Developing foliar growth of ivy geraniums (Pelargonium peltatum) bleaches white after exposure to temperatures greater than 30°C. This study investigated chloroplast development in ivy geraniums under heat stress comparing a heat sensitive cultivar, Temprano™ Lavender, and a heat tolerant cultivar, Contessa™ Red. Using transmission electron microscopy and spectrophotometry, foliar bleaching under heat stress was found to be due to an absence of developed chloroplasts within the bleached new growth accompanied by lower chlorophyll content. To determine whether heat stress related foliar bleaching could be prevented, cytokinin and gibberellins were applied in combination, at different rates before, during or after a heat stress event. Applying 50 to 100 ppm gibberellins before heat stress reduced bleaching in new growth. Gibberellins applied at 50 ppm within a week of a heat stress event decreased bleaching. Net photosynthesis and chlorophyll fluorescence was greater in non-heat stressed plants than heat stressed plants.

The development of whitening of the youngest leaves of actively growing ivy geranium (Pelargonium peltatum L.) has been observed as the season changes from late spring to summer. This study was conducted to determine the specific environmental causes of whitening, if micronutrients deficiencies cause similar whitening, whether low night temperatures can reverse whitening, and whether salicylic acid affects growth and whitening in ivy geraniums. Two cultivars, ‘Beach’ and ‘Butterfly’, with different susceptibility to whitening were chosen for this study. Elevated air temperature, but not elevated root-zone temperature, was found to be the environmental cause of whitening in ivy geranium. Elevated air temperatures severely reduced plant growth, leaf area, fresh weight, and dry weight in both cultivars. Elevated air temperature reduced photosynthetic pigments and their ratios in ivy geranium. Carotenoids and pheophytins decreased in ‘Butterfly’ at elevated air temperature. Foliar total Fe levels indicated no inhibition of Fe-uptake at elevated temperatures. Applications of Fe-chelate at elevated temperatures helped chlorophyll synthesis in ivy geraniums. Deficiency treatments of all micronutrients, Fe, Mn, Zn, S or Mg did not result in whitening in either cultivar of ivy geraniums. Salicylic acid did not affect whitening of ivy geraniums. It did not affect growth, leaf area, fresh or dry (stem, leaf or total) weight, fresh: dry weight ratio, leaf area ratio, specific leaf area or foliar nutrient (Fe, Mn, Zn, Mg and S) content in either cultivar. Cultivars varied in their response to low night temperature. ‘Beach’ reduced its plant growth and fresh to dry weight ratio whereas ‘Butterfly’ did not. Fe-chelate application did not reduce growth, leaf area, fresh weight, dry weight or fresh:dry weight ratio of either cultivar. Although Fe-application did not reduce whitening in ivy geraniums, it helped to preserve chlorophyll, particularly chlorophyll b as indicated by Chl a:b ratio in ‘Beach’. Whitening in ivy geranium is a heat stress response initially exhibited by young, developing leaves and is caused by elevated air temperatures. Whitening is the result of impaired photosynthetic pigments synthesis and/or degradation.

Pelargonium peltatum, ivy geranium, experiences foliar bleaching at temperatures exceeding 30° C. Contessa™ Red (heat tolerant) and Temprano™ Lavender (heat susceptible) were compared. Established plants underwent temperature treatments of 15/20° C or 25/30° C night/day with moisture treatments of 80% or 30% substrate volumetric water content (VWC). Photosynthesis, leaf greenness and growth data were collected at days 0, 7 and 11. No differences in photosynthetic rate nor a decrease in greenness in developed leaves occured in either cultivar due to high temperature or drought. Contessa™ Red had overall greater growth and leaf greenness than Temprano™ Lavender. Greenness and growth increased similarly for both cultivars at 80% VWC. Any decrease in foliar bleaching due to drought was likely due to a decrease in growth. A second study using Temprano™ Lavender indicated foliar bleaching occurs in newly emerging, developing leaves.

Master of Science / Department of Horticulture, Forestry, and Recreation Resources / Kimberly A. Williams / Ivy geranium (Pelargonium peltatum L’Herr ex. Ait.) and tomato (Solanum lycopersicum L.) are two economically important greenhouse crops known to be affected by non-pathogenic lesions on leaf tissues. These physiological disorders are often termed edema (oedema) or intumescences, but several other names have been used including enations, non-pathogenic galls or tumors, and neoplasms. These lesions, characterized by small protrusions on leaf tissues that become necrotic over time, are considered to be the result of environmental factors. Our research focused on determining what environmental factors affect these disorders on ivy geranium and tomato. The physiological disorder of ivy geranium is thought to be the result of water uptake exceeding transpiration, resulting in a build-up of water and solutes in leaf tissue that results in the blister-like protrusions in the epidermal layer. Current convention suggests that susceptible plants be grown in an environment that promotes transpiration with low humidity and infrequent watering. Over four experiments, we evaluated the effects of four root medium water contents, five rates of supplemental calcium application and two vapor pressure deficit (VPD) environments on three cultivars of ivy geranium. Our results indicate that high root medium water contents do not increase the incidence of edema on ivy geranium, but increase overall plant growth. Supplemental calcium had no affect on edema or growth, while our VPD results were inconclusive. These results suggest that current convention regarding cultural practices that abate the disorder be revisited. In tomato var. hirsutum ‘Maxifort’, the physiological disorder is characterized by individual epidermal cells swelling, which is unlike the disorder in ivy geranium where solutes build up across a group of epidermal cells. The environmental factors we focused on were two root medium water contents and supplemental UVB light. Our results suggest that root medium water content may play a role in development of tomato intumescences based on visual observation, and UVB light supplementation helps prevent the lesions from forming.

Ivy geranium (Pelargonium peltatum) is a heat susceptible species with its heat tolerance varying among varieties. Reactive oxygen species (ROS) and in-vivo defense systems are related to plant heat damage and heat tolerance. Application of chelated-iron has also been reported to enhance ivy geranium heat tolerance; however, the correlation of ROS, relative enzyme stability, and iron content to differences in heat tolerance in ivy geraniums is unknown. Here we show that the H2O2 content and ROS scavenging enzyme stability in ivy geranium varies with varieties and active iron is not related to heat tolerance in ivy geranium. H2O2 content in mature leaves in both heat tolerant 'Beach' and sensitive 'Butterfly' increased under heat stress, but 'Butterfly' had a relatively greater increase of this toxic compound. Catalase (CAT) activities in young leaves in both varieties decreased. In young leaves of 'Butterfly', CAT activities decreased to a level significantly lower than that in old leaves while this did not occur in 'Beach'. Superoxide dismutase (SOD) activities in 'Butterfly' young leaves were also decreased. All these phenomenon coincided with the heat tolerance differences of the two varieties. Active iron content only changed with leaf age and did not vary between varieties or treatments. Our results demonstrated that ROS scavenging ability and relative enzyme stability may indicate heat tolerance in ivy geranium and that iron deficiency was not the cause of heat damage. Cell Membrane Themostability (CMT) and Triphenyl Tetrazolium Chloride (TTC) cell viability tests are alternative, laboratory-based screening methods for screening for heat-tolerance. Both CMT and TTC tests can represent the variance in heat tolerance observed in ivy geraniums. The results of both CMT and TTC tests correlated well with plant width and growth indexes although their correlations to plant chlorosis were low. Unlike TTC, CMT strongly correlated with plant width. CMT and TTC tests are complementary laboratory-based methods that can be applied to cultivar screening for heat tolerance in ivy geraniums.