Abstract:

Four genotypes of capsicum (Capsicum annuum L.) with distinct low light tolerance were used to study the response of anatomical structure, ultrastructure of chloroplast，stomatal characteristics and photosynthetic characteristics of leaves to low light (75~100 μmol·m^-2·s^-1) in a phytotron. The results revealed that leaves treated with low-light became thinner with an increase of palisade/spongy tissue ratio, and two genotypes including hot pepper ‘Fudijian’and sweet pepper ‘Shanghai Yuanjiao’had higher ratio than the other two. Under low light, the number of chloroplast decreased, but the size of chloroplasts and starch grains, the number of grana, grana lamella, starch grain and grana thickness of chloroplast increased. Comparing with sweet pepper, hot pepper had more chloroplasts and granas. In sweet pepper, the stomatal density in lower epidermal descended during leaf unfolding, but no more changes of stomatal index and number of stomata per leaf occurred after 14 day of leaf unfolding. Low light treatment resulted in decreases of stomatal density, stomatal index and number of stomata per leaf, and increases of the stomatal size, the vertical and horizontal axes of stomata in ‘Shanghai Yuanjiao’. Simultaneously, the longer horizontal axes of stomata were also observed in another genotype ‘20078’. The epidermal cells of sweet pepper grown under low light were large and distorted. Meanwhile the stomata and lower epidermal cell were equal height or the stomata slightly protruded out of the epidermal wall. The cycle wrinkle could be obviously observed on cuticle of guard cell in lower epidermis of leaves under low light. Photosynthetic parameters including light saturated rate of the CO₂ assimilation (A_sat), dark respiration rate (R_d), night respiration rate (Rn), light respiration rate (R_p), light compensation point (LCP), light saturation point (LSP), CO₂ compensation point (Г), CO₂ saturation point (CSP), carboxylation efficiency (CE) and maximum RuBP regeneration rate decreased, while apparent quantum yield (Фi) increased under low light. The A_sat decreasing, LCP, LSP, R_d, R_n and R_p were lower in hot pepper than those in sweet pepper. The start time of photosynthesis (namely, the net photosynthetic rate i.e. Pn from zero to maximum) in low-light leaves was relatively prolonged, especially in low light-sensitive than in low light-tolerant genotypes. Hence, some adaptive response of photosynthetic characteristic to low light have happened in capsicum leaves.

Key words: capsicum, low light, anatomical structure, ultrastructure of chloroplast, stomata, photosynthetic characteristics