Growth of Citrus Rootstock (Carrizo Citrange) Seedlings During and After Long-term CO2 Enrichment

Abstract

Abstract Carrizo citrange seedlings [Citrus sinensis (L.) Osbeck × Poncirus trifoliata (L.) Raf.] were grown in field enclosures for 17 weeks during 1984 at 330, 660, or 990 µl·liter-1 CO2 and then transferred to ambient air for 10 weeks to examine growth during and after these treatments. Immediately after the CO2 treatment period, plants grown at the highest CO2 level had accumulated about 120% more total dry matter than those at 330 µl·liter-1 CO2, compared to a mean increase of 67% at 660 µl·liter-1. Shoot length, leaf area, and leaf number were also increased 71%, 55%, and 39%, respectively, at 990 µl·liter-1 CO2 and somewhat less at 660 µl·liter-1. Percentage gains from CO2 enrichment decreased during the posttreatment period, but absolute differences were maintained or increased for at least 10 more weeks. During the treatment period, dry matter partitioning among leaves, stems, and roots did not vary with CO2 level, but root growth after transplanting and transfer to ambient air occurred at the expense of shoot growth in plants that had been grown at ambient CO2 levels. Plant growth, in terms of dry weight, shoot length, leaf area, and leaf number, was advanced 1.0 to 3.5 weeks after 17 weeks at elevated CO2 levels, and fresh weight per centimeter of stem length was about 1 month ahead. This last parameter can be used as an approximate indicator of readiness for scion budding, suggesting that CO2 enrichment could represent a considerable value to the citrus nursery industry. Data are also consistent with the hypothesis that a greater response to elevated CO2 is likely to occur where a high sink demand and/or low levels of leaf starch are maintained. Both such demand and low leaf starch occur in citrus seedlings, which have a pronounced juvenility period and capacity for continuous flush-type growth.