Phosphorus deficiency affects the I-step of chlorophyll a fluorescence induction curve of radish

Phosphorus (P) is an essential nutrient for plant development and its fertilisation efficiency is an important issue in agriculture. Difficulties in diagnosing P deficiency in plants have created a demand for a quick and noninvasive method/tool for assessing plant P status on-site. It has been well documented that chlorophyll fluorescence measurement is a very useful and sensitive tool for screening nutrients deficiency in plants. Although impact of P deficiency on photosynthesis process has been widely examined, its effect/s on photosynthetic electron transport between the two photosystems is still not clear. In this work, we studied the response of radish plants to P deficiency throughout stress conditions and recovery period. Leaf phosphorus content, growth rate, and photosynthetic activity and efficiency were monitored. Under P deficiency, a decrease in all of these physiological features was observed. Moreover, some changes in chloroplast structure were explored, including a reduction in chloroplast grana amount and loose arrangement. Additionally, the I-step disappeared from chlorophyll fluorescence induction curves. This can be linked to the inactivation of PSI and suppression of the cyclic phosphorylation. The observed decreases in quantum yield and the efficiency of electron transport chain in P-deficient plants could be due to downregulation mechanisms in the photosynthetic apparatus. However, both the inhibition of net photosynthetic rate, as well as the severe effect on the I-step were reversible when plants were resupplied with phosphorus. Loss of I-step can be used as a specific bioindicator for early detection of phosphorus deficiency in radish plants.