EFFECTS OF NITROGEN SUPPLY ON THE ANATOMY AND CHEMICAL-COMPOSITION OFLEAVES OF 4 GRASS SPECIES BELONGING TO THE GENUS POA, AS DETERMINED BY IMAGE-PROCESSING ANALYSIS AND PYROLYSIS MASS-SPECTROMETRY

Previous experiments have shown that the anatomy and chemical composit ion of leaves of inherently fast- and slow-growing grass species, grow n at non-limiting nitrogen supply, differ systematically, The present experiment was carried out to investigate whether these differences pe rsist when the plants are grown at an intermediate or a very low nitro gen supply, To this end, the inherently fast-growing Poa annua L. and Poa trivialis L., and the inherently slow-growing Poa compressa L. and Poa pratensis (L.) Schreb, were grown hydroponically at three levels of nitrate supply: at optimum (RGR(max)) and at relative addition rate s of 100 and 50 mmol N (mol N)(-1) d(-1) (RAR(100) and RAR(50)), respe ctively. As expected, at the lowest N supply, the potentially fast-gro wing species grew at the same rate as the inherently slow-growing ones , Similarly, the differences in leaf area ratio (LAR, leaf area:total dry mass), specific leaf area (SLA, leaf area:Ieaf dry mass) and leaf mass ratio (LMR, leaf dry mass:total dry mass) disappeared, Under opti mal conditions, the fast-growing species differed from the slow-growin g ones in that they had a higher N concentration, There were no signif icant differences in C concentration, With decreasing N supply, the to tal N concentration decreased and the differences between the species disappeared, The total C concentration increased for the fast-growing species and decreased for the slow-growing ones, i.e. the small, but i nsignificant, difference in C concentration between the species at RGR (max) increased with decreasing N supply. The chemical composition of the leaves at low N supply, analysed in more detail by pyrolysis-mass spectrometry, showed an increase in the relative amounts of guaiacyl l ignin, cellulose and hemicellulose, whereas those of syringyl lignin a nd protein decreased. The anatomy and morphology of the leaves of the four grass species differing in RGR(max) were analysed by image-proces sing analysis, The proportion of the total volume occupied by mesophyl l plus intercellular spaces and epidermis did not correlate with the a mount of leaf mass per unit leaf area (specific leaf mass, SLM) at dif ferent N supply, The higher SLM at low N supply was caused partly by a high proportion of non-veinal sclerenchymatic cells per cross-section and partly by the smaller volume of epidermal cells. We conclude that the decrease in relative growth rate (and increase in SLM) at decreas ing N supply is partly due to chemical and anatomical changes, The dif ferences between the fast- and slow-growing grass species at an optimu m nutrient supply diminished when plants were growing at a limiting ni trogen supply.