Physiological Investigation of Drought Stress Tolerance of ‘W. Murcott’ Mandarin Grafted onto ‘Carrizo’, ‘Sour Orange’, and ‘Volkameriana’ Rootstocks

This study investigated the effects of rootstock selection and deficit irrigation on the growth, physiological, and photosynthetic performance of young ‘W. Murcott’ mandarin trees. A two-way ANOVA was conducted to evaluate the impact of rootstocks (sour orange, Carrizo citrange, and Volkameriana) and deficit irrigation treatments (40%, 50%, and 70% of field capacity as control) on various plant parameters. Results revealed that rootstock diameter, scion diameter, leaf chlorophyll concentration (Chl), fresh weight, total dry weight, and photosynthetic rate (P<i>_N</i>) were significantly influenced by rootstocks and/or deficit irrigation treatments. Deficit irrigation significantly reduced plant height, fresh and dry weights, rootstock diameter, Chl, <i>Fv’</i>/<i>Fm’</i> (chlorophyll fluorescence), and P<i>_N</i>, while scion diameter, stomatal conductance, and water-use efficiency (WUE) remained unaffected. Among rootstocks, sour orange exhibited the highest Chl and <i>Fv’</i>/<i>Fm’</i> values under water stress, indicating greater drought tolerance, despite showing lower growth compared to Carrizo and Volkameriana. Conversely, Carrizo and Volkameriana rootstocks demonstrated higher fresh and dry weights under optimal irrigation but were more sensitive to water stress. Photosynthetic rate was highest in sour orange-grafted plants under deficit irrigation, while transpiration rates were highest in control plants. These findings suggest that sour orange rootstock may enhance drought resilience by maintaining photosynthetic efficiency and chlorophyll integrity, albeit at the cost of reduced vegetative growth. At the end of this study, it was determined that W. Murcott seedlings grafted onto sour orange rootstock were more tolerant compared to the other two rootstocks.