STARCH ACCUMULATION DURING HYDROPONIC GROWTH OF SPINACH AND BASIL PLANTS UNDER CARBON-DIOXIDE ENRICHMENT

The effects of CO2 enrichment, photoperiod duration, and inorganic phosphate levels on growth and starch accumulation by spinach and basil plants were studied in a commercial hydroponic facility. During a 3-week growth period, both species exhibited increased whole-plant fresh weight as a result of an increase in atmospheric CO2 concentration from 400 to 1500 mul/l. However, basil leaves exhibited a 1.5- to 2-fold greater increase in specific leaf weight (SLW), and accumulated starch to much greater levels than did leaves of spinach. At 1500 mul CO2/l, starch accounted for up to 38% of SLW with basil compared to < 10% of SLW with spinach. The maximum ratio of starch/chlorophyll was 55.0 in basil leaves vs 8.0 in spinach leaves. High ratio values were associated with the appearance of chlorotic symptoms in leaves of basil grown under CO2 enrichment (WALLICK and ZINNEN (1990) Plant Disease 74, 171-173), whereas spinach did not exhibit chlorosis. Increasing inorganic phosphate concentrations from 0.7 to 1.8 mM in the hydroponic medium did not appreciably affect leaf starch accumulation in either species. Starch accumulation in basil leaves was not consistently related to the duration of the photoperiod. However, photoperiod-induced changes in leaf starch levels were much greater in basil than spinach. The results clearly indicate that different horticultural crops can show diverse responses to CO2 enrichment, and thus highlight the need to develop individual growth strategies to optimize production quality of each species.