MICROCLIMATE AND TRANSPIRATION EFFICIENCY IN DRYLAND CROPS IN RELATION TO PLANTING GEOMETRY, GROWTH STAGE, AND CULTIVAR
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Cultivar selection, planting geometry, and plant population are the key factors determining grain sorghum yields in water deficit areas. When soil resources such as water are non-limiting, uniform cropping will provide the greatest efficiency in light interception and photosynthesis, but when resources are limiting, non-uniform treatment of the land or the crop can be an advantage. A 2-yr sorghum (Sorghum bicolor L. Moench) greenhouse study was conducted to investigate whether clump geometry (three plants clustered) improves microclimate within crop canopy when plants are grown under varying water levels. Plants were grown at two geometries (clump and conventional evenly spaced planting; ESP), two water levels (high and low representing well-watered and drought condition), and three soil surface treatments (lid covered, straw-mulched, and bare surface). Air temperature and relative humidity (RH) within the plant canopy were measured every five minutes at different growth stages. Mean vapor pressure deficits (VPDs) within the clumps were consistently lower than those for ESPs, indicating that clumps improved the microclimate. Clumps had significantly higher harvest index (HI) compared to ESPs (0.48 vs. 0.43), which was largely due to clumps having only 0.4 tillers per plant compared to 1.2 tillers per plant for ESPs. Grain yield was not different between clumps and ESPs. However, results suggest that improved microclimate was likely a reason for clumps producing significantly higher grain yields in previous studies reported in the literature. Corn (Zea mays L.) field studies were conducted in Gruver (Gruver field study, GFS) and Bushland (Bushland field study, BFS), Texas to compare plant canopy temperature, within canopy VPD, grain yield, yield components, and water use efficiency (WUE) for clump (3 plants clustered) and ESP geometries with the same plant populations. At different growth stages for both studies, thermal images were taken for calculating canopy temperature, and temperature and relative humidity within the plant canopy were measured. As a whole, canopy temperatures were significantly lower for clumps compared to ESPs, and mean VPDs within the clumps were consistently lower than those for ESPs, indicating that clumps improved the microclimate. WUE and grain yield showed mixed results, but HI was significantly higher for clumps than that for ESPs in both studies (0.56 vs. 0.54 in GFS and 0.48 vs. 0.45 in BFS). In GFS, plants were grown under three water levels (high, medium, and low). With decreasing irrigation level, canopy temperature and VPD increased and aboveground biomass, grain yield, and HI decreased. Corn plants with medium irrigation level had the highest WUE (1.83 kg m-3) compared to plants at high (1.34 kg m-3) and low (1.22 kg m-3) irrigation levels. Results suggest that growing corn in clumps may be a useful strategy under semi-arid climatic conditions because they improved microclimate, reduced number of tillers, and increased HI with comparable grain yield compared to conventional ESP. Transpiration efficiency (TE) is an important physiological trait in plants for maintaining soil moisture longer and producing high yield with limited water supply. In contrast to other major food crops, little is known about the sorghum TE and its dynamics in relation to environmental VPD. Two simultaneous studies in each of the greenhouse and plant growth chamber were conducted to compare sorghum TE at different growth stages, and to determine the effects of VPD on TE. Plants were grown using lid covered boxes and harvested at six-leaf stage (S1), flag leaf stage (S2), grain filling stage (S3), and grain maturity stage (S4). For all studies, shoot biomass increased linearly with cumulative water used in transpiration. Root biomass increased up to S3 and remained constant thereafter, but shoot biomass as well as shoot: root (S:R) ratio increased consistently from S1 through S4. The overall mean VPDs and shoot transpiration efficiency (TEshoot) for different growth stages were similar within each study. VPDs were different from one study to the other. When data from all studies were combined, ETshoot showed an inverse linear relationship with crop growing period VPD, suggesting that TE decreases as the crop growing period VPD increases. The yield of wheat (Triticum aestivum L.), one of the major crops grown in the Texas High Plains, is mainly affected by drought. Under drought conditions, TE is often considered an important determinant of plant growth and grain yield, which may differ from one cultivar to the other. A greenhouse wheat study was conducted to compare TE among six wheat cultivars namely, Triumph 64 and Scout 66 (released during 1960s), TAM W 101 and TAM 105 (released during 1970s), and TAM 111 and TAM 112 (released after 2000). Plants were grown at high and low water levels with four replications and harvested before anthesis at 62 days after planting. Aboveground dry matter showed a significant linear relationship (P < 0.0001, R2 = 0.93) with cumulative water used during the crop growing period. On average, wheat plants produced ~2.8 kg of aboveground dry matter per cubic meter of water use. WUE was not significantly different among the cultivars, but there was a trend that the older cultivars had higher WUEs. Plants growing at high water had significantly higher WUE (2.40 kg m-3) and leaf chlorophyll (55) than those at low water level (2.15 kg m-3 WUE, and 52.6 chlorophyll).