Schynder (1) also discusses the problems associated with designing experiments to accurately measure the stem contributions to accumulation of grain dry weight. Some of these problems can be illustrated by examining field data collected in the 1999 season. Wheat cv. Patterson was collected once or twice weekly from 9 days prior to anthesis through the conclusion of caryopsis weight gain. Ketose sugars (fructose, sucrose and fructan) accumulated and were remobilized from the stem twice during this period (Fig. 1). At first glance this data supports the remobilization of stored
Days Relative To Anthesis/Date
Figure 1. Wheat plants were frozen in the field at the Purdue Agricultural Research Station in 1999, freeze dried, ground, extracted and the total ketose sugars determined as fructose equivalents using cold anthrone. Kernel dry weight was determined on 12 kernels from the 6 central spikelets of six different main stems. Plants were marked at anthesis when the anthers extruded from the central portion of the head.
reserves (mostly fructan) to the grain. For example in the 3rd internode from the top of the plant at 10 days after anthesis the ketose sugar content reached a maximum of 15.8% then dropped to 3% at 33 days (Fig. 1). It would be tempting to say that the loss of ketose sugars from the three internodes analyzed occurred during the linear phase of dry weight gain of the caxyopsis and so these reserves contributed to the dry weight of the kernel. Unfortunately this may not be true since ketose sugars are a heterologous mixture whose relative quantification is more easily achieved than determining an absolute value. Even with advances in procedures for separating and for detecting the purified polymers, quantification still involves bulking numbers and averaging to describe a response (5, 6, 7, 8, 9). Values obtained are no erroneous, just relative, so that partitioning stem loss to the grain or to respiration or to another use is difficult. Mass balance data will suffer from this inability to link loss in stem mass to gain in kernel mass. Mass balance first used by Pierre (2) in 1867, contributed to the estimates presented by Schnyder (1) in his review and is still the method most often exploited since 1993 (10-26). Although it may be logical to suggest that losses in stem reserves during peak periods of grain weight gain are related it must be remembered that there is not a cause and effect relationship between the two. Simultaneous decreases of fructan with an increase in fructan hydrolytic activity represents a closer association (10, 11). By labeling the carbon used for CO2 fixation it is possible to show carbon transfer from the stem to the grain (26, 27), but determining amounts of carbon moved is more difficult because of the time lapse between labeling and repartitioning. Respiration of radiolabeled carbohydrates and subsequent refixation could provide error in calculations of repartitioning. The research reported since the 1993 review have used mass balance and labeling of reserves as methods of choice, although both are somewhat imprecise.
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