THE CROP RESPIRATION RATE OF TULIPS

Transcription

1 New PhytoL (1967) 66, THE CROP RESPIRATION RATE OF TULIPS BY A. R. REES Glasshouse Crops Research Instittite, Littlehampton, Sussex {Received 18 October 1966) SUMMARY The shading methods developed by Watson and Hayashi (1965) for differentiating the photosynthetic and respiratory components of net assimilation rate were used on a field scale for estimating the respiratory losses in a tulip crop. By selecting a period in the plant's life which may be described by a linear regression, variation due to weather in growth rate under different sfiading treatments may be averaged. The mean growth rates under each shading treatment may in turn be fitted by a regression line whose intercept with zero light estimates respiration rate, A value of 2o"/o of gross crop growth rate was arrived at; this agrees closely with previous estimates of dry weight losses in bulb crops before emergence. INTRODUCTION Few comprehensive data exist for the respiration rates of a crop, and it is often the case that carefully determined estimates of net photosynthesis, or crop growth rate, are converted to gross or potential rates of photosynthesis using an arbitrary figure for respiration rate, A value of 20/^ has widely been used for this purpose, although this may be open to criticism (Black, 1964), An analysis of potential growth rates in southern England in relation to observed growth of the tulip has already been attempted (Rees, 1965); the present work was undertaken to assess whether the factor used to correct for respiration (assumed at 20% following de Wit (1959)) was reasonable. A recent paper by Watson and Hayashi (1965) describes a method of separating the photosynthetic and respiratory components of net assimilation rate. This was achieved by shading experimental plants for different periods of time (usually whole days) to prevent photosynthesis, and measuring net assimilation rates over the whole period. The regression coefficient of net assimilation rate on the number of days when photosynthesis was allowed measures the contribution of i day's photosynthesis to net assimilation rate. The photosynthetic component of the net assimilation rate is the product of the regression coefficient and the total number of days over which net assimilation rate is measured, and the respiratory component is then estimated by difference, or by extrapolating the regression line to zero days photosynthesis. The limitations of the method pointed out by the authors are that the plants must be kept in an environment uniform on all days of the experimental period, although they do suggest that 'the method might possibly be used on field crops in situations when the climate is exceptionally stable'. In addition to its use in the study of net assimilation rate, the method could also be applied to analyses of crop growth rate. In this case, a broader assessment of mean crop growth rate is possible, and respiration losses may be considered as a percentage of the 251

2 252 A. R. REES positi\l- gain in dry weigbt. The assessment of crop growtb rate from successive samples to which is fitted a regression line is the first stage in the analysis. For outdoor grown crops the rate of photosynthesis is dependent upon day-to-day variation in the weather. If a period is chosen when plant weight, determined on a number of sampling occasions, may adequately be described by a linear regression with time, then the slope of the line will represent mean growth rate over the period. Use of a number of shading treatments will provide a series of estimates of growth rate, which may be used to determine, by extrapolation, the respiration rate. Tbe measurement of leaf area, usually tedious and time consuming, is not necessary, and the pitfalls associated with the use of the expanded formula for net assimilation rate (which assumes a linear relationship between plant dry weight and leaf area) are avoided. This is particularly valuable for bulbous plants where the relationship is not linear. MATERIALS AND METHODS Graded 12 cm bulbs of the cultivar 'Rose Copland' were planted in the autumn at 3 in. spacing within rows which were 9 in. apart (giving 174 cm^/plant). The complete hlock, measuring 23 ft 3 in. by 5 ft 3 in. (7.1 x 1.6 m) was orientated north-south, and plots of twenty plants were marked off so that they were surrounded on all sides by guard plants. Shading was started on 21 April 1966 and continued for 4 weeks. Previous experience had indicated approximate linearity of total plant dry weight witb time over this period (Rees, 1966). Flower heads were removed shortly before shading was started. Cardboard boxes sufficiently large to enclose the whole twenty-plant plot were used for shading. The tops of the inverted boxes were covered by polythene sheeting to make the boxes more weather-resistant, and tbey were anchored in position by a single guy rope over the top of the box. The plots were shaded every other day, every 3rd day or were left unshaded. There was no evidence for excessively high temperatures under the shades at plant level; this may have been because the weather was generally rather dull and the boxes were almost twice the height of the plants. The appearance of the plants was also unaffected by tbe shading treatments. An initial sample (one plot) was taken on the first day of shading, and four samples of twenty plants were harvested at w eekly intervals from each of the three treatments. The plants were oven-dried for 48 hours at 80 C in a ventilated oven and weighed individually. Root weight was ignored because of difficulties of recovery; the proportion of total plant weigbt represented by the roots is less than 6 ^ even at the time of minimum total plant dry weight (Rees, 1966). RESULTS The mean plant dry weight for each of the four sampling occasions and the initial sample are shown in Fig. i, together wdth the regression lines fitted to the three shading treatments. Tbe regression coefficients and their standard errors are , and g/plant/week for the treatments in order of decreasing shade. These correspond to crop growth rates of 2.7, 6.5 and 8.8 g/m^/day respectively. The value for full daylight is considerably lower than that of 15.4 g/m^/day obtained for tbe corresponding period in a previous year (Rees, 1966), but the difference probably reflects differences in

