PHOTOSYNTHETIC IRRADIANCE RESPONSE CURVES OF PHASEOLUS-VULGARIS UNDERMODERATE OR SEVERE MAGNESIUM-DEFICIENCY

Bean plants were cultured in nutrient solution with varied magnesium s upply, and gas exchange was measured in a clamp-on cuvette connected t o a differential infrared gas analyzer. Moderate magnesium deficiency slightly affected leaf chlorophyll (Chi) content, caused accumulation of sugars and starch and slight decrease of stomatal conductance (g(s) ) and did not affect net photosynthetic rate (P-N) under low measuring irradiance. A highly significant decrease of P-N was found in these p lants under high measuring irradiance. Severe Mg deficiency with prono unced Chi loss was related to almost complete closure of stomata, loss of irradiance response of P-N and drastically reduced CO2-fixation ra tes. Resupply of Mg to deficient plants resulted in decrease of accumu lated saccharides and rapid restoration of P-N and g(s).