Evaluation of Photosystem II Activity in Two Almond Genotypes under Drought Stress by Exploring the OJIP Test

Introduction  Almond (Prunus amygdalus) is considered to be able to tolerate drought stress fairly well during all stages of growth. Water shortages are very frequent in many countries, and, together with the rising demand for the industry, the growth of the human population, climate change and specifically the trend towards irrigated agriculture, have led to widespread problems of water scarcity in most countries. Plant responses to water deprivation are usually monitored through selected morphological and physiological parameters which have been proven to be good indicators of drought in different studies. Chlorophyll a (Chl a) fluorescence, produced by the Chl a molecule after excitation by light, is a non-invasive and rapid biomarker for the assessment of stress (microbial and environmental) effects on PSII, as well as its structure and function. Fluorescence induction patterns and derived indices have been used as empirical diagnostic tools in stress physiology. The aim of this study was to investigate the effect of water stress on chlorophyll fluorescence parameters in two almond genotypes. It is known that the kinetics of fluorescence transients are polyphasic when plotted on a logarithmic time scale labeled as OJIP. This curve rises from an initial low-value F0 (minimal fluorescence) to FJ (fluorescence value at 2 ms) and FI (fluorescence value at about 20–30 ms) and a peak of fluorescence FP (maximal fluorescence or Fm). Materials and Methods  The Experiment was carried out under a completely randomized design with split arrangement having three replications. Chlorophyll and carotenoid contents were determined by the method of Lichtenthaler (1986). Fresh leaves (1 g) were triturated in 80% acetone. The absorbance of the extracts was measured at 645, 663, and 470 nm using a spectrophotometer. Chlorophyll fluorescence was measured 40 days after the start of drought treatment. Full expanded leaves were selected from each plant for measurements. They were measured with a portable photosynthetic efficiency analyzer model (Hansatech, United Kingdom). Calculations were made with computer-assisted analysis using the SPSS 25 software. Results and Discussion The results showed that the maximum total fresh and dry mass was recorded in the Mamaei cultivar. The drought stress caused a significant reduction in a, b, total chlorophyll and carotenoids in Rabie (R) as well as Mamaei (M) cultivars but in general, M plants had higher content of pigments in comparison with R plants under drought stress. In both of cultivars, the potential efficiency of PSII photochemistry (Fv/Fm) was reduced with an increasing drought intensity. The reduction of Fv/Fm was accompanied by a decline in Fv and Fm. The VJ, relative variable fluorescence at J step (2 ms), was increased with increase in drought levels. Plants response to drought depends on PSII ability to respond to this stress. It has been reported that water limitation reduces the quantum yield of PSII electron transport, which in turn decreases the amount of light energy reaching the reaction centers. PSII plays a pivotal role in mediating oxygen evolution activity. In our study, we observed an interaction between cultivar and drought treatment, particularly evident in parameters such as Fm, Fv, Fv/Fm, and PI. Additionally, our findings revealed a robust correlation between the Pi index and total chlorophyll content (0.647), as well as the fresh (0.685) and dry (0.695) weight of plants. Furthermore, our results indicate that drought stress significantly impairs the growth of cultivars grafted on GN15 rootstock. This may be the outcome of the inhibition of water shortage on the photosynthetic apparatus. The results discovered that carotenoids were higher in M cultivar than R cultivar, carotenoids protect the photosynthetic apparatus from photooxidative damage. Protection is afforded by quenching of the triplet state of chlorophyll, thereby preventing the formation of harmful oxidative species. We also found the performance index is the parameter that better reflects the responses of the studied cultivars to progressive drought stress. Conclusion We applied chlorophyll fluorescence as a biomarker to assess the growth response and PSII behavior and performance of two almond cultivars to different drought levels. In conclusion, Mamaei was less affected by drought stress in terms of total Chl, Fv/Fm, PI, and total fresh and dry weight followed by Rabie. Differential responses among cultivars under drought stress treatments were observed regarding their capacity to induce PSII activity. Parameters derived from the JIP test proved effective in characterizing the degree of response to drought stress, with PI serving as a particularly responsive multi-parametric expression.