Evaluation of models of leaf water ¹⁸O enrichment using measurements of spatial patterns of vein xylem water, leaf water and dry matter in maize leaves

The effectiveness of several leaf water models ('string-of-lakes', 'desert river' and the Farquhar-Gan model) are evaluated in predicting the enrichment of leaf water along a maize leaf at different humidities. Progressive enrichment of both vein xylem water and leaf water was observed along the blade. At the tip, the maximum observed enrichment for the vein water was 17. 6‰ at 50% relative humidity (RH) whereas that for the leaf water was 50‰ at 34% RH and 19‰ at 75% RH. The observed leaf water maximum was a fraction (0.5-0.6) of the theoretically possible maximum. The 'string-of-lakes' and 'desert river' models predict well the variation of leaf water enrichment pattern with humidity but overestimate the average enrichment of bulk leaf water. However, the Farquhar-Gan model gives good prediction for these two aspects of leaf water enrichment. Using the anatomical dimensions of vein xylem overestimates the effective longitudinal Péclet number (P ₁). Possible explanations for this discrepancy between the effective and the xylem-based estimate of P₁ are discussed. The need to characterize the heterogeneity of transpiration rate over the leaf surface in studies of leaf water enrichment is emphasized. The possibility that past atmospheric humidity can be predicted from the slope of the Δ ¹⁸O spatial variation of leaf macrofossils found in middens is proposed.