GROWTH OF LYCHEE (LITCHI-CHINENSIS SONN) IN SAND CULTURE UNDER VARIABLE NITROGEN SUPPLY

Lychee seedlings were grown in sand culture to study the effects of N supply on growth, and to determine whether the leaf N concentration required for maximum shoot growth was the same as that for yield. Seedlings of cv. Tai So were grown in sand with 0 (control), 0.5, 2.5, 5 or 10 mM N treatments as KNO3 and harvested after 46 weeks. In a second experiment, seedlings of cv. Wai Chee were grown in a mixture of sand, peat and soil with 0 (control) or 10 mM N as KNO3 (nitrogen) treatments and harvested after 5, 12, 16, 25 or 38 weeks. The inclusion of peat and soil delayed the onset of N deficiency. In the N rate experiment, shoot growth in the control stopped within a month of withholding N from the sand (average of 0.3 +/- 0.1 flushes per plant over 46 weeks) and was markedly reduced in the 0.5 mM N treatment (1.0 +/- 0.3 hushes per plant) compared with 2.4 +/- 0.2, 3.2 +/- 0.5 and 5.0 +/- 0.4 flushes per plant in the 2.5, 5 and 10 mM N treatments, respectively. Leaf, stem and total dry-matter production were greatest at 5 mM and 10 mM N, while maximum root dry weight occurred at 2.5 and 5 mM N. Shoot growth (x 70 fold) was more sensitive than root growth (x 2 fold) to increases in N supply and was virtually eliminated without N, whereas root growth was about half of values recorded at 5 mM N. Increasing N supply increased N concentration in young leaves (0.8 to 1.7%), index leaves (youngest fully expanded leaf plus 4 older leaves) (0.7 to 1.4%) and old leaves (0.6 to 1.2%), while the stems.(0.2 to 0.4%) and roots (0.2 to 0.5%) were less responsive. Stem and leaf dry-matter production increased with increasing N in index leaves from 0.7 to 1.4%, whereas 95% of maximum root dry weight occurred over the lower range of 1.0 to 1.2%. These compare with a leaf N concentration of about 1.5% for maximum yields in the field. In the multiple harvest experiment with a mixture of sand, peat and soil, leaf and stem dry weight in the controls were about 50% of that in the plants receiving N after 38 weeks, while root weight was about 75% of those plants receiving N. At the end of the experiment, the concentration of N was 0.8 +/- 0.1% in the leaves and 0.3 +/- 0.1% in the rest of the plant. Average N concentrations in the plants receiving N were 1.5 +/- 0.1% in the young leaves, 1.7 +/- 0.1% in the index leaves, 1.9 +/- 0.1% in the old leaves, 0.8 +/- 0.1% in the stems and 0.9 +/- 0.1% in the roots. There was a reduction in the increment of plant dry weight in the controls compared with plants receiving N after week 16, when the concentration of N in the index leaves had fallen below 1.2%. Differences in shoot growth in both experiments above and below 1.2% N in index leaves were not reflected in large differences in leaf colour. These results suggest that productivity will be affected long before leaves yellow.