Effect of Humic Acid On Some Morpho-Physiological and Bio-Chemical Attributes of Kinnow Mandarin (Citrus reticulata Blanco)

Pakistan is one of the top thirteen citrus producing countries of the world. However, average yield of Kinnow in Pakistan is about 9.0 tons ha^-1 which is 3 fold lower than other developed citrus growing countries. Malnutrition practices coupled with poor soil organic matter (0.5-0.7%) and improper plant protection measures are the major reasons for the low yield. Use of humic acid to improve nutrient efficiency of applied fertilizers is an activity still in its infancy in Pakistan. Humic acid is one of the major components of humic substances which enhance organic matter contents of the soil. Liquid humic acid having pH 5.9 was applied @ 40, 60 and 80 ml tree^-1 to 12 year old Kinnow mandarin plants in single, two and three equal splits as well. The humic acid significantly affected morpho-physiological and bio-chemical attributes of Kinnow mandarin plants compared with untreated plants. Maximum photosynthetic rate (42.38 µmol CO₂ m^-2 S^-1), stomatal conductance (52.09 mmol m^-2 S^-1) and total chlorophyll contents (3.092 mg g^-1 of fresh matter) were recorded in plants where  a total of 80 ml humic acid per plant was applied  in three equal splits in February (before flowering), April (after fruit setting) and August. There was also minimum fruit drop (50.43%) in these plants as compared to control (84.06%). Maximum sugar contents (16.21 mg g^-1 of fresh matter) with higher juice percentage were also noted in fruits of these plants.  Humic acids also affected the biochemical attribute as it decreased reducing sugar and total titrable acidity of fruit juice. Moreover, application of humic acid in three splits significantly increased fruit size, fruit weight, while decreased leaf drop percentage. Similarly all the humic acid treatments surprisingly enhanced photosynthetic rate, transpiration rate and stomatal conductance along with chlorophyll contents. So, it was concluded that humic acid application in three equal splits at different growth stages play a vital role in improving the growth and yield of citrus and also improve fruit taste and quality.