Effects Of Far-red Light On Growth And Quality Physiology Of Lettuce In Plant Factory And Its Application Strategy

In nature,plants detect the presence of neighbors by sensing changes in the ratio of red（R）to far-red（FR）light in surroundings.Phytochrome could sense changes of R/FR and initiate a series of developmental responses known as shade avoidance syndrome（SAS）,including elongated growth of stems and petioles and elevated leaf angles,which adjust plants to optimize resource acquisition and affect their photomorphogenesis.Such morphological changes could promote light interception in the plant canopy and are particularly important for seedlings whose leaf area index is too small to adequately capture incident radiation.However,previous studies lack an overall understanding of how FR light affects plant morphology and physiological properties,and it still needs to be further explored that how to apply FR light reasonably in horticultural production.In this thesis,we investigated the effects of FR light on plant morphology,physiological characteristics and postharvest quality,explored the appropriate supply and dose of FR light,explained the morphological changes from the perspective of material distribution,and understood the physiological essence of FR light regulation of lettuce growth.The main results obtained are as follows:（1）Two different FR treatments were applied.Red and blue light(200μmol m^-2s^-1,16 h)was considered as Control.FR light was supplemented during the day(FR-Day,50μmol m^-2s^-1,16 h)or end of the day(FR-EOD,50μmol m^-2s^-1,1 h).The FR-Day and FR-EOD treatments increased dry weight by 39%and 25%,leaf area by 49%and 27%,and radiation use efficiency by 11%and 23%,respectively,compared to the control.Although temporarily adding FR light increased net leaf photosynthetic rates,FR light treatments resulted in a significantly lower chlorophyll and total nitrogen content,as well as lower leaf absorbance,which led to lower maximum leaf photosynthetic rate.（2）It was demonstrated that FR light was more efficient in promoting growth when applied alone at the end of day.Six FR intensities(0,15,30,50,75,100μmol m^-2s^-1)with the same duration were applied.It was found that FR light-induced effect was clearly intensity-dependent,while the required intensity to induce the maximal response was different for each trait.When the dose of FR light remained constant,no significant differences were observed between different FR light intensities.（3）The effect of FR-Day and FR-EOD on the postharvest lettuce quality was addressed.Shelf-life of minimally processed and packaged（polyethylene film）leaves was monitored during dark storage at 4or 16℃.FR light did not alter visual quality at harvest,but decreased the content of major health-promoting metabolites,especially under FR-Day.At 4℃,shelf-life was not affected by the cultivation light regime.At 16℃,instead,FR light was associated with increased postharvest deterioration rate and reduced overall visual quality throughout storage.The reduced shelf-life following FR light at preharvest coincided with elevated O₂^·-content along with decreased activity of enzymatic（superoxide dismutase）and reduced levels of non-enzymatic（carotenoids,total phenolics,and flavonoids）antioxidants.（4）Morphological changes and matter partitioning of lettuce plants from the seedling stage to the maturity stage were monitored.After far-red light irradiation in the first time,the rate of carbohydrate consumption was accelerated in leaves and petioles of lettuce seedlings.In the early growth period,when resources were limited,FR light increased the dry mass partitioning to the petiole and reduced the dry mass partitioning to the leaves,changed plant morphology without affecting total biomass.In the later growth period,plant leaf area and biomass increased significantly under FR light treatment.To meet the rapid growth and consumption requirements,FR light increased the accumulation of soluble sugars in the leaves and then carbohydrates were present in a form that facilitates transport and metabolism.In summary,FR light optimized the plant morphology by adjusting matter partitioning during seedling stage,and effectively improved the yield and radiation use efficiency of lettuce.FR-EOD can be chosen,the duration and intensity can be selected flexibly,and the postharvest storage temperature can be optimized in order to maximize benefits from both yield and quality together.