Growth Analysis Based on Degree Days

Comparisons of growth analysis functions within and among experiments are often confounded by sources of variation other than those imposed by treatment. w e suggest use of a temperature index, such as modified growing degree days, as the divisor in growth functions to facilitate treatment comparisons within certain experiments and to reduce the effects of differing temperature regimes among experiments on these comparisons. Three experiments were identified to provide data to analyze this new approach. Mean absolute growth rate 0 and mean relative growth rate @3X) were compared in two experiments with maize (Zea mays L.) conducted in eastern Nebraska. Previously published values of KCR and mean net assimilation rate 0 of barley (Hordeum vulgare L.) grown under controlled environments in a soil temperature and P fertility study were also evaluated. Use of modified growing degree days, rather than days, as the divisor in these growth functions led to the recognition of physiological differences due to or associated with treatment, which were previously masked by normal crop response to temperature, and clarified other treatment differences by reducing the effect of temperature. Additional index word^ Barley, Growth functions, Growth rate, Heat units, Hordeum vulgare L. , Maize, Net assimilation rate, Relative growth rate, Zea mays L. R ATES of most biological processes are affected markedlv bv temnerature. Growth and devel/ I 1 opment of whole organisms show a temperature response which results from the integrated effect of temperature on the many individual physiological processes involved. Neild and Seeley (22) quoted the report by Reamur in 1735 that plant development was not as closely related to time as to accumulated temperature. Numerous studies have demonstrated the usefulness of temperature indices, like growing degree days or heat units, for predicting crop growth and development, classifying crop s ecies, hybrids and varieties, or evaluating climates For specific crop-management combinations (2, 7, 9, 11, 22, 25, 28). Most proposed temperature indices show si nificantly greater cor2 relation with plant growth an development than does accumulated time, although differences in the relationshi among temperature indices are slight (6, 7, 9, 12, ! 0, 27). Plant growth and development are certainly affected by factors other than temperature, such as flux and duration of photosynthetically active radiation, availability of nutrients and water, and loss of photosynthetic tissue. Day length plays a well-known, integral part in induction and initiation of flowering in many species (4). However, even with maize (Zea mays L.) grown under field conditions, for example, temperature indices alone can often explain over 95% of the variability in development (20, 22). Despite general acceptance of this close relationship, the use of temperature indices has not been general1 extended to growth analysis. Growth analysis has e p f e s s o r , d e b . of Univ. of Nebr. -Lincoln, Lincoln, ebr. ; and research where W1 and W2 are weights plant k%::%bA-ARS, Lincoln, Nebr. at two successive sampling times, and MI and M2 are RUSSELLE ET AL. : GROWTH ANALYSIS BASED ON DEGREE DAYS 2 9 a tem erature index, such as modified growing degree d' ays, from some common date such as planting or emergence to the respective dates of sampling. Growth analysis formulae calculated in this manner should be desi nated by the subscript "M" for modified rowin fegree days to difFerentiate them from time% ased f ormulae. The NARM should be calculated only after establishing the relationships between leaf area (A) and dry weight (W), since the form of the equation depends upon this (24). However, the chosen temperature be used as the divisor. The objective of this paper is to demonstrate the use of temperature index as opposed to time as the divisor in growth analysis functions, using data from three previously completed experiments. MATERIALS AND METHODS To show the effect of using a temperature index rather than time in the calculation of growth functions, we applied standard analyses of variance to data from two field experiments conducted in eastern Nebraska by the authors, both involving maize. Experiment I was conducted near Lincoln, Nebr. , on a Butler silty clay loam (fine, montmorillonitic, mesic Abruptic Argiaquolls) on rain-fed maize (cv. Neb 620) planted in each of 2 years on plots subjected to three different combinations of tillage and manure application. All plots received a uniform application of 70 kg N ha-' as NH,NO, and were replicated four times. Plant samples (four from each replication) were procured from each treatment for dry matter determination at 5-leaf, 11to 12-leaf, blister, hard-dough, and physiological maturity stages in 1977, and 4-leaf, 12to 13-leaf, blister, harddough, and physiological maturity stages in 1979. Experiment I1 was conducted near Mead, Nebr. , on a Sharpsburg silty clay loam (fine, montmorillonitic, mesic Typic Argiudolls) and included early and late planting (late April and late May 1980, respectively) of irrigated maize (cv. Neb 714) with a factorial combination of two N rates at four application times. Three plants from each of four replications were sampled at %leaf, 12-leaf, silking, soft-dough, and physiological maturity [stages 2,3,5,7, and 10, respectively (14)l Aboveground dry weights were determined for individual plants after d at 70" C, and calculations of m, RGR, GR,, and RGR, were made for both experiments. Standard analyses of variance were computed for all functions. In Exp. I, treatments were sampled on the same date, so only the comparisons of growth functions between years are discussed here. In Exp. 11, all N treatments within a plagng date were sampled at the same time, so conversion of GR and RGR to GR, and m,, respectively, did not change these comparisons. Because differences between the analyses were statistically significant only between planting dates, only these means are examined here. Also evaluated were data published by Power et al. (23) on barley (Hordeum vulgare L.) grown in controlled environment chambers on Parshall fine sandy loam (coarseloamy, mixed Pachic Haploborolls) topsoil at two P fertilization rates (8.8 and 44.0 mg P kg-'), constant air temperature (22"C), and constant soil temperatures of 9.0, 15.5, or 22.0°C (Exp. 111). Dry-matter and leaf-area determinations were made at the 3-leaf, 4-leaf, tillered, headed, soft-dough, and maturity s t a e o r r e s p o n d i n g to stages 1 to 6, respectively (23)l. The NAR and RGR, calculated by Table 1. V&es of r u d m, calculated on the basis of days (dl, and GRM and RGRM, calculated on the basis of modified growing degree days (M), for the indicated intervals between growth stages for tillage experiment with maize at Lincoln, Nebr. (Exp. I). Day basis Growing degree day basis Growth stage interval? Year 0-1 1-3 3-6 6-8 8-10 0-1 1-3 3-6 6-8 8-1