Chloroplast avoidance movement is not functional in plants grown under strong sunlight

Chloroplast movement in nine climbing plant species was investigated. It is thought that chloroplasts generally escape from strong light to avoid photodamage but accumulate towards weak light to perform photosynthesis effectively. Unexpectedly, however, the leaves of climbing plants grown under strong sunlight showed very low or no chloroplast photorelocation responses to either weak or strong blue light when detected by red light transmittance through leaves. Direct observations of Cayratia japonica leaves, for example, revealed that the average number of chloroplasts in upper periclinal walls of palisade tissue cells was only 1.2 after weak blue-light irradiation and almost all of the chloroplasts remained at the anticlinal wall, the state of chloroplast avoidance response. The leaves grown under strong light have thin and columnar palisade tissue cells comparing with the leaves grown under low light. Depending on our analyses and our schematic model, the thinner cells in a unit leaf area have a wider total plasma membrane area, such that more chloroplasts can exist on the plasma membrane in the thinner cells than in the thicker cells in a unit leaf-area basis. The same strategy might be used in other plant leaves grown under direct sunlight. Chloroplast movement is a crucial phenomenon for plant survival under fluctuating light conditions. However, leaves of climbing plants that spread their leaves under direct sunlight rarely show chloroplast avoidance response. We analysed climbing plants' leaves and found their sophisticated strategy for surviving under and usage of strong sunlight. Under strong light, palisade cells become narrower and more columnar than leaves grown under low light condition. In a narrow cell, almost all chloroplasts in the cell stay at the anticlinal wall in any light condition, as is the position of chloroplast avoidance response. Accordingly, light can penetrate deep into spongy cells, so that photosynthetic activity increases. This chloroplast arrangement protects most chloroplasts from the exposure to direct strong light. Moreover, as our schematic model indicates, narrower cells have the wider plasma membrane, so that the more chloroplasts are possible to stay in a unit leaf area and higher efficiency of CO2 exchange is performed.