Field measurement of temperature and leaf growth on maize/bean inter-crop

English: Notwithstanding the emphasis of research on the intensification of sole-crop systems, the practice of inter-cropping remains widespread. Evidence is accumulating that indicates that under many situations it may represent a more efficient use of natural resources. Much of the basic information on the response of leaf growth to a single environmental factor was obtained during the 1960s when controlled environment facilities became available, yet it proved difficult to extrapolate results obtained in a controlled environment to the field situation. From this background emerged the notion that temperature constitutes one of the main environmental factors influencing leaf growth at the field level for both monocotyledonous and dicotyledonous crops. Sole- and inter-crop maize (Zea mays L.) and dry beans (Phaseolus vulgaris L.) were grown in order to examine the mechanisms by which temperature influences leaf growth during the early growth stage using of three consecutive planting dates in summer. For daily measurements of leaf growth 15 individual plant samples were measured from each replicated plot. Temperature variations were observed during the three planting dates, namely in November, January and March, from the automatic weather station at the experimental site. Generally the temperature increased gradually from the first planting in November until late January during the second planting and thereafter decreased from the beginning of February to reach the lowest temperature in May. Due to the difference in temperature at the consecutive planting dates the seedling emergence in the third planting showed took longer. From daily leaf length measurements of sole and inter-crop maize the leaf length proved to be almost linear with time (days after planting). During the first planting, the leaf growth was more rapid and the largest leaf size was recorded. In the case of the third planting it took a longer time to reach the same length due to low temperatures, while in the second planting heat stress caused the maize crop to grow at a slower rate and reach a smaller size compared to the other planting dates. For sole and inter-crop beans during the first planting, the leaf growth displayed some form of sigmoid curve, whereas In the second planting due to the high temperatures the growth appeared to have two sigmoidal cycles during the growing period. For simplicity in the analysis, the mean leaf growth rate, and the slope (rate) of a linear regression was applied for each leaf length. In maize, both approaches showed an increase in its rate with increasing leaf number with the exception of leaf 11 in first planting, whereas in the third planting the leaf growth was lower and fewer leaves resulted. In beans, these two approaches showed some differences during the growth period for all planting dates but they followed the same general trend of growth rate. Comparing the two approaches, the slope of the linear regression could render a more representative rate provided the leaf growth was linear with time . On the other hand, the behaviour of leaf growth as a function of temperature was recorded by searching for the most appropriate thermal responses by curve fitting, using the Richards function model. This gave the highest correlation of maize leaf growth with thermal time. Generally, in all planting dates and cropping systems there was a significant correlation between the leaf growth variables and thermal time after emergence when using 10°C and 30°C as Tbase and extreme temperatures respectively. In contrast, for the bean crop the estimates displayed a weak correlation and it became important to consider other environmental factors along with the temperature variations. The study also assessed the field measurements of hourly leaf extension rate versus leaf temperature for sole- and inter-cropped maize plants. On each cropping system 6 auxanometers were installed to measure hourly leaf extension rate along with leaf temperatures for three days during warm and cool periods. It was shown that the leaf extension rate (LER) is one of the first components of plant growth to be affected by short period changes in temperature. Its importance led to the measurement of hourly growth rate in conjunction with leaf temperature. In this study the LER of maize as an average for three hours was used during both warm and cool periods. The measured rate was higher during the warm period, yet declined sharply above 29.5°C. Nonetheless, most of the data concentrated on temperatures up to 24°C with very few measurements In the range of 29- 29.5°C of temperatures. These values were used as common values for both fitting lines. The combination of data from both periods produced two linear regression equation. LER reached a maximum (3.2 mm h-1) at 27.8°C and was expected to be zero at the lower temperature of 6.2°C and the higher temperature of 35.3°C. These measurements of leaf growth and temperature show how temperature variations during the early growth stage of sole- and inter-crop maize/bean influence the leaves' subsequent expansion to final size. It was also observed that temperature greatly influences the rate of leaf expansion in chronological time, particularly for leaves in the field. It is difficult to resolve leaf growth data without recourse to thermal time analysis. From the study it was seemed, that accurate estimation of Tbase and Tmax as well as the method of calculating the thermal time play a great role in assessment of possible variation of leaf growth in different planting dates.