Effects of Boron Deficiency on Geranium Grown under Different Nonphotoinhibitory Light Levels

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 (P) 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 xhortorum L.H. Bailey cv. Nittany Lion Red) plants were grown hydroponically and were then exposed to normal (45 mu m) or deficient (0 mu M) Bat two light levels [100 or 300 mu mol.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 P-n 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 P-n by higher light was confirmed in a different cultivar (Maverick White) grown at 100, 300, and 500 mu mol.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.