Short and long term fluctuations of the leaf mass per area of tomato plants - Implications for growth models

The leaf mass per unit leaf area (LMA) is a key variable in many growth models, since it is often used to predict leaf area expansion from leaf dry weight increase, or vice versa. Influences of source-sink balance on leaf area, leaf dry weight, LMA, and leaf content in non-structural carbohydrates were investigated in glasshouse tomato crops. The source-sink balance was manipulated by artificial shading, CO2 enrichment or fruit removal using different tomato cultivars. Leaf area was hardly affected by competition for assimilates except under extreme conditions. Iri contrast, leaf dry weight, and consequently LMA, underwent large and rapid fluctuations in response to any factor that changed source and sink activities. A 60% reduction of photosynthetically active radiation involved a 24% decrease in LMA after 10 d. Carbon dioxide enrichment and fruit removal induced about a 45% and 15% increase in LMA, respectively, on plants with two fruiting trusses, but hardly affected LMA of producing plants. No significant cultivar effect could be identified. Changes in starch and soluble sugar content in leaves accounted for only 29% of diurnal variations in LMA, suggesting regular fluctuations of other components. We propose that structural LMA varies between a maximum and a minimum value according to the ratio of assimilate supply and demand during leaf development. Leaf area is independent of the supply of assimilates when the minimum structural LMA is realised. When the maximum structural LMA is attained, a storage pool of assimilates may accumulate in leaves during periods of high supply and low demand. We present a model including these hypotheses, which predicts structural and non-structural LMA variations of plants with different source-sink ratios. (C) 1998 Annals of Botany Company.