“To manipulate growth, hemp is usually cultivated under prolonged photoperiods or continuous light that could cause photo-oxidative damage and adjustments of photosynthetic reactions.” In a recent study on the effects of photoperiod, researchers explain both the positive and negative effects. Besides ﬂowering, photoperiod also inﬂuences daily photosynthesis, growth, and starch metabolism. The quality and quantity of light affect the excitation of PSI and PSII, key components in photosynthetic processes. The study showed that photosynthetic capacity is increased by longer photoperiods which in turn induces growth.
The negative effects
“However, numerous negative and adverse effects can also be induced in conditions of continuous light or prolonged photoperiods, such as chlorosis, decrease in plant growth and its productivity and yield, high starch production, and stress induction. Plants grown under long days contain chloroplasts with smaller grana stacks and increased chlorophyll content. Photoperiod can also inﬂuence plants’ resistance to drought and salt stress and, according to recent evidence, the length of light period can have an impact on the response to pathogen infection.”
Finding the best condition
To determine the extent of changes in photosynthetic response caused by prolonged light exposure, the researchers employed chlorophyll aﬂuorescence measurements accompanied with level of lipid peroxidation (TBARS) and FT-IR spectroscopy on two Cannabis cultivars. Plants were grown under white (W) and purple (P) light at different photoperiods (16/8, 20/4, and 24/0). The results showed diverse photosynthetic reactions induced by the different light type and by the duration of light exposure in two cultivars.
According to the study, the most beneﬁcial condition was the 16/8 photoperiod, regardless of the light type, since it brought the most efﬁcient physiological response and the lowest TBARS contents suggesting the lowest level of thylakoid membrane damage. These ﬁndings indicate that different efﬁcient adaptation strategies were employed based on the type of light and the duration of photoperiod. “White light, at both photoperiods, caused higher dissipation of excess light, causing reduced pressure on PSI. Efﬁcient dissipation of excess energy and formation of cyclic electron transport around PSI suggests that P20/4 initiated an efﬁcient repair system. The P24/0 maintained functional electron transport between two photosystems suggesting a positive effect on the photosynthetic reaction despite the damage to thylakoid membranes.”
To read the complete study, go to www.researchgate.net