Biomass and yield

The presence of large sinks for assimilates in tuber and root crops makes these crops good candidates for large growth and yield responses to rising [CO2]. In the early 1980s, literature on the effects of rising [CO2] on growth and yields of several crop species were reviewed and summarized (Kimball, 1983). These data confirmed that root and tuber crops are very responsive to elevated [CO2]. Yield increases were higher than those of the other crop groups except for those reported for fibre crops (Fig. 9.7). More recent studies have also reported substantial increases in yield for root and tuber crops grown with CO2 enrichment. For example, both CO2 enrichment experiments with potato in the Netherlands and in Italy showed considerable yield increases. In the OTC experiment (A.H.C.M. Schapendonk, personal communication, 1995), tuber growth was stimulated by 24-49%, depending on maturity class and weather conditions. Leaf area effects, however, were only small or even negative. In that case, in contrast to the predictions of the basic model, the benefit of elevated [CO2] on tuber yield was higher for the late cultivar than for the early maturing varieties and higher in a temperate year than in a warm year. Thus the extent of the CO2 effect is not determined by earliness or lateness of the cultivar but mainly by interaction effects of [CO2] and temperature/drought on: early leaf area development, senescence of leaves later in the season, the remaining buffer capacity of photosynthetically active leaves and partitioning of carbon to different plant organs. In the FACE experiment, tuber growth and final yield were also stimulated by rising [CO2] levels (Fig. 9.8). Yield stimulation by CO2 was as large as 10% for each 100 |mmol mol-1 increase, which was equal to 40% yield increase with doubling of ambient [CO2]. This

Fig. 9.7. Mean relative yield increase (ratios) of CO2-enriched to control crops for (a) immature and (b) mature plants. Immature plants are agricultural crops for which yield was taken as total plant height or weight. (For more detail, see Kimball, 1983.)

was due to more tubers per plant (1.5 tubers for each 100 |mmol mol-1 increase in [CO2]) rather than to a greater mean tuber mass or size. Studies in the USA to assess the suitability of this crop for growth under very high [CO2] in the CELSS (Controlled Ecological Life Support System; Wheeler et al., 1994) showed that

Fig. 9.8. Relationship between tuber dry mass (g plant-1) and atmospheric [CO2] during growing season, measured at three different time intervals. □ = day 248, y = 83.5 + 0.275x, r = 0.97; O = day 228, y = 49.1 + 0.447x, r = 0.99; I = day 200, y = 2.83 +0.123x, r = 0.65. (Slope coefficients all significant at P< 0.01.)

, concentration i

Fig. 9.8. Relationship between tuber dry mass (g plant-1) and atmospheric [CO2] during growing season, measured at three different time intervals. □ = day 248, y = 83.5 + 0.275x, r = 0.97; O = day 228, y = 49.1 + 0.447x, r = 0.99; I = day 200, y = 2.83 +0.123x, r = 0.65. (Slope coefficients all significant at P< 0.01.)

potato yield response to [CO2] was continuously increased up to 1000 |mmol mol-1. The positive effects of CO2 enrichment on tuber yields may decrease if tuber sink strength is limited because of a down-regulation of the assimilation rate (Wheeler et al., 1991). In this experiment, CO2 enrichment increased tuber yield and total biomass dry weight by 39% and 34%, respectively, under short-day and low-light conditions, by 27% and 19% under short-day and high-light conditions, by 9% and 9% under long-day (24 h) and low-light conditions, and decreased dry weight by 9% and 9% under long-day and high-light conditions. This shows that where tuber growth was limited, whether by longer days in which the tuber initiation and growth were retarded or by higher irradiation (from high light intensity and/or long days), the response to elevated [CO2] was also limited. In other studies, yields of carrot and kohlrabi (Sritharan et al., 1992; Mortensen, 1994; Wheeler et al., 1994) increased with [CO2] doubling, despite significant interactions between the availability of phosphorus and the [CO2].

The optimal temperature range for tuber growth (between 16 and 22°C) is small (Kooman, 1995). Daily temperatures that are outside this optimal range result in reduced tuber growth, a lower allocation of assimilates to the tubers, and thus a lower harvest index and tuber yield. With climatic change, the prevailing temperature during tuber growth will likely be different, but whether higher or lower and how much is uncertain.

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