Leaf Growth and Photosynthetic Response to Nitrogen and Phosphorus in Seedling Trees of Gmelina arborea

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Plants were grown in sand cultures under controlled conditions using nutrient solutions which were adjusted with respect to either nitrogen (N) or phosphorus (P) concentrations (two separate experiments) with other elements non-limiting. Leaf expansion at successive nodes was measured non-destructively and analysed according to the Richard's function, revised to provide geometrically meaningful parameters that could be linked with the dynamics of leaf growth. The reparameterised form describes sigmoidal increase in area (W) over time (t) as follows: W(t)= Wx(1+der(1+d)(to-t))-(1/d) where W, (cm2) is the asymptotic value of W for large t, to (days) is the time at which W(t) undergoes its point of inflexion, r (days-1) is the relative rate of lamina expansion of W(t) at to, and d determines the shape of the curve W versus t, so that the inflexion point occurs further up the curve with larger d. A comparison between N and P effects on leaf expansion parameters in plants showing similar net reduction in leaf mass and stem volume showed that Wx was more influenced by low N, whereas r was more sensitive to low P. A given reduction in canopy area due to nutrient deficiency was thus mainly attributable to smaller leaves on low N, but to fewer leaves on low P. Leaf photosynthesis (light-saturated in air) showed a curvilinear increase with tissue N and P, saturating above c. 2.5 mmol N (g dm)-1 and 100 μmol P (g dm)-1 in N and P experiments respectively. Leaf growth parameters Wx and r showed a similar dose response and highlight a potential use of those indices for analysis of growth response to N and P nutrition.