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 growt h models, since it is often used to predict leaf area expansion from l eaf dry weight increase, or vice versa. Influences of source-sink bala nce on leaf area, leaf dry weight, LMA, and leaf content in non-struct ural carbohydrates were investigated in glasshouse tomato crops. The s ource-sink balance was manipulated by artificial shading, CO2 enrichme nt or fruit removal using different tomato cultivars. Leaf area was ha rdly affected by competition for assimilates except under extreme cond itions. Iri contrast, leaf dry weight, and consequently LMA, underwent large and rapid fluctuations in response to any factor that changed s ource and sink activities. A 60% reduction of photosynthetically activ e radiation involved a 24% decrease in LMA after 10 d. Carbon dioxide enrichment and fruit removal induced about a 45% and 15% increase in L MA, respectively, on plants with two fruiting trusses, but hardly affe cted LMA of producing plants. No significant cultivar effect could be identified. Changes in starch and soluble sugar content in leaves acco unted for only 29% of diurnal variations in LMA, suggesting regular fl uctuations of other components. We propose that structural LMA varies between a maximum and a minimum value according to the ratio of assimi late supply and demand during leaf development. Leaf area is independe nt of the supply of assimilates when the minimum structural LMA is rea lised. 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 pred icts structural and non-structural LMA variations of plants with diffe rent source-sink ratios. (C) 1998 Annals of Botany Company.