3 Crop respiration of tulips 253 weather. In 1966 the mean daily value for incoming radiation over this 4-week period was only 302 g/cal/cm^/day, compared with 467 g/cal/cm^/day in The regression coefficients are plotted in Fig. 2 against the shading treatment. The Weeks Fig. I. Changes in mean plant dry weight for the three shading treatments. Symbols, with regression coefficients in parentheses: ; = unshaded (1.06 ±0.18); x = plots shaded onethird of time ( ); n = plotsshadedhalfthe time (0.33 ±0.10); = point common to all three treatments. 1-0 ± Light as proportion of totol Fig. 2. Relation between growth rate and light as a proportion of full light. Equation of line y = V The vertical lines represent 95",; confidence limits. intercept of the fitted regression line with zero ligbt indicates a respiration rate of 0.27 g dry matter per plant per week, which is equivalent to 2.2 g/m^/day on a land area basis, or 20% of the gross rate of dry matter production in full light. H N.P.

4 254 A. R. REES DISCUSSION In an earlier study an estimate was obtained of the loss of dry weight experienced by a tulip plant in the period from planting in the autumn until the start of net increase in weight the following spring (Rces, 1966). Expressed as a (negative) relative growth rate, an identical value of g/g/week was obtained for each of three cultivars. In the present study, assuming a mean plant dry weight of 16 g the relative dry weight loss is g/g/week. The very close agreement between these two figures, despite differences in season, although reassuring, is no doubt partly fortuitous. Some estimates of crop respiration rate are higher than the value of 20 found with tulips and assumed to be general (de Wit, 1959). Monteith (1966), using a model of light transmission and photosynthesis, arrived at a value for respiration of 44% of gross production in a sugar beet crop from July to October. The mean loss by respiration expressed on a leaf area basis was not \'ery dissimilar from that found for young sugar beet plants grown in a controlled environment by \\ atson and Hayashi (196^) (2.1 compared with 1.4 g/m-/day respectively), but the \alues determined by the latter authors for respiration as a percentage of gross production was 12.4 and 18.3 for sugar beet and barley respectively. Gaastra (1963) reviewed other work on respiration rates of field crops and concluded that the total respiration as a percentage of gross photosynthesis could be between 25 and ^o, considerably larger than the 20-2s",j often assumed for field crops. Clearly the use of shading for restricting photosynthesis and thereby arriving at an assessment of respiration rate has possibilities even on a field scale and under changeable weather conditions, pro^ ided a means of averaging the effects of weather on growth ean be achieved. The method is a useful tool for the better understanding of a relatively neglected subject. REFERENCES BLACK, J. N, (1964)..\n analysis of the potenti.il production of swards of subterranean clo^er {Trifolium subterraneum L.) at Adelaide, South Australia. J. appl. Ecol., 1, 3. GAASTRA, P. ^1963). Climatic control of photosynthesis and respiration. Environmental Control of Plant Gro-iLth (Ed. by L. T. Evans), p Academic Press, Xew York. MoNTEiiH, J. L. (1966). The photosynthesis and transpiration of crops. Expl Agric. 2, i. REES,.A. R. (1965). Potential rates of dry-matter production in Southern England with special reference to the tulip. Rep. Glasshouse Crops Res. List., 1964, 132. REES, A. R. (1966). Dry matter production by field-crown tulips. J. hort. Sci., 41, 19. WATSON, D. J. & HAYASHI, K-I. (1965). Photosynthetic and respiratory components of the net assimilation rates of sugar beet and barley. Neiu PhytoL, 64, 38. DE WIT, C. T. (1959). Potential photosynthesis of cnip surfaces. Xetli. J. a^ric. Sci., 7, 141.

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