Leaf Area Partitioning as an Important Factor in Growth

Abstract

Despite continuing efforts to correlate unit area rates of photosyn-thesis of crop varieties with growth rates, there has been little or no success. It is reasonable to assume that partitioning of photosynthate into new leaf area is an important component of growth. Accordingly, an expression was developed to measure leaf area partitioning. Using growth analysis techniques, relative growth rates were compared to net assimilation rates, partitioning of daily weight gain into new leaf area, and partitioning of daily weight gain into new leaf weight of nine species grown in growth chambers under three temperature regimes. Day/night temperatures of 21/10, 32/21, and 38/27 C caused large differences in relative growth rates. Relative growth rates were closely correlated with leaf area partitioning in seven of the nine species, but were inversely correlated with leaf weight partitioning for six of the nine species. Relative growth rates were poorly correlated with net assimilation rates for five of the nine species. The product of net assimilation rate times leaf area partitioning is shown to be equal to the relative leaf area expansion rate. These results indicate that growth responses due to temperature shifts were more sensitive to changes in leaf area partitioning or relative leaf area expansion rates than to net assimilation rates. Because changes in leaf area partitioning or relative leaf area expansion rates can have an effect on relative growth rates that overshadow changes in net assmila-tion rates, and because net assimilation rates are largely a function of unit area rates of photosynthesis, the correlation of unit area rates of photosynthesis with growth should include consideration of leaf area partitioning or relative leaf area expansion rates. For more than a decade, considerable effort has been expended in the search for crop varieties or ecotypes that have high unit leaf area photosynthetic rates (3-6, 11, 13, 14, 16, 22). These investigations have been based on the assumption that high unit area photosynthetic rates should correlate well with growth. Generally, it has been found that growth is not well correlated with unit area rates of photosynthesis (4, 13, 15, 16, 18, 23), although some authors have reported otherwise (9). However, good correlations between growth and rates of leaf area expansion (5-7, 11, 22, 24, 26) have been reported. As early as 1938, Heath and Gregory (12) concluded that ' Present address: Ornamental Plants Research Laboratory, Agricultural Research Service, United States Department of Agriculture, Cor-vallis, Oregon 97330. 2 Mississippi Agricultural and Forestry Experiment Station cooperating. rates of leaf area expansion have a greater influence on dry matter production than do net assimilation rates. The NAR3 is primarily a function of photosynthesis, particularly during the first few weeks following seedling emergence. Extensive studies later verified the predominant role of leaf area expansion over NAR as a factor in plant growth (25, 26). However, as Evans (8) points out, two recent developments have renewed the interest in the role of photosynthesis in plant growth. First, IR gas analysis has permitted the accurate and rapid measurement of short term photosynthetic rates. Second, the discovery that the pathways and rates of photosynthesis differ greatly among species has led plant scientists to reexamine the role of photosyn-thesis in plant growth. Coupled with this renewed interest in photosynthesis is a growing awareness that partitioning of photo-synthate into new leaf growth is also an important factor in plant growth (2, 10, 17, 19, 21). This paper is an assessment of the relation between growth, NAR, and the partitioning of daily weight gain into new leaf area and new leaf weight. MATERIALS AND METHODS Nine species (maize, Zea mays L. var. Trucker's Favorite; cotton, Gossypium hirsutum L. var. Stoneville 213; soybean, Glycine max [L.] Merr. var. Pickett 71; velvetleaf, Abutilon theophrasti Medic.; spurred anoda, Anoda cristata [L.] Schlecht; prickly sida, Sida spinosa L.; common cocklebur, Xanthium pensylvanicum Wallr.; johnsongrass, Sorghum halepense [L.] Pers.; redroot pigweed, Amaranthus retroflexus L.) were grown from seed in 15-cm-diameter pots of soil in growth chambers under three day/night temperatures (21/10, 32/21, and 38/ 27 C). Chamber conditions were 70 + 5% relative humidity, with 100 w m-2 of photosynthetically active radiation at the plant canopy top for 14 hr each day. Leaf areas were measured with a Hayashi Denko automatic area meter model AAM-54 (Yen Enterprises, Inc., 2912 Terminal Tower, Cleveland, Ohio 44113), and dry weights of leaves, stems, and roots of two plants of each species were determined three times a week for 4 weeks after emergence or until the total plant weight exceeded 5 g. The plant material was dried at 70 C until it reached constant weight.