Phosphate Deficiency in Maize. I. Leaf Phosphate Status, Growth, Photosynthesis and Carbon Partitioning

Maize plants (Zea mays L.) were cultured with nutrient solutioncontaining 0.001 or 0.5 mM orthophosphate (Pi). Effects of lowphosphate (low-P) nutrition on growth, leaf phosphate status,photosynthesis, and carbon partitioning were investigated. Withlow-P treatment, the fresh weight of aerial parts decreasedby about 40% by 24 days after planting. Detailed studies ofthe effects of low-P treatment on the other characteristicsof maize leaves-were done using the middle part of the thirdleaf, counting from the base. Low-P treatment had almost noeffect on specific leaf weight or soluble protein content measured13 to 21 days after planting. Low-P treatment decreased Chicontent slightly (by 15% 19 days after planting). Twenty onedays after planting, low-P treatment had greatly decreased thelevels of leaf acid extractable Pi (by 77%) and photosynthesisrates (by 68%). The detrimental effects of low-P treatment onthe rates of photosynthesis and the amounts of acid extractablePi became progressively greater with time. There was a strongcorrelation between levels of leaf acid extractable Pi and ratesof photosynthesis. The minimum level of Pi necessary to sustainthe maximum photosynthesis rate was 0.6 mmol m^–2. Belowthis minimum content of Pi, the rate of photosynthesis decreasedsharply with decreasing Pi. To investigate the direct effectof Pi depletion on photosynthate partitioning at equivalentrates of photosynthesis, the rates in controls were reducedto almost the same as those in 18 or 19 day old low-P plants(about 50% of those in controls) by lowering light intensityand/ or ambient CO₂ concentration. The data clearly indicatesthat low-P treatment had a direct effect in lowering photosynthatepartitioning into starch. Starch mobilization during the nightwas also inhibited under low-P conditions. (Received January 7, 1991; Accepted March 5, 1991)