Relationship Between Physiological and Morphological Characteristics and Yield of Nondormant Alfalfa Clones

Alfalfa (Medicago sativa L.) breeders are constantly striving to improve the productivity of alfalfa. Yields have been increased primarily through the selection of plant materials resistant to insects and diseases. The objective of this study was to evaluate physiological characteristics which might be used as selection criteria in alfalfa improvement. Alfalfa clones were grown under field conditions on a Mojave clay loam soil. Carbon dioxide flux was evaluated in a closed system using an infrared gas analyzer. The physiological variables measured in these studies did not account for the varialion in yield among the clones. Apparent photosynthetic rates, dark respiration rates, and postillumination CO, burst rates expressed as mg CO, dm-% hour-I were not correlated with dry matter production. However, when these physiological factors were multiplied by leaf area and expressed as total CO, exchange per plant per hour, there was a significant relationship between yield and these calculated variables, in both studies. Regression analyses of more than 30 factors indicated that leaf area, leaf to stem-petiole ratio, and leaf weight per plant accounted for more than 95% of the variation in yield among alfalfa clones. The data from these studies suggest that morphological factors were more reliable indicators of alfalfa productivity than physiological factors. Additional index words: Photosynthesis, Respiration, Leaf Area, Leaflet to stem-petioles ratio, Leaflet weight. Y I E L D is a function of the quantity of the photosynthetic products available for plant growth (1 1). However, little evidence exists that photosynthetic rates per unit leaf area are correlated with yield of individual genotypes within the same species. Cultivars may have high potential photosynthetic activity per unit leaf area and yet not produce a high yield because of genetic or environmental factors which lcontribution from the Dep. of Agronomy and Plant Genetics, Univ. of Arizona, Tucson. This paper is part of a Ph.D. dissertation submitted by the senior author in partial fulfillment of the requirements for the Ph.D. degree. Arizona Agriculture Exp. Stn. journal paper no. 2480. Received 7 July 1975. ,Former graduate research associate (now assistant professor of crop science, California Polytechnic State Univ., San Luis Obispo, CA 93407); former graduate research assistant (now graduate assistant, Dep. of Agronomy, Univ. of Missouri, Columbia, MO 65201), and professor, Dep. of Agronomy and Plant Genetics, Univ. of Arizona, Tucson, AZ 85721. would limit the utilization of this potential. Heichel and Musgrave (9) observed differences in mean photosynthetic rates of 100 and 20070 among several inbred lines and open pollinated cultivars of maize (Zea mays L.). Carlson et al. (2) suggested that differences in photosynthetic rates in alfalfa (Medicago sativa L.) were heritable and increased yields could be obtained by selection for increased photosynthetic potential. Yoshida (15) suggested that dry matter production of a single plant must be correlated with the product of leaf area and photosynthetic rate. Delaney and Dobrenz (4) found that dry matter forage yield of a single alfalfa plant was significantly correlated with the product of apparent photosynthesis and leaf area per plant. Several researchers (5, 7, 11) found that dry matter production was dependent on leaf area rather than photosynthetic rate. Alberta (1) indicated that respiration was an important factor which contributed to the dry matter production of closed canopies, but that it had received far less attention than photosynthesis. Ishizuka (10) suggested that a simple way to obtain higher yields would be to lower respiration as much as possible. However, Delaney and Dobrenz (4) found that dark respiration rates per unit leaf area were not significantly associated with dry matter production in alfalfa. Decker (3) first demonstrated photorespiration in 1955 as a post-illumination burst of COz. Since then many researchers (12, 14, 16) have studied photorespiration and noted considerable variation in photorespiratory rates among species. Zelitch (17), Zelitch and Day (18), and Wilson (14) suggested that net photosynthesis was decreased in many plant species because of high rates of photorespiration. They concluded that increased COz uptake could be achieved by selection for plants with low photorespiratory rates. Zelitch (17) noted that the carbon dioxide compensation point was a function of photorespiration. Decker (3) stated that net photosynthetic rate and photorespiratory rate measured by the C 0 2 compensation point was probably the most useful index of photosynthetic efficiency available to plant breeders. Heichel (8), on the other hand, suggested that the COz com588 AGRONOMY JOURNAL, VOL. 68, JULY-AUGUST 1976 pensation point may not reflect respiratory activity. He concluded that GOz compensation points would be of little value for screening inefficient species for lines low in photorespiration. Researchers (6, 14) have reported differences in COz compensation points among species of the genera Lolium, Triticum, and Aegilops. They concluded that the investigation of these genera for low COz compensation points would be valuable. Photosynthesis by leaves accounts for most of the carbon dioxide incorporated into a plant. Researchers have, therefore, looked at leaf characteristics for a basis of selection of plants with high photosynthetic rates. Pearce et al. (13) and Carlson et al. (2) observed positive correlations between specific leaf weight and net photosynthesis. Differences in plant populations indicated that specific leaf weight was heritable and that there was a possibility of increased net photosynthesis in plants selected for high specific leaf weight. Delaney and Dobrenz (4) concluded that palisade tissue thickness of alfalfa leaflets could be used as selection criterion for photosynthetic potential. The obiectives of this research were to evaluate Table 1. Means of morphological variables measured on five alfalfa clones in the space-planted study in 1972.