Physiological disorders in citrus

 Physiological disorders in citrus:

Article by - Rupa kumari

Introduction:

Citrus belongs to the genus Citrus of the family Rutaceae, and it is rich source of neutraceuticals such as ascorbic acid, carotenoids, limonoids, flavanones and vitamin-B complex (Ladaniya, 2008a). The Indian production of Citrus with an area of 1003 thousand hectares (15.41% of total fruit area) and production of 12546 thousand MT (12.88% of total fruit production) with a productivity of 12.50 t/ha. The states Gujarat, Madhya Pradesh and Andhra  Pradesh  are  the  leading  producers  of  lime/lemon,  mandarin  and  sweet  orange  respectively (Anonymous, 2018). Physiological disorders are the result of dysfunction or malfunction of physiological processes of the fruit tissues due to abiotic stresses (temperature, relative humidity, moisture/water stress, chemicals, and nutrient excesses and deficiencies) and  are  therefore distinct  from disorders caused by biotic factors  such  as disease-causing pathogens and  insect-pests. The  physiological  disorders caused  by preharvest factors  are granulation, fruit cracking, puffiness, superficial rind pitting, creasing and sunburn, post-harvest factors cause the chilling injury, oleocellosis, rind staining, peteca, stylar end breakdown and stem end rind breakdown .

Citrus decline:

Citrus decline is the integrated expression of many disorders and it is characterised by retarded growth and unproductive  plants,  in  severe  cases  plants  may  die.  The  causes  of  this  disorder  are  improper  nutrition, excessive irrigation, pest incidence ( leaf miner, fruit sucking moth, lemon butterfly, aphid, psylla and mites), disease incidence (fungal and viral diseases), incidence of citrus nematode and burrowing nematode. Proper fertilisation, drainage, and use of resistant rootstocks and disease-free certified bud woods helps in reducing citrus decline .

Sunscald:

The sunburn in citrus fruits is caused by excess light and solar radiation. The severity of sunburn is more in heat or draught stressed plants. Sunburn disrupts the photosynthetic systems, ruptures the oil glands, it will lead to subsequent water loss and reduction in growth and yield of citrus fruits. The fruits affected by sunburn have the increased fruit flesh firmness, dry matter content, reduced relative water content and reduced titratable acidity. Anti-transparent, reflectants, kaolin spray, paper bags and shade nets are used to minimise water loss and appearance of sunscald in citrus.

. Creasing:

The creasing disorder of citrus fruit cause cracks in cell layers of albedo tissue of peel, depressions on rind and healthy areas of fruit becomes bulky with loss of turgor pressure, cell wall  collapse,  pectin degradation and promotion  of  pectin methyl  esterase activity  of albedo  tissue.  Cultural practices, climatic  and endogenous factors are the cause of  creasing disorder. Application of GA3, synthetic auxins or mixture of both will reduce the intensity of physiological disorders like creasing, splitting, puffing and peel pitting. The application of GA3 10-20 mg/l at fruit size of 30-40 mm or just prior to colour breaking stage of fruit will reduce the incidence of creasing disorder.  Gibberellins make long  term formative effects  in albedo tissue,  delay the  senescence of albedo tissue and increase the compactness of albedo tissues.

Fruit cracking:

Citrus fruit cracking is a serious pre-harvest physiological disorder and the range of incidence is 10–35%, which severely affects yield and fruit quality. There are three citrus fruit-cracking patterns viz. flavedo-splitting, inner-cracking  and  albedo-splitting  (fruit  creasing  or  pitting).  The  factors  like  cultivar  characteristics,  weather conditions,  rootstock,  peel  thickness,  peel  hardness  and  growth  regulators  cause  the  fruit  cracking.  The application of 2,4-D and NAA will reduce the incidence of fruit cracking .

 

Granulation:

Granulation  is also  known as  vesicle drying,  kaosan  and  crystallisation.  The juice  sacs of  granulated  fruits become  hard, dry,  get enlarged,  greyish in  colour with  little juice  content.  The  frequent  irrigation,  higher application  of  nitrogenous  fertilizers,  luxuriant  growth  following  heavy  pruning  or  fertilization,  delayed harvesting, climatic factors, rootstock, fruit size, tree age and enzymes like peroxidase, superoxide dismutase are associated with this disorder.

Chilling injury:

When tropical and subtropical fruits held below 10–15oC for certain period of time will cause the chilling injury. Limes, lemons and grapefruits are more susceptible to chilling injury than mandarins and oranges. The common symptom of chilling injury in citrus is the peel pitting and peel becomes brownish colour.