Chlorophyll fluorescence parameter as a predictor of tomato growth and yield under CO2 enrichment in protective cultivation

Greenhouse crops production has been improved by CO2 enrichment commercially. The impact of a threshold level of elevated CO2 (e[CO2]) on the effective quantum yield of photosystem II (ФPSII, chlorophyll fluorescence parameter), which is linked to crop yield, crop growth and fruit quality, is still uncertain. Many previous studies cultivated C3 crops under 550–700 µmol·mol-1, not beyond this range. Hence, tomato plants were grown under ambient CO2 (a[CO2], 500 µmol·mol-1) and e[CO2] (700, 850, and 1000 µmol·mol-1): named as EC700, EC850, and EC1000, respectively, under uniform environmental conditions for two growing seasons to investigate the response of ФPSII along with four photosynthetically active radiation intensities and plant growth to continuous CO2 concentration [CO2]. Under four constant PAR intensities during autumn-winter 2020 and spring-summer 2021, the order of mean ФPSII values under e[CO2] were explored as EC700 > EC1000 > EC850 > a[CO2] and EC700 > EC850 > EC1000 > a[CO2], respectively. The optimum total fruit yield was stimulated under EC700 by 74.04% and 55.86% during autumn-winter 2020 and spring-summer 2021, respectively, as compared to a[CO2]. The results showed that water use efficiency related to yield (WUEy) were enhanced by reducing the stomatal conductance under e[CO2] as compared to a[CO2]. Optimum WUEy was enhanced by 55.91–210.87% under EC700 compared to a[CO2]. The principal component analysis showed strong correlation of ФPSII with plant height, yield and water use efficiency in autumn-winter 2020 and spring-summer 2021, LAI in spring-summer 2021, weak correlation with stem diamter in autumn-winter 2020 and spring-summer 2021, indicated the strong effect of ФPSII on plant growth and yield under e[CO2]. The EC1000 in autumn-winter 2020 and EC700 in spring-summer 2021 were found more influential by the analytic hierarchy process to get optimum crop production, no significant difference between the ranking scores of EC1000 and EC700 during autumn-winter 2020 was found. Thus, the overall optimum impact of e[CO2] on tomato growth was explored under EC700. The findings of this study are beneficial to greenhouses crops production commercially and could be used as references for validating the climate change impact modelling.