Of the mineral nutrients requiring management, nitrogen (N) is generally required in greatest amounts by plants. N is a building block of 22 of the 23 amino acids, and thus comprises parts of almost all proteins. It also comprises parts of each of the nucleic acids. Its availability influences both cell division and elongation. N is also a building block of the chlorophyll molecule. Its availability is well known to have a direct influence on photosynthesis. N is a component of most enzymes. It comprises parts of the enzyme, ribulose-diphosphate-carboxylase (RUDP), which largely influences the rate of carbon-dioxide fixation by the leaf. It is not surprising that N availability markedly influences plant growth and development. Due to its chemistry in the soil, it is particularly important that N be applied in direct accordance with demand, which in turn bears a direct relation with growth rate. If not available in sufficient amounts during the period of active growth, growth is immediately slowed. Young trees deprived in N specifically, take longer to become productive units. In mature fruit trees, N-deprivation reduces the tree's ability to produce new productive wood during the yearly vegetative-generative growth cycle.
N is highly mobile in the plant body, moving from one part to another in amide or amino acid form. Deciduous tree leaves return about 50% of their N to the tree body prior to them dropping. During flowering, when the demand for N is high, N is known to move preferentially to the developing flowers from other parts of the tree.
N-deficiency is first indicated by a slight reduction in green colour intensity of the leaves. If N becomes increasingly deficient, intensity decreases. If extreme, the entire leaf, including the veins, becomes yellow. Yellowing of the old leaves precedes that of the younger leaves due to N-extration by the old leaves. In many species reduced green colouration is accompanied by orange or reddish pigmentation of the entire leaf or parts thereof. This typically occurs in peach, apple, pear and cherry. On occasions the stems also become red. In lemon, N-deficiency may be noted by the sudden incidence of vein yellowing in mature leaves. N-deficiency may also effect a reduction in leaf size.
Images from the book Symptons of Nutrient Imbalances in fruit trees, Bruno Razeto.
'Granny Smith' apple leaves: Left- leaf from a N-deficient tree. Right- leaf from a non-N-deficient tree.
'Red Delicious' apple trees: Front- tree with severe N-deficiency. Back- trees having received soil-applied N fertilizer.
'Granny Smith' apples: Left- harvested from a healthy tree. Right- harvested from a N-deficient tree.
Peach leaves: Left- N-deficient leaf. Right- normal leaf concerning N status.
Left- N-fertilized peach tree in a highly N-deficient orchard.
Symptoms of severe N-deficiency in peach shoots developing in spring.
Grape leaves: Right- leaf with symptoms of N-deficiency.
Grape shoots: Left- normal shoot concerning N status. Right- N-deficient shoot.
Lemon leaves: Left- N-deficient leaf. Right- normal leaf concerning N-status.
Severe N-deficiency in lemon.
Avocado leaves: Left- N-deficient leaf. Center- leaf low in N. Right- normal leaf concerning N status.
Avocado nursery: Front- plants growing in a substrate devoid of N. Back- plants having been supplied with N fertilizer.
Mango tree deficient in N.
Symptoms of N-deficiency in coffee ('Arabica') (Photo courtesy INPOFOS).
Sections of a banana leaf: Left- leaf section from a N-deficient plant. Right- leaf section from a normal plant concerning N-status.
Front- strawberry plant growing in a N-deficient substrate. Back- strawberry plants having been fertilized with N.