Technical Abstract: Apart from an essential role in cell-wall structure, specific functions for boron (B) in plants are unclear; hence, early responses to B stress are debated, and adaptations to B stress are incompletely understood. We tested hypotheses that (1) photosynthesis is an early target of B deficiency, and (2) increased light availability can mitigate B deficiency. Geranium (Pelargonium x hortorum cv. Nittany Lion Red) plants were grown hydroponically from cuttings until rooted, and then exposed to normal or deficient levels of B (45 or 0 'M) and low or medium light levels (100 or 300 'mol m-2 s-1 PAR). Photosynthesis (net CO2 uptake, carboxylation and PSII efficiency) was monitored for 5 d, as were levels of B, chlorophyll, soluble sugars, total protein, and several photosynthetic and stress proteins (rubisco, rubisco activase, OEC23, Cu/Zn-SOD, Mn-SOD, eiF5-A). After 5 d, plant mass was greater in medium vs. low light, and was decreased by B deficiency only in leaves in medium light. B deficiency decreased [B] in all tissues, especially in new leaves and at medium light. Net photosynthesis and carboxylation efficiency decreased within 1 day of B deficiency in low-light plants, but not until 5 days in high-light plants. Total chlorophyll decreased, and Mn-SOD increased transiently, with B deficiency (both light levels), but no other effects of low B were observed. Together, these results indicate that photosynthesis is indeed an early and specific target of B deficiency, and higher light can ameliorate B deficiency, perhaps due in part to enhanced carbohydrate status.