Drought-induced Changes in Shoot and Root Growth of Young Cotton Plants

An understanding of the response of plants to water deficits is important in efforts to model cotton (Gossypium hirsutum L.) growth, estimate irrigation needs, and breed drought-resistant cultivars. This study examined shoot and root growth of a longand a short-season cotton cultivar after a brief drought and subsequent recovery period. Seeds were planted in fritted clay-filled pots in a growth room under fluorescent lights at about 27 (C. Plants were divided at 36 d after planting into drought-treatment and watered-control groups. Plants were sampled after a 13-d drought and again after a 10-d recovery period. There were no treatment-by-genotype interactions. At the end of the drought and recovery, height, leaf P.F. Pace, DEKALB Genetics Corporation, 3100 Sycamore Road, DeKalb, IL 60115; Harry T. Cralle, Department of Soil and Crop Sciences, Texas AM Sherif H. M. El-Halawany (deceased), Cotton Research Institute, Agricultural Research Center, Ministry of Agriculture, Giza, Egypt; J. Tom Cothren , Department of Soil and Crop Sciences, Texas AM Scott A. Senseman, Department of Soil and Crop Sciences, Texas AM Masle and Passioura, 1987). Several studies have shown that drought inhibits cotton canopy development. Krieg and Sung (1986) determined that drought decreases the number of leaves on sympodial branches of cotton. Leaf area of glasshouse-grown cotton also was inhibited when the percentage of soil-available water was less than 51 ± 15% (Rosenthal et al., 1987). Cutler and Rains (1977) concluded that predawn leaf water potentials below $0.5 MPa were accompanied by decreased leaf elongation rate. Leaf expansion of 55-d-old cotton plants slowed after 2 d of withholding water, which meant that leaf growth was more sensitive than root elongation to drought (Ball et al., 1994). Similarly, McMichael and Quisenberry (1991) found that terminal drought decreased the shoot:root ratio. Drought also reduced the growth, development, and distribution of cotton roots (Malik et al., 1979; Taylor, 1983). Root growth of 55-d-old cotton was reduced after 6 d of withholding water (Ball et al., 1994). The number of roots elongating decreased by 35% during the drought. Planting early-maturing cultivars can decrease the amount of water used by cotton, and other traits in future cotton cultivars may further decrease the amount of water used. Quisenberry et al. (1981) found considerable variability for heat tolerance, root growth, dry matter accumulation, and water use efficiency among exotic cotton strains under dryland conditions. Gerik and co-authors (1996) compared two short-season cotton cultivars and found that one, Tamcot HQ95, yielded more than other, GP74, regardless of the level of water stress. They concluded that the photosynthetic capacity of Tamcot HQ95 might be greater than that of GP74. Root elongation during drought may help plants get deeper water, thus avoiding water deficits near the soil surface. Elongation also could reduce the water lost by drainage when precipitation allows recovery after the drought (Ludlow and Muchow, 1990). If, however, water is unavailable deeper in the soil profile, longer roots may reduce shoot dry weight and harvest index by allowing the preferential partitioning of photosynthate to roots at the expense of shoots. This study examined various measures of shoot and root growth of one longand one short-season cotton cultivar after a drought of limited duration and a subsequent recovery period. MATERIALS AND METHODS A long-season cotton, ‘Stoneville 506,’, and a short-season cotton, ‘Tamcot HQ95,’ were planted in pots (9-L volume, 20 cm deep) filled with fritted clay (Absorb-N-Dry, Balcones Co., Flatonia, TX). Filter paper at the bottom of the pots retained the fritted clay while allowing for drainage. Two plants were seeded per pot and were supplied with distilled water every other day for 10 d. The pots were then watered with a nutrient solution of 0.90 g L of 2020-20 NPK fertilizer (Peters Professional All Purpose Plant Food, Spectrum Group, Division of United Industries Corp., St. Louis, MO) until 36 d after planting. This fertilizer was selected because soil tests showed that the fritted clay had very low levels of N, P, and K and that nutrients would be quickly leached from this well-drained soil. Water or nutrient solution, when applied, was added until an excess drained from the bottom of the pot. The experiment 185 JOURNAL OF COTTON SCIENCE, Volume 3, Issue 4, 1999 was conducted in a growth room under fluorescent lights providing a photosynthetic photon flux density of 700 )mol m s for 16 h d. Temperature was maintained at 127 (C. At 36 d after planting, plants were randomly divided into drought-treatment and watered-control groups. The drought-treated plants were not watered for 13 d. At the end of this drought treatment (49 d after planting), control and drought-treated plants were sampled and height, number of nodes, leaf area, and taproot length were measured. Secondary root length of fresh roots was measured by a Comair Root Length Scanner (Commonwealth Aircraft Corp. Ltd, Melbourne, Australia). Leaves, stems, and tap and secondary roots were dried for 48 h at 90 (C before dry weights were determined. The shoot:root weight ratio was calculated from the dry weights. At 49 d after planting, the remaining droughttreated plants were watered during a 10-d recovery period. At 59 d after planting, both treatments were sampled as described above. The experiment was a randomized complete block design with two blocks. Each block had four pots of each cultivar. Each pot had two plants. The experiment was repeated twice. There was no interaction between treatment and experimental run, so data from the two runs were pooled for statistical analysis. The cultivar primary effect was insignificant, so data also were pooled across cultivar. This analysis used the SAS System (SAS Institute, Cary, NC). RESULTS AND DISCUSSION At the end of the drought treatment, droughttreated plants had significantly (P < 0.05) lower height, less leaf area, fewer nodes, and lower dry weights of stems and leaves than did the controls (Table 1). Additionally, the drought-treated plants had a lower shoot:root ratio (Table 2) than did the controls at this sampling, 49 d after planting. There were no differences between the two treatments in the lengths of the secondary roots or in the dry weight of the secondary or taproots at the end of the drought period (Table 3). However, the droughttreated plants had a significantly (P < 0.01) greater tap root length than did the controls at this time (49 d after planting). The taproot dry weight in the drought-treated plants was identical to that of controls, so the drought-related elongation occurred at the expense of taproot thickening. While the drought-treated plants had a taproot dry weight per length of only 0.011 g cm, the corresponding measurement for the well-watered controls were Table 1. Heights and dry weights of stem, leaf area and dry weight, and node number in drought-treated and control plants of Stoneville 506 and Tamcot HQ95 at the end of the drought, 49 d after planting.† Means are followed by standard errors of the mean in parentheses.