THE DYNAMICS OF HYDROPONIC CROPS FOR SIMULATION STUDIES OF THE CELSS INITIAL

Phenolo_icll development affects canopy structm_ radiation interception, and dry matter prodoefiom most crop simulation models therefore incorporate leul"emergence rate as • basic parameter. A recent study examined lea/emergence rate •s • function of temperature and daylength among wheat ( Triticmm at_ivmm L) and barley (Hordam vulgmre L) culttv•re. Leaf emergence rate and phyllochron were modeled as functions of temperature alone, daylength alone, and the interaction between temperature and daylength. The resulting equations contained an unwieldy number of constants. Here we simplify by reducing the constants by >70%, and show leaf emergence rate as • single response surface with temperature and daylength. In addition, we incorporate the effect of photosynthetic photon flux into the model Generic fits for wheat and barley show cnirirar differences less than ±5% for wheat and less than -10% (or barley. Barley is more sensitive to dayiength changes than wheat for common environmental values of daylength, which may be related to the difference in sensitivity to daylength between spring and winter coitirars. Differences in lea/emergence rate between cultivnrs can be incorporated into the model by means of • single, nomdimcensional factor for each cultivar. N A SER]_ of three papers (Cap and Moss, 1989a, b,c; hereafter C&M-a, C&M-b, and C&M-c), Cap and Moss computed the leaf emergence rate (LER, leaves d-I) for wheat and barley by counting the number of leaves on the man stem as a function of time. The slope of leaves vs. time was used to estimate the emergence rate. They found a linear relationship; i.e., LER is constant for a g_ven environment from the time of seedling emergence and up to 20 d, or approximately the third leaf. They also were concerned with a related quanuty, the phyilochron (P in degree-days leaf-t), which is the thermal time required for the emergence of each leaf. As they stated, these quantifies are important in many dynamic simulation models and need refinement. They conducted experiments using four soft-white winter wheat cultivars and four spring barley genotypes. Temperature and daylight hours were varied to determine the effect on LER and P. Temperature (7") vaned from 7.5 to 25 °C. Daylight hours (D), wlth a photosynthetic photon flux density of 400 _mol m-: s -i, vaned from 6 to 24 h. Cap and Moss fit curves for LER and P as functions of T {C&M-a), as functions of D (C&M-b), and as functions of a synthenc quantity they called the thermal-photo ratio, T'D-', in degree-days per hour (C&Mc). Their equations are useful, but have important inconsistencies. Our objective was to combine their findings mto a smgle equation for LER that will simT. Volk. Dep. of Applned Science. 26 Stuvvesant St., New York Umv. New _ork. NY 10003: and B. Bugbee. Plants, Soils, and Biometeorology Dep. Utah State Umv. Logan. UT 84322-4820. Support provuded by NASA Cooperatxve Agreements NCC2-608 Ifor T. Volk)and NCC2-139 (for B. Bugbee). Received 31 May 1990. *CorresDondung author. Published m Crop Sen. 31 : 1218-1224 ( 1991 ). plify the application in larger models. For symbols and variables used, see Table 1. RESULTS FROM CAP AND MOSS Temperature Effects The results of an experiment to isolate temperature effects were reported in C&M-a. With D -14 h, T was varied between 7.5 and 25 °C at intervals of 2.5 °C. Data points for LER and P fell along smooth curves. They fit LER and P with the following equations: LER = c_ + c2Tc3T "z [la] e = c, exp(csT) lib] The constants, A to c_, were fit for each of the eight cultivars of wheat and barley. Daylength Effects Photoperiod effects were examined in C&M-b. With T -15 °C, D was varied at 8, 10, 12. 14, 16, 18, 21, and 24 h. As with the T variation, the resulting LER and P were smooth curves. They fit these data with the following equations: D LER = []c] c_ + cTD p = cs + c,D D lid] These equations require four new constants. Note that c6 _ ca and c7 _: c9, because, as explained more fully below, P and LER vary by a factor that must include T. Temperature and Daylength Interactions In C&M-c, Cap and Moss examined the value for the phyllochron in experiments in which T -10, 15, and 20 °C, and D = 6, 10, 14, and 18 h. When these 12 points were plotted against a thermo-photo ratio, a term proposed by Cap and Moss with the umts of degree-days per hour of photoperiod (TD-L), there was a linear correlation. The foUowmg equation was used to fit this data: 7" P = clo + c,,_ lie] Since C&M-c emphasized the phyllochron, the data and potential models for LER were not shown. Choices among Models There are several inconsistencies in the results reported by Cap and Moss. For example, consider Eq. [la] and lib]. Presumably, a modeler could select either Eq. [la] for LER or Eq. lib] for P, depending on the needs of the model. However, P is usually defined as a function of LER, 7", and a constant base temperature ( Tb): TTb P = _ [2] LER Abbreviations: LER. leaf emergence rate: PPF, photosynthetic photon flux. See also Table 1. 1218 PREC. ,II_NG PAGE. BLANK NOT FIi.MIEIJ VOLK & BUGBEE; MODELING LEAF EMERGENCE RATE IN WHEAT AND BARLEY 1219 Table i. List of symbols used in modeling light and temperature effects on leaf emerllence in wheat and barley.