Calcium Physiology and Its Application Management in Citrus Orchards

It is the objective of the present study to explore calcium fertilization management in citrus orchards in an attempt to improve fruit quality via the more profound understanding thus gained about plant calcium requirements, proper amount and timing of fertilization, and processes of calcium absorption and transfer to leaves and fruits in the trees. It is well established that calcium concentration in citrus fruits is only second to potassium and nitrogen but that it is the first in terms of quantity in the biomass of citrus trees (compared to potassium and nitrogen). The amount of calcium harvested by fresh citrus fruit is about 550 to 600 gr per ton. The post-setting developing fruits have an immediate and stable need for calcium, which is supplied by the xylem vessels and strongly depends on transpiration from the surface of xylem tissues and organs. It, therefore, follows that unfavorable climatic conditions and management practices leading to such adverse situations as drought stress, flooding stress, heat stress, lack of sunny hours, high relative humidity, or high nitrogen and potassium fertilizer application in the first stage of fruit growth (I) only intensify calcium deficiency in the developing tissues. Study has shown that the most appropriate timing for fertilizer application to increase calcium content in citrus fruits is the first and early second stages of fruit growth, beyond which time calcium efficiency and transfer to fruits undergo drastic drops. On the other hand, calcium enjoys the greatest impacts not only on enhancing fruit tissue firmness but also on preventing their softening, reduced fruit weight, physiological disorders, and fruit rotting percentage during storage. Thus, to achieve these goals of calcium fertilization, previous research in different regions of the world and in Iran recommend that the first stage of fruit growth (from post-setting to June drop) and early to mid-second stage of fruit growth (II) may be the best timing for calcium fertilizer application (foliar application or fertigation) to increase calcium content in citrus fruits. It is further recommended that Ca application during late second stage of fruit growth and pre-harvest period or post-harvest fruit immersion in calcium solutions must be avoided.