Kinetics of nitrogen and phosphorus uptake by litchi under different temperatures and nitrogen forms

Objectives Temperature affects nutrient uptake by plant directly, and plant has a preference for nitrogen (N) form. The N and P uptake of litchi with varied temperatures and N forms was investigated, aiming to provide a basis for selecting suitable N source and P source with season (phenology) in litchi.

Methods A hydroponic experiment was conducted using litchi seedlings of cultivar ‘Heiye’as test materials. The designed growth temperature was 10℃, 15℃, 20℃, 25℃, 30℃ and 35℃, and the supplied N forms included nitrate (NN), 1/2 nitrate + 1/2 ammonium (1/2NN + 1/2AN) and ammonium (AN). After the seedlings were kept in hungry for 48 h, they were loaded into the treatment solution. At the 0, 1, 2, 3, 4, 6, 8, 10 and 12 h since the treatment, the N and P contents of the nutrient solution were measured, the uptake kinetic parameters of N and P nutrients were determined.

Results Temperature and N form had significant effects on uptake of N and P nutrition by litchi (P < 0.01). With the increase of temperature, the maximum absorption rate (I_max) of N by litchi under treatments of nitrate and 1/2NN + 1/2AN supply fluctuated, but that was not impacted significantly under treatment of AN. The I_max of N uptake under AN at 15℃ and 30℃ was significantly higher than those under the other two N nutrition treatments, and that under 1/2NN + 1/2AN condition was the highest at any other temperatures. When a sole N source was fed, both the affinity to N (A_m) and ion compensation concentrations (C_min) in litchi fluctuated with increasing temperature; while fed with 1/2NN and 1/2AN, the A_m was increased with temperature(10℃–30℃) but C_min decreased. The I_max of N uptake of litchi was related to temperature, however, the lowest I_max and C_min and the highest A_m were all obtained in litchi seedlings fed with sole nitrate. Under 1/2NN + 1/2AN nutrition, litchi plant showed significantly higher I_max with higher C_min and lower A_m for NH₄⁺ absorption, compared with NO₃^– uptake. The role of N sources on H₂PO₄^– absorption by litchi was related to temperature as well. There was no significant difference for the I_max of H₂PO₄^– uptake among N sources at various temperatures, except that at 15℃ and 25℃, the I_max ranked as 1/2NN + 1/2AN > AN > NN nutrition. The A_m and C_min of H₂PO₄^– uptake were similar under different N nutrient conditions, with the exception of significantly higher C_min or lower A_m under nitrate supply at 15℃.

Conclusions Litchi prefers nitrate, but its absorbing capability for ammonium is higher. Supplying sole ammonium at 15℃ and 30℃ or supplying nitrate and ammonium simultaneously at other tested temperatures are beneficial to N uptake by litchi. The simultaneous addition of both nitrate and ammonium shows promotion of H₂PO₄^– uptake by litchi only at 15℃ and 30℃, followed by sole ammonium nutrition. In order to improve the absorption of N and P nutrients, nitrate fertilizer should not be applied in one time heavily in litchi production.