Apart from a role in cell wall structure, specific functions for boron (B) in plants are unclear; hence, responses and adaptations to B stress are incompletely understood. We tested hypotheses that net photosynthesis (Pn) decreases with B deficiency before visible foliar symptoms and that higher nonphotoinhibitory light levels enhance soluble carbohydrate status and therefore mitigate B deficiency. Geranium (Pelargonium ×hortorum L.H. Bailey cv. Nittany Lion Red) plants were grown hydroponically and were then exposed to normal (45 μm) or deficient (0 μm) B at two light levels [100 or 300 μmol·m−2·s−1 photosynthetically active radiation (PAR)]. Photosynthesis [net CO2 uptake, carboxylation, and photosystem II (PSII) efficiency] was monitored for 5 days, as were concentrations of B, chlorophyll, soluble sugars, total protein, and several photosynthetic and stress proteins [ribulose 1,5-bisphospate carboxylase oxygenase (rubisco), rubisco activase, oxygen-evolving complex-23 (OEC23), Cu/Zn-superoxide dismutase (SOD), Mn-SOD, and eukaryotic translation initiation factor 5A-2 (eIF5A-2)]. Biomass and sugar concentration were greater in high light, and mass was decreased by B deficiency only in leaves in high light. Boron deficiency decreased [B] in all tissues, especially in new leaves. Carboxylation efficiency and Pn decreased within 1 day of B deficiency in low light, but not until 5 days in high light. Chlorophyll concentration decreased, and Mn-SOD increased transiently, with B deficiency in both light levels, but no other effects of low B were observed. Protection of Pn by higher light was confirmed in a different cultivar (Maverick White) grown at 100, 300, and 500 μmol·m−2·s−1 PAR. Thus, in geranium, photosynthesis is affected by B deficiency before effects on leaf growth, and higher light can at least temporarily ameliorate B deficiency, perhaps partly due to enhanced carbohydrate status.
Sasmita Mishra, Scott Heckathorn, Jonathan Frantz, Futong Yu and John Gray
Jonathan Frantz, Dharmalingam Pitchay, David Tay, Jennifer Ehrenberger, John Gray, Scott Heckathorn and Scott Leisner
Zonal geranium (Pelargonium ×hortorum) and scented geranium (Pelargonium sp.) together are among the top-selling floriculture plants in the United States today, with several hundred cultivars and species available each year. With such diversity in appearances, growth habits, and developmental traits, we hypothesize a correspondingly wide range of nutritional uptake and partitioning characteristics. Mature leaves from 55 cultivars or breeding lines of zonal geranium and 60 species of Pelargonium sp. were sampled from paired plants twice throughout the year from the Ornamental Plant Germplasm Center. Their tissue was analyzed for essential macro- and micronutrients using ICP-OES. Generally, macronutrients varied by a factor of 3–5, whereas micronutrients varied by up to 100-fold. The micronutrient boron was a notable exception with variation across the samples of only about a factor of 5. With this information, attempts will be made to correlate tissue nutrient concentrations with genetic source (cultivars and breeding lines) and environmental conditions from the origin of the different Pelargonium species from South Africa. This work illustrates the challenges in associating specific sufficiency or deficiency values for nutrient concentrations in tissue of plants based on only a few cultivars, species, or locations.