drip-irrigated plants did not result in improved whitefly control. Imidacloprid efficacy was better in subirrigated, bottom-treated plants than in drip-irrigated, drenched plants. Overall, results from these studies indicate that imidacloprid is very effective when applied to the bottom of subirrigated pots.

Far-red light is needed for efficient photochemistry and photosynthesis

The efficiency of monochromatic light to drive photosynthesis drops rapidly at wavelengths longer than 685nm. The photosynthetic efficiency of these longer wavelengths can be improved by adding shorter wavelength light, a phenomenon known as the Emerson enhancement effect. The reverse effect, the enhancement of photosynthesis under shorter wavelength light by longer wavelengths, however, has not been well studied and is often thought to be insignificant. We quantified the effect of adding far-red light (peak at 735nm) to red/blue or warm-white light on the photosynthetic efficiency of lettuce (Lactuca sativa). Adding far-red light immediately increased quantum yield of photosystem II (ΦPSII) of lettuce by an average of 6.5 and 3.6% under red/blue and warm-white light, respectively. Similar or greater increases in ΦPSII were observed after 20min of exposure to far-red light. This longer-term effect of far-red light on ΦPSII was accompanied by a reduction in non-photochemical quenching of fluorescence (NPQ), indicating that far-red light reduced the dissipation of absorbed light as heat. The increase in ΦPSII and complementary decrease in NPQ is presumably due to preferential excitation of photosystem I (PSI) by far-red light, which leads to faster re-oxidization of the plastoquinone pool. This facilitates reopening of PSII reaction centers, enabling them to use absorbed photons more efficiently. The increase in ΦPSII by far-red light was associated with an increase in net photosynthesis (Pn). The stimulatory effect of far-red light increased asymptotically with increasing amounts of far-red. Overall, our results show that far-red light can increase the photosynthetic efficiency of shorter wavelength light that over-excites PSII.